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Module 3 Chapters . Electron Theory 2. Static Electricity and Conduction 3. Electrical Terminoloov 4. Generation of Elect#ity 5. DC Sources of Electricity 6. DC Circuits 7. Resistance/Besistor 8. Power 9, Capacitance/capacitor 1

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1. lnductance/inductor

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Magnetism DC Motor/Generator Theory

13. ACTheory 14. Flesrstive (R). Capacitive (C) and lnductive (L) Circuits 15. Transformers

16, Filters 17. AC Generators 18. AC Motors

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1: |; Module 3 Preface

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3.1 Electron Theory

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Copyright Notice copyright. All worldwide rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form by any other means whatsoever: i.e. photocopy, electronic, mechanical recording or otherwise without the prior written permission of Total Training Support Ltd.

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Basic knowledge for categories A, B1 and 82 are indicated by the allocation o{ knowledge levels indicators (1, 2 or 3) against each applicable subject. Category C applicants must meet either the category Bt or the category 82 basic knowledge levels. The knowledge level indicators are defined as lollows:

LEVEL

1

A familiarisalion with the principal elements of the subject. Objectives: The applicant should be familiar with the basic elements of the subject. The applicant should be able to give a simple description of the whole subject, using common words and examples. The applicant should be able lo use typical terms.

LEVEL 2 A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge. Objectives: The applicant should be able to understand the theoretical fundamentals ol the subject. The applicant should be able to give a general description of the subject using, as appropriate, typical examples. The applicant should be able to use mathematical lormulae in conjunclion with physical laws describing the subject. The applicant should be able to read and understand sketches, drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using detailed procedures.

LEVEL 3 A detailed knowledge of the theoretical and practical aspects ol the subjecl. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner. Objectives: The applicant should know the theory of the subject and interrelationships with other subjects. The applicant should be able to give a detailed description of the subject using theoreticil fundamentals and specific examples. The applicant should understand and be able to use mathematical formulae related to the subject. The applicant should be able to read, understand and prepare sketches, simple drawings and schematrcs describing the subject. The applicant should be able to apply his knowledge in a practical manner using manufacturers instructions. The applicant should be able to interpret results from various sources and measurements and apply corrective action where appropriate.

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Table of Contents

Module 3.1 Electron Theory Matter Elements and Compounds Molecu les Atoms Energy Levels Shells and Sub-shells Valence Compounds lonisation Conductors, Semiconductors, and lnsulators

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Module 3.1 Enabling Objectives and Certification Statement Certif ication Statement These Study Notes comply with the syllabus of EASA Regulation 2O42|2OO3 Annex lll (Part-66) ix l. and the associated Levels as ed below:

Structure and distribution of eleckical charges within: atoms. molecules. ions. comoounds

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Module 3.1 Electron Theory Matter ',latler is defined as anything that occupies space and has weight; that is, the weight and :'nensions of matter can be measured. Examples of matter are air, water, automobiles, : cthing. and even our own bodies" Thus, we can say that matter may be found in any one of :^ree states: solid, liquid, and gaseous.

Elements and Compounds

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ELEMENT is a substance which cannot be reduced to a simpler substance by chemical -eans. Examples of elements with which you are in everyday contact are iron, gold, silver, ::lper. and oxygen. There are now over 100 known elements. All the different substances we .1":','; about are composed of one or more of these elements. ,',

'en hvo or more elements are chemically combined, the resulting substance is called a

compound. A compound is a chemical combination of elements which can be separated by :-:nical but not by physical means. Examples of common compounds are water which .:^s sts of hydrogen and oxygen, and table salt, which consists of sodium and chlorine. A mixture, on the other hand, is a combination of elements and compounds, not chemically ::-c,ned. that can be separated by physical means. Examples of mixtures are air, which is --:a up of nitrogen, oxygen, carbon dioxide, and small amounts of several rare qases, and sea yrhich consists chiefly of salt and water. 'r a:e'.

Molecules

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molecule is a chemical combination of two or more atoms, (atoms are described in the next

::'a3raph). In a compound the molecule is the smallest particle that has all the characteristics

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Water is matter, since it occupies space and has weight. on the temperature, it may exist as a liquid (water), a solid (ice), or a gas (steam). =::ai-d ess ol the temperature, it will still have the same composition. lf we start with a quantity :' ,,, a:er. divide this and pour out one half , and continue this process a sufficient number of : -:s. ,1'e will eventually end up with a quantity of water which cannot be further divided without ::ls r'J io be water. This quantity is called a molecule of water. lf this molecule of water : , r:r. instead of two pafts of water, there will be one parl of oxygen and two parts of

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Atoms r.t: 3:- es are made

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up of smaller parlicles called atoms. An atom is the smallest particle o{ an

::-er: ihal retains the characteristics of that element. The atoms oi one element, however, : -:' '-3''n the atoms of all other elements. Since there are over '100 known elements, there - - s: :e o,rer 100 different atoms, or a different atom for each element.

Module 3.1 Electron Theory

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words can be made by combining the proper letters of the alphabet, so thousands of different materials can be made by chemically combining the proper atoms.

Any particle that is a chemical combination of two or more atoms is called a molecule. The oxygen molecule consists of two atoms of oxygen, and the hydrogen molecule consists of two

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hydrogen, and oxygen. These atoms are combined into sugar molecules. Since the sugar molecules can be broken down by chemical means into smaller and simpler units, we cannot have sugar atoms.

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The atoms of each element are made up of electrons, protons, and, in most cases, neutrons, which are collectively called subatomic particles. Furthermore, the electrons, protons, and neutrons of one element are identical to those of any other element. The reason that there are different kinds of elements is that the number and the arrangement of electrons and protons

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withintheatomaredifferentforthedifferentelements The electron is considered to be a small negative charge of electricity. The proton has a positive charge of electricity equal and opposite to the charge of the electron. Scientists have measured the mass and size of the electron and proton, and they know how much charge each possesses. The electron and proton each have the same quantity of charge, although the mass of the proton is approximately 1837 times that of the electron. ln some atoms there exists a neutral padicle called a neutron. The neutron has a mass slightly greater than that of a proton, but it has no electrical charge. According to a popular theory, the electrons, protons, and neutrons of the atoms are thought to be arranged in a manner similar to a miniature solar system. The protons and neutrons form a heavy nucleus with a positive charge, around which the very light electrons revolve. Figure 1.'1 shows one hydrogen and one helium atom. Each has a relatively simple structure. The hydrogen atom has only one proton in the nucleus with one electron rotating about it. The helium atom is a little more complex. lt has a nucleus made up of two protons and two neutrons, with two electrons rotating about the nucleus. Elements are classified numerically according to the complexity of their atoms. The atomic number of an atom is determined by the number of protons in its nucleus.

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PFIOTONS

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Structule of Hydrogen and Helium

ln a neutral state, an atom contains an equal number of protons and electrons. Therefore, an atom of hydrogen - which contains one proton and one electron - has an atomic number of 1; and helium, with two protons and two electrons, has an atomic number of 2. The complexity of atomic structure increases with the number of protons and electrons.

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Energy Levels

of energy' By since an electron in an atom has both mass and motion, it contains two types position it also contains virtue of its motion the electron contains kinetic energy. Due to its the factor pot"nti"iln"tgy. The totai energy contained by an electron (kinetic plus potential) is this orbit' to remain in which determines the radius of th"e electron orbii' ln order for an electron it must neither GAIN nor LOSE energy. in which this energy exists It is well known that light is a form of energy, but the physical form not

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photons' one accepted theory proposes the existence of light as tiny packets of energy.called colour of the

photons can contain uuriou" quuniitles of energy.-The amount depends upon the the electron iigr'1-r"otu;0. Should a ptroioii ot sufficient en6igy collide with an orbital electron, a greater has now which *]ft aOsorU the photon's energy, as shown in figure t.Z. The electron, new The first nucleus' than normal amount ot energ-y, witt lump to a nlw orbit farther from the r-1ui a radius four times as large as the radius of the original orbit to which the electron to which it "Ji'iutnri orbit. Had the electron ,"""V"i ft"utut amount of energy, the next possible orbit " orbit may be considered to could jump would have u ruJir. niie times the original. Thus, each lt must be iepresent'one of a large nrrb"r of energy levels that the electron may attain' will remain in its lowest emphasized that the e]ectron cannot jum-[ to iusl any orbit' The electron will accept the orbit until a sufficient amou;t of energy is auuil"bl", at which time the electron exist in the space unO jump to one of a series oipermissible orbits. An electron cannot photon of energy unless "n"igy netwL'en enLrgy levels. This indicates that the electron will not accept a Heat energy and it contains enough energy to elevate itself to one of the higher energy levels. jump orbits. collisions with oi-her partictes can also cause the electron to

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Energy levels in an atom

lowest possible energy Once the electron has been elevated to an energy level higher than the th" atom is said to be in an excited state. ihe electron will not remain in this excited excess energy and return aonOition for more than a fraction of a second before it will radiate the electron has just to a lower energy orbit. To illustrate this principle, assume that a normal energy level' ln a short received a phoion ot energy iufficient to'raise it from the first to the third

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period of time the electron may jump back to the first level emitting a new photon identical to the one it received. A second alternative would be for the electron to return to the lower level in two jumps; from the third to the second, and then from the second to the first. ln this case the electron would emit two photons, one for each jump. Each of these photons would have less energy than the original photon which excited the electron.

This principle is used in the fluorescent light where ultraviolet light photons, which are not visible to the human eye, bombard a phosphor coating on the inside of a glass tube. The phosphor electrons, in returning to their normal orbits, emit photons of light that are visible. By using the proper chemicals for the phosphor coating, any colour of light may be obtained, including white. This same principle is also used in lighting up the screen of a television picture tube. The basic principles just developed apply equally well to the atoms of more complex elements. ln aioms containing two or more electrons, the electrons interact with each other and the exact path of any one electron is very difficult to predict. However, each electron lies in a specilic energy band and the orbits will be considered as an average of the electron's position.

Shells and Sub-shells The ditference between the atoms, insofar as their chemical activity and stability are concerned, is dependent upon the number and position of the electrons included within the atom. How are these electrons positioned within the atom? ln general, the electrons reside in groups of orbits called shells. These shells are elliptically shaped and are assumed to be located at fixed intervals. Thus, the shells are arranged in steps that correspond to fixed energy levels. The shells, and the number of electrons required to fill them, may be predicted by the employment of Pauli's exclusiol principle. Simply stated, this principle specifies that each shell will contain a maximum of 2n'electrons, where n corresponds to the shell number starling with the one closest to the nucleus. By this principle, the second shell, for example, would contain 2(2\2 or I electrons when f ull. ln addition to being numbered, the shells are also given letter designations, as pictured in figure '1-3. Starting with the shell closest to the nucleus and progressing outward, the shells are labelled K, L, M, N, O, P, and Q, respectively. The shells are considered to be full, or complete, when they contain the following quantities ol electrons: two in the K shell, eight in the L shell, 18 in the M shell, and so on, in accordance with the exclusion principle. Each of these shells is a major shell and can be divided into sub-shells, of which there are four, labelled s, p, d, and f. Like the major shells, the sub-shells are also limited as to the number of electrons which they can contain. Thus, the "s" sub-shell is complete when it contains two electrons, the "p" sub-shell when it contains 6, the "d" sub-shell when it contains 10, and the "f" sub-shell when it contains 14 electrons.

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Figure 1 .3 - Shells in an atom ln as much as the K shell can contain no more than two electrons, it must have only one subshell, the s sub-shell. The M shell is composed of three sub-shells: s, p, and d. lf the electrons in the s, p, and d sub-shells are added, their total is found to be 18, the exact number required to fill the M shell. Notice the electron configuration for copper illustrated in figure 1.4. The copper atom contains 29 electrons, which completely fill the first three shells and sub-shells, leaving one electron in the "s" sub-shell of the N shell.

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Figure 1.4 - The copper atom

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Valence The number of electrons in the outermost shell determines the valence of an atom. For this reason, the outer shell of an atom is called the valence shell; and the electrons contained in this shell are called valence electrons. The valence of an atom determines its ability to gain or Iose an electron, which in turn determines the chemical and electrical properties of the atom. An atom that is lacking only one or two electrons from its outer shell will easily gain electrons to complete its shell, but a large amount of energy is required to free any of its electrons. An atom having a relatively small number of electrons in its outer shell in comparison to the number of electrons required to fill the shell will easily lose these valence electrons. The valence shell always refers to the outermost shell.

Compounds Pure substances made up more than 1 element which have been joined together by a chemical reaction therefore the atoms are difficult to separate. The propedies of a compound are different from the atoms that make it up. Splitiing of a compound is called chemical analysis. Note that a compound: consists of atoms of two or more different elements bound together, can be broken down into a simpler type of matter (elements) by chemical means (but not by physical means), has properties that are different from its component elements, and always contains the same ratio of its component atoms.

lonisation When the atom loses electrons or gains electrons in this process of electron exchange, it is said to be ionized. For ionisation to take place, there must be a transfer of energy which iesults in a change in the internal energy of the atom. An atom having more than its noimal amount of electrons acquires a negative charge, and is called a negative ion. The atom that gives up some of its normal electrons is left with less negative charges than positive chargei and is called a positive ion. Thus, ionisation is the process by which an atom loses or gains electrons.

Conductors, Semiconductors, and lnsulators ln this study of electricity and electronics, the association of matter and electricity is important. Since every electronic device is constructed of parts made from ordinary matter, the e{fects o{ electricity on matter must be well understood. As a means of accomplishing this, all elements of which matter is made may be placed into one ol three categories: conductors, semiconductors, and insulators, depending on their ability to conduct an electric current. conductors are elements which conduct electricity very readily, insulators have an extremely high resistance to the flow of electricity. All matter between these two extremes may be called sem iconductors.

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Electrical Fundamentals 3.2 Static Electricity and Conduction

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Category A, 81, 82 and C Aircraft Maintenance

('l' 2 or by the allocation o{ knowledge levels indicators Basic knowledge {or categories A, B1 and 82-are indicated or the category 82 c"i"gorv c applicants must meet either the category 81 3) againsl each applicable "ror""i. basic knowledge levels. are defined as follows: indicators level The knowledge

LEVEL

1

A familiarisation wilh the principal elements of the subject' Objectives: o{ the subiect' The applicant should be familiar with the basic elements of the whole subject, using common words and description gi"" simple a The applicant shoutd be Ji" t" examples. The applicant should be able to use typical terms'

LEVEL 2 the subiect' A general knowledge of the theoretical and practical aspects of An ability to apply that knowledge' Objectives: ol the subject' The applicant should be able to understand the theoretical fundamentals i" giue a generat description ol the subject using, as appropriate, typical The applicant shorro o" "[i" examples.

Theapplicantshouldbeabletousemathematicalformulaeinconjunctionwithphysicallawsdescribingthe subject.

TheapplicantShouldbeabletoreadandunderstandSketches,drawingsandschematicsdescribingthe subject.

Theapplicantshouldbeabletoapplyhisknowledgeinapracticalmannerusingdetai|edprocedures.

LEVEL 3 subject' A detailed knowledge oi the theoretical and practical aspects o{ the elements of knowledge in a logical and comprehensive A capacity to combine and apply th" ""p"rui" manner. Objectives:

TheapplicantshoUldknowthetheoryofthesubjectandinterrelationshipSWithothersUbjects.lundamenlals the subject using theoretical The applicant sho"ro n" uorL io giue a d"taiteo description of and sPecific examPles.

Theapplicantshouldunderstandandbeabletousemathematicalformulaerelatedtothesubject' TheapplicantShourooeaotetoread,understandandpreparesketches,simpledrawingsandschematics describing the

subiect.

practical manner usrng manu lacturer,s The applilant sho;ld be able to apply his knowledge in a instructions.

TheapplicantShouldbeabletointerpretresultsfromVarioussourcesandmeasurementsandapply corrective action where appropriate'

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Table of Contents

Module 3.2 Static Electricity and Conduction lntroduction Static Electricity Nature of Charges Charged Bodies Coulomb's Law of Charges Unit of Charge Electric Fields Conduction of Electricity in Solids, Liquids and a Vacuum

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Module 3.2 Enabling Obiectives and Certification Statement Certif ication Statement These Study Notes comply with the syllabus of EASA Regulation 2O42|2OO3 Annex lll (Part-66) dix l. and the associated Knowledqe Levels as

Static Electricitv and Conduction Static electricity and distribution of electrostatic Electrostatic laws of attraction and Units of charqe, Coulomb's Law Conduction of electricity in solids, liquids, gases and a vacuum

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lntroduction Electrostatics (electricity at rest) is a subject with which most persons entering the field of electricity and electronics are somewhat familiar. For example, the way a person's hair stands on end after a vigorous rubbing is an effect of electrostatics. While pursuing the study of electrostatics, you will gain a better understanding of this common occurrence. Of even greater significance, the study of electrostatics will provide you with the opportunity to gain important background knowledge and to develop concepts which are essential to the understanding of electricity and electronics.

lnterest in the subiect of static electricity can be traced back to the Greeks. Thales of Miletus, a Greek philosopher and mathematician, discovered that when an amber rod is rubbed with fur, the rod has the amazing characteristic of attracting some very light objects such as bits of paper and shavings of wood. About 1600, William Gilbert, an English scientist, made a study ol other substances which had been found to possess qualities of attraction similar to amber. Among these were glass, when rubbed with silk, and ebonite, when rubbed with fur. Gilbert classified all the substances which possessed properties similar to those of amber as electrics, a word of Greek origin meaning ambe

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Because of Gilbert's work with electrics, a substance such as amber or glass when given a vigorous rubbing was recognized as being electrified, or charged with electricity. ln the year 1733, Charles Dufay, a French scientist, made an impodant discovery about electrif ication. He found that when a glass was rubbed with fur, both the glass rod and the fur became electri{ied. This realization came when he systematically placed the glass rod and the fur near other electrified substances and found that certain subslances which were attracted to the glass rod were repelled by the fur, and vice versa. From experiments such as this, he concluded that there must be two exactly opposite kinds of electricity. Benjamin Franklin, American statesman, inventor, and philosopher, is credited with first using the terms positive and negative to describe the two opposite kinds of electricity. The charge produced on a glass rod when it is rubbed with silk, Franklin labelled positive. He attached ihe term negative to the charge produced on the silk. Those bodies which were not electrified or charged, he called neutral.

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Static Electricity ln a natural or neutral state, each atom in a body of matter will have the proper number of electrons in orbit around it. Consequently, the whole body of matter composed of the neutral atoms will also be electrically neutral. ln this state, it is said to have a "zero charge." Electrons will neither leave nor enter the neutrally charged body should it come in contact with other neutral bodies. lf, however, any number of electrons is removed from the atoms of a body of matter, there will remain more protons than electrons and the whole body of matter will become electrically positive. Should the positively charged body come in contact with another body having a normal charge, or having a negative (too many electrons) charge, an electric current will flow between them. Electrons will leave the more negative body and enter the positive boo\ This electron flow will continue until both bodies have equal charges. When two bodies of matter have unequal charges and are near one another, an electric force is exerted between them because of their unequal charges. However, since they are not in contact, their charges cannot equalize. The existence of such an electric force, where current cannot flow, is referred to as static electricity. ("Static" in this instance means "not moving.") lt is also referred to as an electrostatic force. One of the easiest ways to create a static charge is by friction. When two pieces of matter are rubbed together, electrons can be "wiped off" one material onto the other. lf the materials used are good conductors, it is quite difficult to obtain a detectable charge on either, since equalizing currents can flow easily between the conducting materials. These currents equalize the charges almost as fast as they are created. A static charge is more easily created between nonconducting materials. When a hard rubber rod is rubbed with fur, the rod will accumulate electrons given up by the fur, as shown in figure 2.1. Since both materials are poor conductors. very little equalizing current can flow, and an electrostatic charge builds up. When the charge becomes great enough, current will flow regardless of the poor conductivity of the materials. These currents will cause visible sparks and produce a crackling sound.

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Nature of Charges When in a natural or neutral state, an atom has an equal number of electrons and protons. Because of this balance, the net negative charge of the electrons in orbit is exactly balanced by the net positive charge ol the protons in the nucleus, making the atom electrically neutral. An atom becomes a positive ion whenever it loses an electron, and has an overall positive charge. Conversely, whenever an atom acquires an extra electron, it becomes a negative ion and has a negative charge. Due to normal molecular activity, there are always ions present in any material. lf the number of positive ions and negative ions is equal, the material is electrically neutral. When the number of positive ions exceeds the number of negative ions, the material is positively charged. The material is negatively charged whenever the negative ions outnumber the positive ions. Since ions are actually atoms without their normal number of electrons, it is the excess or the lack of electrons in a substance that determines its charge. ln most solids, the transfer of charges is by movement of electrons rather than ions. The transfer of charges by ions will become more significant when we consider electrical activity in liquids and gases. At this time, we will discuss electrical behaviour in terms ol electron movement.

Charged Bodies One of the fundamental laws of electricity is that like charges repel each other and unlike charges attract each other. A positive charge and negative charge, being unlike, tend to move toward each other. ln the atom, the negative electrons are drawn toward the positive protons in the nucleus. This attractive force is balanced by the electron's centrifugal force caused by its rotation about the nucleus. As a result, the electrons remain in orbit and are not drawn into the nucleus. Electrons repel each other because of their like negative charges, and protons repel each other because of their like positive charges.

The law ol charged bodies may be demonstrated by a simple experiment. Two pith (paper pulp) balls are suspended near one another by threads, as shown in figure 2.2.

Figure 2.2 - Repulsion and attraction of charged bodies

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and is then.held against the.. lf a hard rubber rod is rubbed with fur to give it a negative charge right-hand ball ,ighi-h""Jbiri in parl (A), y'" i"J *iir givE off u n"gi1u" charge to the.ball. The *itt'i""pe-ct to the leftlhand ball. When released, the two balls will will have a negative until the "6urg" .nounn in tig ui,re 2.2 (A\. They will touch and remain in contact be drawn together, u" ball, at.which time they will left-hand ball gains a portion'oi tn"e ;d"ti"; charge of the right-hand charge is placed on both balls swing aparl as shown in tigure 2.2 Q\: ll a positive or a negitive $igule 2-2 (B)), the balls will repel each other'

Coulomb's Law of Charges

was first discovered and written The relationship between attracting or repelling-charged bodies Law states that uUouin' u French scientisi nur"iCnurt"s A.boulomb. Coulomb's

directly proportional to the charged bodies attract or repel each other.with a force that is pr"Ji"iot ttt"ir individual c-harges, and is inversely proportional to the square of the distance between them. electrically charged bodies The amount of attracting or repelling force which acts between two (2) the distance between them' in free space depends on i*oif'tgi - (.1) their charges and

Unit of Gharge

when certain combinations The process of electrons arriving or leaving_is exactly what happens material are forced by the of materials are rubbed togeth"i electrons"from the atoms of one and transfer over to the atoms of the other material' ln rubbing to leave their resp"ective atoms if'" ;tfriO' hypothesized by Beniamin Franklin. The operational other words, etectrons generated between definition of a coulomb as ihe unit of electrical charge (in terms of force about. of was found to be equal to an excess or deficiency point electron has a charge of "hutg"") 6,280,000,000,000,000,000 etectrons. or, stated in reverse terms, one is the smallest known a6out 0.00000000000000000016 coulombs" Being that one electron as lhe elementary carrier of electric charge, this last figure of chargelor the electron is defined charge.

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Electric Fields

felt is called an The space between and around charged bodies in which their influence is fields and electrostatic electric field of force. lt can exist inlir, glass, paper, or a vacuum. dielectric fields are other names used to refer to this region of force. and, in general' Fields of force spread out in the space surrounding their point o{ origin diminish in proportion to the square of the distance from their source' are.referred to as The field about a charged body is generally represented by lines which to represent lhe electrostatic lines of t"i"". irt"si lines ire imaginary and are used merely by a positive force exerted direction and strength ot irr" ti"ro. To avoid confuJion, ihe lines of tnlV are shown a negative ur" always lhown leaving the charge, and for 9!a1Oe about charged bodies' "frurg" entering. Figure 2.3 illustrates th6 use of lines to represenfthe lield

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Conduction of Electricity in Solids, Liquids and a Vacuum Solids Electric current is the movement of valence electrons. Gonduction is the name of this process. It is more fully described in Chapter 1 of this Module. Generally, only metals conduct electricity. Some conduct better than others.

The exception to this is graphite (one ol the forms of the element Carbon). Carbon is a nonmetal which exhibits some electrical conductivity.

Liquids The only liquid elements which conduct are the liquid metals. At room temperature liquid mercury is a conductor. Other metals continue to conduct electricity when they are melted. Non-metals such as water, alcohol, ethanoic acid, propanone, hexane and so on, are all non conductors of electricity. However, it is possible to make some non-conducting liquids conduct electricity, by a process called ionization. lonized substances are called ionic substances.

lonic substances are made of charged particles - positive and negative ions. ln the solid state they are held very firmly in place in a lattice structure. ln the solid state the ions cannot move about at all. When the ionic solid is melted, the bonds holding the ions in place in the lattice are broken. The ions can then move around f reely. When an electric current is applied to an ionic melt the electricity is carried by the ions that are now able to move. ln an ionic melt the electric current is a flow of ions.

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Taking water as an example. Remember firstly, that water is considered to be a non-conductor of electricity. lt can allow some electricity through it if a high voltage is applied to it. This is due to the presence of a minute concentration of H* and OH- ions in the water. However, electrons cannot f low through water.

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Covalent substances do not conduct at all in solution.

lonic substances are able to conduct electricity when they are dissolved in water. The reason lies again in the fact that ionic substances are made of charged particles - ions. When the ionic solid is dissolved in water the ionic lattice breaks up and the ions become free to move around in the water. When you pass electricity through the ionic solution, the ions are able to carry the electric current because of their ability to move freely. A solution conducts by means of lreely moving ions. An electrolyte is a liquid which can carry an electric current through it. lonic solutions and ionic melts are all electrolytes. tb

Electrolysis describes the process which takes place when an ionic solution or melt has electricity passed through it. Gases A gas in its normal state is one of the best insulators known. However, in a similar way as liquid, it can be forced to conduct electricity by ionisation of the gas molecules. lonisation of the gas molecules can be effected by extremely high voltages. For example, lightning, is electric current flowing through an ionised path through air due to the huge electrical potential difference between the storm cloud and the ground. ln air, and other ordinary gases, the dominant source of electrical conduction is via a relatively small number of mobile ions produced by radioactive gases, ultraviolet light, or cosmic rays. Since the electrical conductivity is extremely low, gases are dielectrics or insulators. However, once the applied electric field approaches the breakdown value, f ree electrons become sufficiently accelerated by the electric field to create additional free electrons by colliding, and ionizing, neutral gas atoms or molecules in a process called avalanche breakdown. The breakdown process forms a plasma that contains a significant number of mobile electrons and positive ions, causing it to behave as an electrical conductor. ln the process, it forms a light emitting conductive path, such as a spark, arc or lightning.

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Plasma is the state of matter where some of the electrons in a gas are stripped or "ionized" 'rcm their molecules or atoms. A plasma can be formed by high temperature, or by application :f a high electric or alternating magnetic field as noted above. Due to their lower mass, the ? ectrons in a plasma accelerate more quickly in response to an electric field than the heavier :csitive ions, and hence carry the bulk of the current.

7 1 Vacu u m : s a common belief that electricity cannot flow through a vacuum. This is however incorrect. t lemember that a conductor is "something through which electricity can flow," rather than 'scmething which contains movable electricity." A vacuum offers no blockage to moving t :r'arges. Should electrons be injected into a vacuum, the electrons will flow uninhibited and t -rretarded. As such, a vacuum is an ideal conductor. t -";s fact is taken advantage of in many situations, from televisions to vacuum valves. A ,t t t rl

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vacuum arc can arise when the sudaces of metal electrodes in contact with a good vacuum 3egin to emit electrons either through heating (thermionic emission) or via an electric field that s sufficient to cause {ield emission. Once initiated, a vacuum arc can persist since the freed 3a(icles gain kinetic energy from the electric field, heating the metal surfaces through high sceed particle collisions. This process can create an incandescent cathode spot which frees -cre particles, thereby sustaining the arc. At sufficiently high currents an incandescent anode soot may also be formed. = ectrical discharge in vacuum is important for certain types of vacuum tubes and for high . o lage vacuum switches.

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Basic knowledge for categories A, Bl and 82 are indicated by the allocation o{ knowledge levels indicators (1, 2 or 3) against eacti applicable subject. Category C applicants must meet either lhe category 81 or the category 82 basic knowledge levels. The knowledge level indicators are defined as lollows:

LEVEL

1

A familiarisation with the principal elements of the subjecl. Objectives: The applicant should be lamiliar with the basic elements of the subject. The applicant should be able to give a simple description of the whole subject, using common words and examples. The applicant should be able to use typical terms.

LEVEL 2 A general knowledge o{ the theoretical and practical aspects of the subiect. An ability to apply thal knowledge. Objectives: The applicant should be able to understand the theoretical fundamentals of the subject. The applicant should be able to give a generat description ol the subject using, as appropriate, typical examples. The applicant should be able to use mathematical formulae in conjunction with physical laws describing the subject. The applicant should be able to read and understand sketches, drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using detailed procedures.

LEVEL 3 A detailed knowledge of the theoretical and practical aspects of the subjecl. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner. Objectives: The applicant should know the theory of the subject and interrelationships with other subjects. The applicanl should be able to give a detailed description of the subject using theoretical {undamentals and specif ic examples. The applicant should understand and be able to use mathematical formulae related to the subject. The applicant should be able to read, understand and prepare sketches, simple drawings and schemat :s describing the sublect. The applicant should be able to apply his knowledge in a practical manner using manufacturers instructions. The applicant should be able to interpret results Jrom various sources and measurements and apply corrective action where appropriate.

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Module 3.3 Enabling Obiectives and Certification Statement

The following terms, their units and factors aflecting them: potential difference, electromotive force, voltage, current, resistance, conductance, charge, conventional

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Module 3.3 Electrical Terminology Electrical Energy ln the field of physical science, work must be defined as the product of force and displacement. That is, the lorce applied to move an object and the distance the object is moved are the factors of work performed. It is imporlant to notice that no work is accomplished unless the force applied causes a change in the position of a stationary object, or a change in the velocity of a moving object. A worker may tire by pushing against a heavy wooden crate, but unless the crate moves, no work will be accomplished.

Energy ln our study of energy and work, we must define energy as the ability to do work. ln order to perform any kind of work, energy must be expended (converted from one form to anotheO. Energy supplies the required force, or power, whenever any work is accomplished. One form of energy is that which is contained by an object in motion. When a hammer is set in motion in the direction of a nail, it possesses energy of motion. As the hammer strikes the nail, the energy of motion is conveded into work as the nail is driven into the wood. The distance the nail is driven into the wood depends on the velocity of the hammer at the time it strikes the nail. Energy contained by an object due to its motion is called kinetic energy. Assume that the hammer is suspended by a string in a position one meter above a nail. As a result of gravitational attraction, the hammer will experience a force pulling it downward. lf the string is suddenly cut, the force of gravity will pull the hammer downward against the nail, driving it into the wood. While the hammer is suspended above the nail it has abitity to do work because of its elevated position in the earth's gravitational field. Since energy is the ability to do work, the hammer contains energy. Energy contained by an object due to its position is called potential energy. The amount of potential energy available is equal to the product of the force required to elevate the hammer and the height to which it is elevated.

Another example of potential energy is that contained in a tightly coiled spring. The amount of energy released when the spring unwinds depends on the amount of force required to wind the spring initially.

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Electrical Charges

that a field of force exists in the space From the previous study of electrostatics, you learned of the field is directly dependent on the force of ih" surrounding any electrical "length "fr"tg". the charge.

Thechargeofoneelectronmightbeusedasa'unitofelectricalcharge,sincechargesare is so small that it is ol"pracement of ele"ctrons; but the charge of one electron impractical to use.

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Thepracticalunitadoptedformeasuringchargesisthecoulomb,namedafterthescientist (six to th6 charge of 6,280,000,000,000,000,000 ii Charles Coulomb. On" "qui "outo*O quadrillion) or 6'28 x 1018 electrons' q"i"tifri"" tto hundred uni "ighty of electrical potential when a charge of one coulomb exists between two bodies, one unit beiween the two bodies' This is referred energy exists, which is caffeJ tfie Jitf"r"n"" of potential is the volt' to aJelectromotive force, or voltage, and the unit of measure

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so that there exists an excess Electrical charges are created by the displacement of electrons, must point .Consequentlyl, of electrons at one point, anJa leticiendy at another -?llSe is electrons of always have either a n"guiiu" ;i positive'polarlty A. b.-ogy *ith 1,9L"-::t^ of electrons is positive. con"iO"r"O to be negative, *f,"r"L. a body witlL a.deficiency u*i.t between two points, or bodies, only if they have different A difference of potential both have a there is no difference in potential between two bodies if t'rr"ig;". i;"itrerwords, ""n o"gree. lf, however, one body is deficient of 6 coulombs deficiency of electrons to tr" (representing 12 volts), there is "ut" (representing 6 volts), unO tf'" otft"t is-def icient by 1 2 coulombs iitt"r""""i,t poteniiat oi o uorti. 11" body with ihe greater deficiency is positive with respect

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points is of importance ln most electrical circuits only the difference of potential between two often it is-convenient to use unJ tn" absolute potentials of the points are of iittle concern. Very piece.of equipment' For this one standard reference to|. uff of the various potentials throughout a with respect to the reason, the potentials ut uuiiors points in a circuit are generally measured is considered to be at metal chassis on which all parts of the circuit are mounled. The chassis to the chassis' zeio potential and alt oher'pot"nii"it ut" either positive or negative with respe,ct potential' ground When used as the reference point, the chassis is said to be at

occasionally,ratherlargevaluesofVoltagemaybeencountered,inwhichcasetheVolt becomestoosmallaunitforconvenience-.lna-situationofthisnature,thekilovolt(kV)'meaning 20 kV' ln other be written as 1,000 volts, is frequently used. As an example, 20,000 volts would small voltages. For this cases, the volt may Oe ioolurg" a unit, as when dealing with very. (pV), meaning p"ip"." tn" rnittivdn lmvf r""unrg o;"-tho.us1ngil of volt, and the microvolt and 1 mV' as be written one-millionth of a volt, url, ,""0. F6r example, 0'001 volt would 0.000025 volt would be written as 25 UV'

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are connected by when a difference in potential exists between two charged bodies that is from the negatively cha.rged body conductor, electrons *irr flow arong the conductor. This-flow and the potential difference to the positively cfrargeO btdy, ,niit tn" two charges are equalized no longer exists.

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An analogy of this action is shown in the two water tanks connected by a pipe and valve in figure 3.1. At first the valve is closed and all the water is in tank A. Thus, the water pressure across the valve is at maximum. When the valve is opened, the water flows through the pipe f rom A to B until the water level becomes the same in both tanks. The water then stops flowing in the pipe, because there is no longer a difference in water pressure between the two tanks.

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Electron movement through an electric circuit is directly proportional to the difference in potential or electromotive force (EMF), across the circuit, just as the flow of water through the pipe in figure 3.1 is directly proportional to the difference in water level in the two tanks. A fundamental law of electricity is that the electron flow is directly proportional to the applied voltage. lf the voltage is increased, the flow is increased. lf the voltage is decreased, the flow is decreased.

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Electric Current Electron f low

It has been proven that electrons (negative charges) move through a conductor in response to an electric field. Electron current flow will be used throughout this explanation. Electron a current is defined as the directed flow of electrons. The direction of electron movement is from potential to a region of positive potential. Therefore electron flow can be said region of negative -negative to positive. Tie direction of current flow in a material is determined by the toilow from polarity of the apPlied voltage.

Conventional Current Flow potential ln the UK and Europe, conventional current flow is said to be from positive to negative - the opposite way to the actual flow of electrons. Conventional current was defined early in the history of electrical science as a flow of positive charge. ln solid metals, like wires, the positive charge carriers are immobile, and only the negatively charged electrons f low. Because the electron carries negative charge, the electron current is in the direction opposite to that of conventional (or electric) current.

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Electric charge moves from the positive side of the power source to the negative.

current ln other conductive materials, the electric current Figure 3'2 - Conventional flow direction is due to the flow of charged parlicles in directions at the same time. Electric currents in electrolytes are flows of electrically charged atoms (ions), which exist in both positive and negative varieties. For example, an elecirochemical cell may be constructed. with salt water (a solution of sodium chloride) on one side of a membrane and pure water on the other. The membrane lets the positive sodium ions pass, but not the negative chloride ions, so a net current results. Electric currents in plasma are flows of electrons as well as positive and negative ions. ln ice and in certain solid electrolytes, flowing protons constitute the electric cuirent. To simplify this situation, the original definition oi conventional current still stands.

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There are also materials where the electric current is due to the flow of electrons and yet it is conceptually easier to think of the current as due to the llow of positive "holes" (the spots that should have an electron to make the conductor neutral). This is the case in a p-type semiconductor. These EASA Part-66 Module 3 notes will use conventional current noiation throughout, unless oiherwise stated, and then only for specif ic reasons'

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Random Drift All materials are composed of atoms, each of which is capable of being ionised. lf some form of energy, such as heat, is applied to a material, some electrons acquire sufficient energy to move to a higher energy level. As a result, some electrons are freed from their parent atom's which then becomes ions. Other forms of energy, particularly light or an electric field, will cause ionisation to occur. The number of free electrons resulting from ionisation is dependent upon the quantity of energy applied to a material, as well as the atomic structure of the material. At room temperature some materials, classified as conductors, have an abundance of free electrons. Under a similar condition, materials classif ied as insulators have relatively few f ree electrons. ln a study of electric current, conductors are of major concern. Conductors are made up of atoms that contain loosely bound electrons in their outer orbits. Due to the effects of increased energy, these outermost electrons frequently break away from their atoms and freely drift throughout the material. The free electrons, also called mobile electrons, take a path that is not predictable and drift about the material in a haphazard manner. Consequently such a movement is termed random drift. It is imporlant to emphasize that the random drift of electrons occurs in all materials. The degree of random drift is greater in a conductor than in an insulator.

Directed Drift Associated with every charged body there is an electrostatic field. Bodies that are charged alike repel one another and bodies with unlike charges attract each other. An electron will be affected by an electrostatic field in exactly the same manner as any negatively charged body. lt is repelled by a negative charge and attracted by a positive charge. lf a conductor has a difference in potential impressed across it, as shown in figure 3.3, a direction is imparled to the random drift. This causes the mobile electrons to be repelled away from the negative terminal and attracted toward the positive terminal. This constitutes a general migration of electrons from one end of the conductor to the other. The directed migration of mobile electrons due to the potential difference is called directed drift.

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Figure 3.3 - Directed drift motion in a The directed movement of the electrons occurs at a relatively low velocity (rate of particular direction). The effect of this directed movement, however, is felt almost, potential is impressed instantaneously, as explained by the use of figure 3.3. As a difference in point A. Point A across the con'ductor, ihe positive terminal of the battery attracts electrons from point B to point A' now has a deficiency of electrons. As a result, electrons are attracted from point B has now developed an electron deficiency, therefore, it will attract electrons. This same instani effect occurs throughoui the conductor and repeats itself from points D to C At the same tfte positive battery"terminal attracted electrons f rom point A, the negative terminal repelled to electrons toward foint D. These electrons are attracted to point D as it gives up electrons point C. This process is continuous for as long as a difference of potential exists across the o" conductor. Though an individual electron mov:es quite slowly through-the conductor, the effect point a directed drift oJcurs almost instantaneously. As an electron moves into the conductor at a light D, an eleciron is leaving at point A. This action takes place at approximately the speed (186,000 Miles Per Second).

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Figure 3.4 - Effect of directed drift.

Magnitude of Current Flow Electric current has been defined as the directed movement of electrons. Directed drift, therefore, is current and the terms can be used interchangeably. The expression directed drift is particularly helpful in differentiating between the random and directed motion of electrons. However, current flow is the terminology most commonly used in indicating a directed movement of electrons.

The magnitude of current flow is directly related to the amount of energy that passes through a conductor as a result of the drift action. An increase in the number of energy carriers (the mobile electrons) or an increase in the energy of the existing mobile electrons would provide an increase in current flow. When an electric potential is impressed across a conductor, there is an increase in the velocity of the mobile electrons causing an increase in the energy of the carriers. There is also the generation of an increased number of electrons providing added carriers of energy. The additional number of free electrons is relatively small, hence the magnitude of current flow is primarily dependent on the velocity of the existing mobile electrons. The magnitude of current flow is affected by the difference of potential in the following manner. lnitially, mobile electrons are given additional energy because of the repelling and attracting electrostatic field. lf the potential difference is increased, the electric field will be stronger, the amount of energy imparted to a mobile electron will be greater, and the current will be increased. lf the potential difference is decreased, the strength of the field is reduced, the energy supplied to the electron is diminished, and the current is decreased.

Measurement of Current The magnitude of current is measured in amperes. A current of one ampere is said to flow when one coulomb of charge passes a point in one second. Remember, one coulomb is equal to the charge of 6.28 x 101b electrons.

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Frequently, the ampere is much too large a unit for measuring current. Therefore, the milliampere (mA), one-thousandth of an ampere, or the microampere (pA), one-millionth of an ampere, is used. The device used to measure current is called an ammeter and will be discussed in detail in a later module.

'l A current of 1 Amp is flowing when a quantity of 1 Goulomb of charge flows for second' The current I in amperes can be calculated with the following equation:

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Electrical Resistance It is known that the directed movement of electrons constitutes a current flow. lt is also known that the electrons do not move freely through a conductor's crystalline structure. Some materials offer little opposition to current flow, while others greatly oppose current flow. This opposition to current flow is known as resistance (R), and the unit of measure is the ohm. The standard of measure for one ohm is the resistance provided at zero degrees Celsius by a column of mercury having a cross-sectional area of one square millimetre and a length ol 106.3 centimetres.

A conductor has one ohm of resistance when an applied potential of one volt produces a current of one ampere. The symbol used to represent the ohm is the Greek letter omega

(). Resistance, although an electrical property, is determined by the physical structure of a material. The resistance of a material is governed by many of the same factors that control current flow. Therefore, in a subsequent discussion, the factors that affect current flow will be used to assist in the explanation of the factors affecting resistance.

Conductance Electricity is a study that is frequently explained in terms of opposites. The term that is the opposite of resistance is conductance. Conductance is the ability of a material to pass electrons. The lactors that affect the magnitude of resistance are exactly the same for conductance, but they affect conductance in the opposite manner. Therefore, conductance is directly proportional to area, and inversely proportional to the length of the material. The temperature of the material is definitely a factor, but assuming a constant temperature, the conductance of a material can be calculated.

The unit of conductance is the mho (G), which is ohm spelled backwards. Recently the term mho has been redesignated siemens (S). Whereas the symbol used to represent resistance (R) is the Greek letter omega ), the symbol used to represent conductance (G) is (S). The relationship that exists between resistance (R) and conductance (G) or (S) is a reciprocal one. A reciprocal of a number is 'one' divided by that number. ln terms of resistance and conductance:

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Electrical Laws Faraday's Law

loop of wire is Faraday's law of induction states that the induced electromotive torce in a closed

directlyproportionaltothetimerateofchangeofmagneticfluxthroughtheloop. Ohm's Law

points is directly An electrical circuit, the current passing through a conductor between two points, and pioportional to the potential differenceli.e. uoltage drop or voltage) across the two inversely proportional to the resistance between them'

Kirchhoff 's Laws in current Law -At any point in an electrical circuit where charge density is- not changing

currents time, the sum of currents flowing towards that point is equal to the sum of flowing awaY f rom that Point.

Voltage Law -The directed sum of the electrical potential differences around any closed circuit must be zero.

Lens's Law

The induced current in a loop is in the direction that creates a magnetic field that opposes the change in magnetic flux through the area enclosed by the loop. That is, the induced current tendJto keeplhe original magnetic flux through the field f rom changing'

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Knowledge Levels Licence

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Category A, 81, 82 and C Aircraft Maintenance

Basic knowledge for categories A, 81 and 82 are indicated by the allocation ol knowledge levels indicators (1, 2 or 3) against each applicable subject. Category C applicants must meet either the category B1 or the category 82 basic knowledge levels. The knowledge level indicators are defined as follows:

LEVEL

1

A familiarisation with the principal elements ol the subject. Objectives: The applicant should be familiar with the basic elements of the subject. The applicant should be able to give a simple description of the whole subject, using common words and examples. The applicant should be able to use typical terms.

LEVEL 2 A general knowledge ol the theoretical and praclical aspects of the subject. An ability to apply that knowledge. Objectives: The applicanl should be able to understand the theoretical fundamentals of the subject. The applicant should be able to give a general description of the subject using, as appropriate, typical examples. The applicant should be able to use mathematical formulae in conjunction with physical laws describing the su bject. The applicant should be able to read and understand skelches, drawings and schematics describing the subject_

The applicant should be able to apply his knowledge in a practical manner using detailed procedures.

LEVEL 3 A detailed knowledge of ihe theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner. Objectives: The applicant should know the theory of the subject and interrelationships with other subjects. The applicant should be able to give a detailed description of the subject using theoretical lundamentals and specific examples. The applicant should understand and be able to use mathematical lormulae related to the subject. The applicant should be able to read, understand and prepare sketches, simple drawings and schematics describing the subiect. The applicant should be able lo apply his knowledge in a practical manner using manufacturer's instructions. The applicant should be able to interpret results lrom various sources and measurements and apply correclive action where appropriale.

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Module 3.4 Generation of Electricity How Voltage is Produced Voltage Produced by Friction Voltage Produced by Pressure Voltage Produced by Heat Voltage Produced by Light Voltage Produced by Chemical Action Voltage Produced by Magnetism

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Module 3.4 Enabling Objectives and Certification Statement Certification Statement i6"iu stuov Notes comply with the syllabus of EASA Regulat'lon 2o42l2oo3 Annex lll (Part-66) l. and the associated

Generation of E Production of electricity by the {ollowing methods: light, heat, friction, pressure' chemical action, maqnetism and motion

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Module 3.4 Generation of Electricity How Voltage is Produced It has been demonstrated that a charge can be produced by rubbing a rubber rod with fur. Because of the friction involved, the rod acquires electrons lrom the fur, making it negative; the fur becomes positive due to the loss of electrons. These quantities of charge c6nstitute a difference of potential between the rod and the fur. The electrons which mike up this difference of potential are capable of doing work if a discharge is allowed to occur.

To be a practical source of voltage, the potential difference must not be allowed to dissipate, but must be maintained continuously. As one electron leaves the concentration of negative charge, another must be immediately provided to take its place or the charge will eventually diminish-to the point where no further work can be accomplished. A voltage source, therefore, is a device which is capable of supplying and maintaining voltage while some type of electrical apparatus is connected to its terminals. The internal action of the source is such that electrons are continuously removed from one terminal, keeping it positive, and simultaneously supplied to the second terminal which maintains a negative charge. Presently, there are six known methods for producing a voltage or electromotive force (EMF). some of these methods are more widely used than olhers, and some are used mosfly ior specific applications. Following is a list of the six known methods of producing a voltage.

Friction - Voltage produced by rubbing certain materials together. Pressure (piezoelectricity) - Voltage produced by squeezing crystals of cerlain

substances Heat (thermoelectricity) - voltage produced by heating the joint (iunction) where two unlike metals are joined. Light (photoelectricity) - Voltage produced by light striking photosensitive (light sensitive) substances. chemical Action - voltage produced by chemicar reaction in a battery cell. Magnetism - voltage produced in a conductor when the conductor moves through a magnetic field, or a magnetic field moves through the conductor in such a manner as to cut the magnetic lines of force of the field.

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Voltage Produced bY Friction

by friction The The first method discovered for creating a voltage was that of generation tl." *3y in which a oeueropment of charges oy ruoning u r5d *ith fJr is a prime example.of voltage is g"""rated"by friction. Beiause of the nature of the materials with which this ""fi"gli" used or maintained. For this reason, very little practical use generated, it cannot o" "onuuni"ntrv generated by this method' tas been iound for voltages of a more practical ln the search for methods to produce a voltage of a larger amplitude.and from one terminal to nature, machines were OevltlpeO in which c-hatges wele transferred of these machines ,""if'dt Uy r"un. of rotating das" 0i""" ot movlng belts. The most notable potentials in lhe..order of millions of is the Van de Graaff gen"r"ioi. lt is used today to produce outside the field of research' their volts for nuclear research. n" tn""" machines have little vaiue theory of operation will not be described here'

Voltage Produced bY Pressure

certain ionic one specialized method of generating an EMF utilizes the characteristics of crystals have the remarkable ;r;t"s quartz, noJfrette salti, and tourmaline. These surface: T1'"' if a crystal o{ "'G;ir ability to generate a voltage whenever stresses are.applied to their of the quu,i, i" iqr"ezeO, charg'es'oi opposite polarity will appear on two opposite surfaces again appear, but will be crystal. lf the force is reversed und thu crystal ii stretihed, charges will is given a oi'tf'" ofpo"it" polarity from those produced by squeezing. lf a crystal of thislypesides' Quartz of its vibratory motion, it wiff produce i ublt"g" of reversing pol-rity between two energy' electrical into or similir crystals can thus be used to convert mechanical energy of the common This phenomenon, called the piezoelectric elfect, is shown in figure 4.1 ' some phonograph.cartridges, and devices that make use of fiezoelectric crystals are microphones, This method of oscillators used in radio transmitters, radio receivers, and sonar equipment. power requirements, generating an EMF is not suitable foi applications having large voltage or voltages can be 6ut is wid6ly used in sound and communications systemi where small signal effectively used. OUARTZ CRYSTAL COMFRE SSED

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crystals of this type also possess another interesting property, the "converse piezoelectric effect." That is, they have the ability to convert electrical energy into mechanical energy. A voltage impressed across the proper surfaces of the crystal will cause it to expand or contract its surfaces in response to the voltage applied.

Voltage Produced by Heat When a length of metal, such as copper, is heated at one end, electrons tend to move away from the hot end toward the cooler end. This is true of most metals. However, in some metals, such as iron, the opposite takes place and electrons tend to move toward the hot end. These characteristics are illustrated in figure 4.2.The negative charges (electrons) are moving through the copper away from the heat and through the iron toward the heat. They cross from the iron to the copper through the current meter to the iron at the cold junction. This device is generally re{erred to as a thermocouple

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Thermocouples have somewhat greater power capacities than crystals, but their capacity is still very small if compared to some other sources. The thermoelectric voltage in a thermocouple depends mainly on the difference in temperature between the hot and c-old junctions. consequently, they are widely used to measure temperature, and as heat-sensing devices in automatic temperature control equipment. Thermocouples generally can be subjected to much greater temperatures than ordinary thermometers, such as the mercury or alcohol types.

Voltage Produced by Light When light strikes the surface ol a substance, it may dislodge electrons from their orbits around the surface atoms of the substance. This occurs because light has energy, the same as any moving force. Some substances, mostly metallic ones, are far more sensitive to light than others. That is, more electrons will be dislodged and emitted from the surface of a highly sensitive metal, with a given amount of light, than will be emitted f rom a less sensitive substance. Upon losing electrons, the photosensitive (light-sensitive) metal becomes positively charged, and an electric force is created. Voltage produced in this manner is referred to as a photoelectric voltage.

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The photosensitive materials most commonly used to produce a photoelectric voltage are various compounds of silver oxide or copper oxide. A complete device which operates on the photoelectric principle is referred to as a "photoelectric cell." There are many different sizes and iypes of photoelectric cells in use, and each serves the special purpose for which it is designed. Nearly all, however, have some of the basic features oi the photoelectric cells shown in figure 4.3. PltsTc$tE5lTlv;

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The cell (figure 4.3 view A) has a curved lighlsensitive surface focused on the central anode. When light from the direction shown strikes the sensitive suface, it emits electrons toward the anode. The more intense the light, the greater the number of electrons emitted. When a wire is connected between the filament and the back, or dark side of the cell, the accumulated electrons will flow to the dark side. These electrons will eventually pass through the metal of the reflector and replace the electrons leaving the lighlsensitive surface. Thus, light energy is converted to a flow of electrons, and a usable current is developed. The cell (figure 4.3 view B) is constructed in layers. A base plate of pure copper is coated with light-sensitive copper oxide. An extremely thin semitransparent layer of metal is placed over the copper oxide. This additional layer serves two purposes: It permits the penetration of light to the copper oxide. It collects the electrons emitied by the copper oxide. An externally connected wire completes the electron path, the same as in the reflectortype cell. The photocell's voliage is used as needed by connecting the external wires to some other device, which amplifies (enlarges) it to a usable level.

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The power capacity of a photocell is very small. However, it reacts to light-intensity variations in an extremely shorl time. This characteristic makes the photocell very useful in detecting or accurately controlling a great number of operations. For instance, the photoelectric cell, or some form of the photoelectric principle, is used in television cameras, automatic manufacturing process controls, door openers, burglar alarms, and so forth.

Voltage Produced by Chemical Action Voltage may be produced chemically when certain substances are exposed to chemical action. lf two dissimilar substances (usually metals or metallic materials) are immersed in a solution that produces a greater chemical action on one substance than on the other, a difference of potential will exist between the two. lf a conductor is then connected between them, electrons will flow through the conductor to equalize the charge. This arrangement is called a primary cell. The two metallic pieces are called electrodes and the solution is called the electrolyte. The voltaic cell illustrated in figure 4.4is a simple example of a primary cell. The difference of potential results from the fact that material from one or both of the electrodes goes into solution in the electrolyte, and in the process, ions form in the vicinity of the electrodes. Due to the electric field associated with the charged ions, the electrodes acquire charges.

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The amount of difference in potential between the electrodes depends principally on the metals used. The type of electrolyte and the size of the cell have little or no effect on the potential difference produced. There are two types of primary cells, the wet cell and the dry cell. In a wet cell the electrolyte is a liquid. A cell with a liquid electrolyte must remain in an upright position and is not readily transportable. An automotive battery is an example of this type of cell. The dry cell, much more commonly used than the wet cell, is not actually dry, but contains an electrolyte mixed with other materials to form a paste. Torches and portable radios are commonly powered by dry cells.

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Batteries are formed when several cells are connected together to increase electrical output.

Voltage Produced by Magnetism Magnets or magnetic devices are used for thousands of different jobs. One of the most useful and"widely empioyed applications of magnets is in the production of vast quantities of electric power from mecninicai sources. The mechanical power may be provided by.a number of different sources, such as gasoline or diesel engines, and water or steam turbines' However, the final conversion of thes-e source energies to electricity is done by generators employing the principle of electromagnetic induction. These generators, of many types and sizes, are discussed in other modules in this series. Theimportant subiect to be discussed here is the fundamental operating principle of all such electromagnetic-induction generators.

To begin with, there are three fundamental conditions which must exist before a voltage can be produced by magnetism. There must be a conductor in which the voltage will be produced' There must be a magnetic field in the conductor's vicinity' There must be relative motion between the field and conductor. The conductor must be moved so as to cut across the magnetic lines of force, or the field must be moved so that the lines of force are cut by the conductor. ln accordance with these conditions, when a conductor or conductors move across a magnetic lield so as to cut the lines of force, electrons within the conductor are propelled in one direction or another. Thus, an electric force, or voltage, is created. ln figure 4.5, note the presence of the three conditions needed for creating an induced voliage.

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A magnetic field exists between the poles of the C-shaped magnet. There is a conductor (copper wire). There is a relative motion. The wire is moved back and forth across the magnetic field. ln figure 4.5 view A, the conductor is moving toward the front ol the page and the electrons move f rom left to right. The movement of the electrons occurs because of the magnetically induced EMF acting on the electrons in the copper. The right-hand end becomes negative, and the left-hand end positive. The conductor is stopped at view B, motion is eliminated (one of the three required conditions), and there is no longer an induced EMF. Consequently, there is no longer any difference in potential between the two ends of the wire. The conductor at view C is moving away from the f ront of the page. An induced EMF is again created. However, note carefully that the reversal of motion has caused a reversal of direction in the induced EMF. lf a path for electron flow is provided between the ends of the conductor, electrons will leave the negative end and flow to the positive end. This condition is shown in part view D. Electron flow will continue as long as the EMF exists. ln studying figure 4.5,it should be noted that the induced EMF could also have been created by holding the conductor stationary and moving the magnetic field back and forth. The more complex aspects of power generation by use of mechanical motion and magnetism are discussed later in Chapter 14 - DC Motor/Generator Theory.

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BasicknowledgeforcategoriesA,Bland82-areindicatedbytheallocationofknowledgelevelsindicators(1,2or 82 c applicants must meet eitrer the category 81 or the category 3) against each applicable "u01""i.-c"i"go.y basic knowledge levels

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LEVEL

1

A lamiliarisation with the principal elements of the subiect' Objectives: o{ the subject' The applicant should be lamiliar with the basic elements ol the whole subiect, using common words and description i" gi"" a simple The applicant sho"ro o" "oi" examPles. The applicant should be able to use typical terms'

LEVEL 2

of the subiect' A general knowledge of the theoretical and practical aspects An ability to apply that knowledge' Objectives: lundamentals ol the subject The applicant should be able to understand the theoretical i" gi"" a general description of the subject using, as appropriate, typical The applicant sho"ro o" examPles.

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Acapacitytocombineandapplytheseparaieelementso{knowledgeinalogicalandcomprehensive manner. Objectives:

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TheapplicantshouldbeabletointerpretresultsfromVarioussourcesandmeasurementsandapply corrective action where appropriate'

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Module 3.5 DC Sources of Electricity lntroduction The Cell Primary and Secondary Cells

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5 5 5 7 7

Electrochemical Action Primary Cell Chemistry Secondary Cell Chemistry Polarization of the Cell Local Action Types of Cells Other Types of Cells Secondary Wet Cells Cell Capacity Cells in Series and Parallel Battery Construction Battery lnternal Resistance Battery Maintenance Capacity and Rating of Batteries Battery Charging Thermocouples Photocells

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15 18 19 20 22 30 31

32 33 35 44

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Construction and basic chemical action of: primary cells, secondary cells, lead acid cells, nickel cadmium cells. other alkaline cells Cells connected in series and parallel lnternal resistance and its affect on a Construction, materials and operation of

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Module 3.5 DC Sources of Electricity lntroduction The purpose of this chapter is to introduce and explain the basic theory and characteristics of batteries. The batteries which are discussed and illustrated have been selected as representative of many models and types which are used in aircraft today. No attempt has been made to cover every type of battery in use, however, after completing this chapter you will have a good working knowledge of the batteries which are in general use. First. you will learn about the building block of all batteries, the cell. The explanation will explore the physical makeup of the cell and the methods used to combine cells to provide useful voltage, current, and power. The chemistry of the cell and how chemical action is used to convert chemical energy to electrical energy are also discussed" ln addition, the care, maintenance, and operation of batteries, as well as some of the safety precautions that should be followed while working with and around batteries are discussed. Batteries are widely used as sources of direct-current electrical energy in automobiles, boats, aircraft, ships, portable electric/electronic equipment, and lighting equipment. ln some instances, they are used as the only source of power; while in others, they are used as a secondary or standby power source.

A battery consists of a number of cells assembled in a common container and connected together to f unction as a source of electrical power.

The Cell A cell is a device that transforms chemical energy into electrical energy. The simplest cell, known as eiiher a galvanic or voltaic cell, is shown in Figure 5.1. lt consists of a piece of carbon (C) and a piece ol zinc (Zn) suspended in a jar that contains a solution of water (Hz0) and sulphuric acid (HzS0+) called the electrolyte.

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; The cell is the fundamental unit of the battery. A simple cell consists of two electrodes placed in a container that holds the electrolyte. ln some cells the container acts as one of the electrodes and, in this case, is acted upon by the electrolyte. This will be covered in more detail later.

Electrodes The electrodes are the conductors by which the current leaves or returns to the electrolyte. ln the simple cell, they are carbon and zinc strips that are placed in the electrolyte; while in the dry cell (Figure 5.2), they are the carbon rod in the centre and zinc container in which the cell is assembled.

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&:1dd* Figure 5.2 - Dry cell, cross-sectional view. ln a discharging battery or galvanic cell (drawing) the cathode is the positive terminal, where conventional current flows out. This outward current is carried internally by positive ions moving from the electrolyte to the positive cathode (chemical energy is responsibie for this "uphill" motion). lt is continued externally by electrons moving inwards, negative charge moving one way amounting to positive current flowing the other way.

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Electrolyte The electrolyte is the solution that acts upon the electrodes. The electrolyte, which provides a path for electron flow, may be a salt, an acid, or an alkaline solution" ln the simple galvanic cell, the electrolyte is in a liquid form. ln the dry cell, the electrolyte is a paste.

Container The container which may be constructed of one of many different materials provides a means of holding (containing) the electrolyte. The container is also used to mount the electrodes. ln the voltaic cell the container must be constructed of a material that will not be acted upon by the electrolyte.

Primary and Secondary Cells Primary Cell A primary cell is one in which the chemical action eats away one of the electrodes, usually the negative electrode. when this happens, the electrode must be replaced or the cell must be discarded. ln the galvanictype cell, the zinc electrode and the liquid electrolyte are usually replaced when this happens. ln the case of the dry cell, it is usually cheaper to buy a new cell. Secondary Cell A secondary cell is one in which the electrodes and the electrolyte are altered by the chemical action that takes place when the cell delivers current. These ceils may be restored to their original condition by forcing an electric current through them in the direction opposite to that of discharge. The automobile storage battery is a common example of the secondary cell.

Electrochemical Action lf a load (a device that consumes electrical power) is connected externally to the eleclrodes of a cell, electrons will flow under the influence of a difference in potential across the electrodes from the anode (negative electrode), through the external conducior to the cathode (positive electrode). A cell is a device in which chemical energy is converted to electrical energy. This process is called electrochemical action.

The voltage across the electrodes depends upon the materials f rom which the electrodes are made and the composition of the electrolyte. The current that a cell delivers depends upon the resistance of the entire circuit, including that of the cell itself. The internal resisiance of the cell depends upon the size of the electrodes, the distance between them in the electrolyte, and the resistance of the electrolyte. The larger the electrodes and the closer together they are in the electrolyte (without touching), the lower the internal resistance of the cell and the more current the cell is capable of supplying to the load.

-e :-a:.:iiosure s t:,). The first dot at the base of the arrow sequence (the left-most dot) represents the capacitor type. This dot is either black, white, silver, or the same colour as the capacitor body. Mica is represented by a black or white dot and paper by a silver dot or dot having the same colour as the body o{ the capacitor. The two dots to the immediate right of the type dot indicate the first and second digits of the capacitance value. The dot at the bottom right represents the multiplier to be used. The multiplier represents picofarads. The dot in the bottom centre indicates the tolerance value of the capacitor.

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Module 3.9 Capaciiance/Capacitor

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Read the multiplier dot and multiply the first two digits by multiplier (Remember that the multiplier is in picofarads).

Lastly, read the tolerance dot.

BLUE

According to the coding (see Table 9.2), the capacitor is a mica capacitor whose capacitance is 1200 pF with a tolerance of +l- 6o/o. The six digits indicate a capacitance ol 2200 pF with a + /-4oo/o tolerance and a working voltage of 44 volts.

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Numeric Codes Like resistors, small capacitors such as film or disk types conform to the BS1852 Standard where the colours are replaced by a letter coded system. The code consists of 2 or 3 numbers and an optional tolerance letter code. Where a two number code is used the value of the capacitor only is given in picofarads (i.e. +Z - 47 pF). A three letter code consists of the two value digits and a multiplier much like a resistor colour code (i.e. 471: 47 xI0 = 470pF). Three digit codes are often accompanied by an additional tolerance letter code.

Figure 9.27 - A ceramic disc capacitor Figure 9.27 is a ceramic disc capacitor that has the code "473J" printed onto its body. This translates to: 47pF x 1,000 (3 zero's) = 47,000 pF, 47nF or 0.047 pF the J indicates a tolerance oI +/- 5o/"

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Module 3.9 Capacitance/Capacitor

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Basic Capacitor Testing Some Digital Multimeters (DMMs) have modes for capacitor testing. These work well to determine approximate pF rating. However, for most applications, they do not test at anywhere near the normal working voltage or test for leakage. Normally, this type of testing requires disconnecting at least one lead of the suspect capacitor from the circuit to get a reasonably accurate reading - or any reading at all. However, newer models may also test capacitors in-circuit. Of course, all power must be removed and the capacitors should be discharged. This will generally work as long as the components attached to the capacitor are either semiconductors (which won't conduct with the low test voltage) or passive components with a high enough impedance to not load the tester too much. The reading may not be as accurate in-circuit, but probably won't result in a lalse negative (calling a capacitor good that is bad).

Caution: For this and any other testing of large capacitors and/or capacitors in power supply, power amplifier, or similar circuits, make sure the capacitor is fully discharged or your multimeter may be damaged or destroyed! Volt-Ohm Meters (VOMs) or DMMs without capacitance ranges can make certain types of tests. For small capacitors (0.01pf or less), all you can really test is for shorts or leakage. (However, on an analogue multimeter on the high ohms scale you may see a momentary deflection when you touch the probes to the capacitor or reverse them. A DMM may not provide any indication at all.) Any capacitor that measures a few ohms or less is bad. Most should test infinite even on the highest resistance range. For electrolytic capacitors in the pF range or above, you should be able to see the capacitor charge when you use a high ohms scale with the proper polarity - the resistance will increase until it goes to (nearly) inlinity. lf the capacitor is shorled, then it will never charge. lf it is open, the resistance will be infinite immediately and won't change. lf the polarity of the probes is reversed, it will not charge properly either.

Note: lt is important to determine the polarity of the meter - they are not all the same. Red is usually negative with (analogue) VOMs but positive with most DMMs. lf the resistance never goes very high, the capacitor is leaky.

The best way to really test a capacitor is to substitute a known good one. A VoM or DMM will not test the capacitor under normal operating conditions or at its full rated voltage. However, it is a quick way of finding major faults. A simple way of determining the capacitance fairly accurately is to build an oscillator using a 555 timer. Substitute the cap in the circuit and then calculate the C value from the frequency.

With a few resistor values, this will work over quite a wide range. Alternatively, using a DC power supply and series resistor, capacitance can be calculated by measuring the rise time to 63% of the power supply voltage from T-RC or C=T/R.

Use and/or disclosure is

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Module 3.9 Capacitance/Capacitor

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0 Magnetism

Module 3.10 Magnetism

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Copyright Notice worldwide rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form by any other means whatsoever: i.e. photocopy, electronic, mechanical recording or otherwise without the prior written permission of Total Training Support Ltd.

@ Copyright. All

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Category A, Bl , 82 and C Aircraft Maintenance

Basic knowledge for categories A, B1 and 82 are indicated by the allocation of knowledge levels indicators ( 1 , 2 or 3) against each applicable subject. Category C applicants must meet either the category Bl or the category 82 basic knowledge levels. The knowledge level indicators are defined as follows:

LEVEL

1

A lamiliarisation with the principal elements of the subject. Objectives: The applicant should be familiar with the basic elements ol the subject. The applicant should be able to give a simple description of the whole subject, using common words and examples. The applicant should be able to use typical terms.

LEVEL 2 A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge. Objectives: The applicant should be able to understand the theoretical fundamentals of the subject. The applicant should be able to give a general description oi the subject using, as appropriate, typical examplesThe applicant should be able to use mathematical formulae in conjunction with physical laws describing the subject. The applicant should be able to read and understand sketches, drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using detailed procedures.

LEVEL 3 A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements ot knowledge in a logical and comprehensive manner. Objectives: The applicant should know the lheory of the subject and interrelationships with other subjects. The applicant shouid be able to give a detailed description of the subject using theoretical lundamentals and specific examples. The applicant should understand and be able to use mathematical formulae related to the subject. The applicant should be able to read, understand and prepare sketches, simple drawings and schemalics describing the subject. The applicant should be able 10 apply his knowledge in a practical manner using manufacturer's instructions. The applicant should be able to interpret results lrom various sources and measurements and apply corrective action where appropriate.

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Module 3..10 Magnetism

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Table of Contents

Module 3.10 Magnetism

5

(a) Magnetic Materials Ferromagnetic Materials Natural Magnets Arlificial Magnets Permeability Types of Magnetism Magnetic Poles The Earth's Magnetism Theories of Magnetism Effect of Breaking a Bar Magnet Magnetic Fields Magnetic Effects Magnetic Flux Magnetic lnduction Magnetic Shielding Magnet Shapes Care of Magnets

5 5 5 5 6 7 B

o 10 13 15 15 18 19 20

2l 22 23

(b) Electromagnetism Force on a Conductor in a Magnetic Field Electromagnets Permeance Electrical and Magnetic Circuit Comparison Hysteresis Summary of Magnetism Terms and Symbols

25 25 26 28 29 30 31 o!)

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Module 3.10 Enabling Obiectives and Certification Statement Certif ication Statement These Study Notes comply with the syllabus of EASA Regulation 204212003 Annex lll (Part-66) below: Levels as l. and the associated Knowl Level

82

Prooerties of a maqnet Action of a magnet suspended in the Eadh's

Maonetic shieldin Various tvpes of maqnetic material Electromagnets construction and principles of Hand clasp rules to determine: magnetic field around current carryinq conductor Magnetomotive force, field strength, magnetic flux density, permeability, hysteresis loop, retentivity, coercive force reluctance, saturation Precautions for care and storaqe of maqnets

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Module 3.10 Magnetism (a)

Magnetic Materials Magnetism is generally defined. as that property of a material which enables it to attract pieces of iron' A material possessing this property is t no*n a magnet. The word originated with the ancient Greeks, who found stones poisesiing this characteristic. "" Materials that ire attracted by a magnet, such as iron, steer, nicker and cobart, have the ability to oecome magnetizeo. rhese are called magnetic materiars. Materiars, such as paper, wood, grass, or tin, which are not atkacted by magnets, are considered nonmagnetic. i\onmagnetic materiars are not abre to become magnetized.

Ferromagnetic Materials The most important group of materials connected with electricity and electronics are the ferromagnetic materiars. Ferromagnetic materiars are those ,hi"h u;" ,;;iiu"ry to magnetize, such as iron, steer, cobalt, and the ailoys Arnico """y is made by ano eermatioy. tni arrov combining two or more erements, one of which b" a metar). These n'ew afloys can be very strongly magnetized, and are capabre of obtaining u Jnirgn to lift 500 times their own weight.

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NaturalMagnets Magnetic stones such as those found by the ancient Greeks are considered to be naturar magnets' These stones had the ability io attract small pieces of iron in a manner similar to the magnets which are common today. However, the magnetic properties attrinutlJ to the stones were products of nature and not the result of the effois of man. ftre Oreets carr"o ,""" substances magnetite.

The Chinese are said to have been aware of some of the effects of magnetism as earry as 2600 B.c. They observed that stones simirar to magnetite, when freely suspended, had a tendency to assume a nearly north and south direction. Because of the dire6tional quality of these stones, they were later referred to as toOesiones or leading stones.

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Artificial Magnets Magnets produced f rom magnetic materials are called artificial magnets. They can be made in a variety of shapes and sizes and are used extensively in electrical apparatus. Artificial magnets are generally made from special iron or steel alloys which are usually magnetized electrically, The material to be magnetized is inserled into a coil of insulated wire and a heavy flow of electrons is passed through the wire. Magnets can also be produced by stroking a magnetic material with magnetite or with another artificial magnet. The forces causing magnetization are represented by magnetic lines of force, very similar in nature to electrostatic lines of force.

Arlificial magnets are usually classified as permanent or temporary, depending on their ability to retain their magnetic properties after the magnetizing force has been removed. Magnets made from substances, such as hardened steel and certain alloys which retain a great deal of their magnetism, are called permanent magnets. These materials are relatively difficult to magnetize because of the opposition offered to the magnetic lines of force as the lines of force try to distribute themselves ihroughout the material. The opposition that a material offers to the magnetic lines of force is called reluctance. All permanent magneis are produced from materials having a high reluctance.

A material with a low reluctance, such as soft iron or annealed silicon steel, is relatively easy to magnetize but will retain only a small part of its magnetism once the magnetizing force is removed. Materials of this type that easily lose most of their magnetic strength are called temporary magnets. The amount of magnetism which remains in a temporary magnet is referred to as its residual magnetism. The ability of a material to retain an amount of residual magnetism is called the retentivity of the material. The difference between a permanent and a temporary magnet has been indicated in terms of reluctance, a permanent magnet having a high reluciance and a temporary magnet having a low reluctance. Magnets are also described in terms of the permeability of their materials, or the ease with which magnetic lines of force distribute themselves throughout the material. A permaneni magnet, which is produced from a material with a high reluctance, has a low permeability. A temporary magnet, produced f rom a material with a low reluctance, would have a high permeability.

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Permeability ln magnetism, permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. Magnetic permeability is represented by the Greek letter p. ln sl units, permeability is measured in henries per metre (H/m), or Newtons per ampere squared (N/A2).

The constant value po is known as the magnetic constant or the permeability of free space (vacuum), and has the exact or defined value po :4nx!0-7 H/m (L.25663T1 H/m). Relative permeability, sometimes denoted by the symbol pr, is the ratio of the permeability of a specific medium to the permeability ol free space given by the magnetic constant p6:

l"=!Fo Materials may have their relative or absolute permeability quoted" From the transposition of the equation above, absolute permeability, trr:

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Nickel

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Platinum

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t.2566650 H/m

Hydrogen

r.2566371, H/m

Vacuum

1..2566371,

Sapphire

7.2566368H/m

Copper

1..2566290 H/m

Water Table 10.1

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H/m (po)

1.2566270 H/m Permeabilities of some materials

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Types of Magnetism Diamagnetism Diamagnetism is a weak repulsion from a magnetic field. lt is a form of magnetism that is only exhibited by a substance in the presence of an externally applied magnetic field. All materials show a diamagnetic response in an applied magnetic field. ln fact, diamagnetism is a very general phenomenon. However, for materials which show some other form of magnetism (such as ferromagnetism or paramagnetism), the diamagnetism is completely overpowered. Substances which only, or mostly, display diamagnetic behaviour are termed diamagnetic materials, or diamagnets. Materials that are said to be diamagnetic are those which are usually considered by non-physicists as "non-magnetic", and include water, most organic compounds such as petroleum and some plastics, and many metals including copper, particularly the heavy ones with many core electrons, such as mercury, gold and bismuth. Diamagnetic materials have a relative permeability that is less than 1, and are therefore repelled by magnetic fields. However, since diamagnetism is such a weak property its effects are not observable in every-day life.

Paramagnetism Paramagnetism is a lorm of magnetism which occurs only in the presence of an externally applied magnetic field. Paramagnetic materials are attracted to magnetic fields, hence have a relative permeability greater than one. The force of attraction generated by the applied field is Iinearin the field strength and rather weak. ll typically requires a sensitive analytical balance to detect the effect. Unlike ferromagnets, paramagnets do not retain any magnetization in the absence of an externally applied magnetic field. Thus the total magnetization will drop to zero when the applied field is removed" Even in the presence of the field there is only a small induced magnetization. This fraction is proportional to the field strength and this explains the linear dependency. The attraction experienced by ferromagnets is non-linear and much stronger, so that it is easily observed on the door of one's refrigerator. Ferromagnetism Ferromagnetism is the "normal" form of magnetism with which most people are familiar, as exhibited in horseshoe magnets and refrigerator magnets. lt is responsible for mosi of the magnetic behaviour encountered in everyday life. The attraction between a magnet and ferromagnetic material is "the quality" of magnetism first apparent to the ancient world, and to us today," according to a classic text on ferromagnetism.

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Ferromagnetism is defined as the phenomenon by which materials, such as iron, in an external magnetic field become magnetized and remain magnetized for a period after the material is no longer in the field. All permanent magnets are ferromagnetic, as are the metals that are noticeably attracied to them. Historically, the term ferromagnet was used for any material that could exhibit spontaneous magnetization: a net magnetic moment in the absence of an external magnetic field. This 10-8 TTS lntegrated Training System O Copyriqht 2011

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general definition is still in common use. More recently, however, different classes of spontaneous magnetisation have been identified. All of these magnetic effects only occur at temperatures below a cedain critical temperature, called the Curie temperature.

Magnetic Poles The magnetic force surrounding a magnet is not uniform. There exists a great concentration of force at each end of the magnet and a very weak force at the centre. Proof of this fact can be obtained by dipping a magnet into iron filings (figure 10.2). lt is found that many filings will cling to the ends of the magnet while very few adhere to the centre. The two ends, which are the regions of concentrated lines of force, are called the poles of the magnet. Magnets have two magnetic poles and both poles have equal magnetic strength.

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Figure 10.2 - A magnet dipped in iron filings Law of Magnetic Poles lf a bar magnet is suspended freely on a string, as shown in figure 10.3, it will align itself in a nodh and south direction. When this experiment is repeated, it is found that the same pole of the magnet will always swing toward the north magnetic pole of the earth. Therefore, it is called the nodh-seeking pole or simply the north-pole. The other pole of the magnet is the southseeking pole or the south-pole.

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Figure 10.3 - North-pole and South-pole A practical use of the directional characteristic of the magnet is the compass, a device in which a freely rotating magnetized needle indicator points toward the North-pole. The realization that

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the poles of a suspended magnet always move to a definite position gives an indication that the opposiie poles of a magnel have opposite magnetic polarity. The law previously stated regarding the attraction and repulsion of charged bodies may also be applied to magnetism if the pole is considered as a charge. The north-pole of a magnet will always be attracted to the south-pole of another magnet and will show a repulsion to a nodhpole. The law for magnetic poles is:

Like poles repel, unlike poles attract

The Earth's Magnetism The fact that a compass needle always aligns itself in a particular direction, regardless of its location on eadh, indicates that the earth is a huge natural magnet. The distribution of the magnetic force about the earth is the same as that which might be produced by a giant bar magnet running through the centre of the earth (figure 10.4). The magnetic axis of the earth is located about 1S"lrom its geographical axis thereb y locating the magnetic poles some distance from the geographical poles. The ability of the north-pole of the compass needle to point toward the north geographical pole is due to the presence of the magnetic pole nearby. This magnetic pole is named the magnetic North-pole. However, in actuality, it must have the polarity of a south magnetic pole since it attracts the north-pole of a compass needle (see figure 10.4). The reason for this conflict in terminology can be traced to the early users of the compass. Knowing little about magnetic effects, they called the end of the compass needle that pointed towards the north geographical pole, the north-pole of a compass. With our present knowledge of magneiism, we know the north-pole of a compass needle (a small bar magnet) can be attracted only by an unlike magnetic pole, that is, a pole of south magnetic polarity.

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Magnetic Variation The earth's magnetic poles are some distance from the geographic or "true" poles. The magnetic lines of force do not pass over the surface in a neat geometric pattern because they are influenced by the varying mineral content of the earth's crust. For these reasons, there is usually an angular difference, or variation, between true north and magnetic north from a given geographic location. Although this variation is not equal at all points on the earth, it does follow a pattern. Points of equal variation can be connected by an isogonic line, which can be plotted accurately on a chart. ln some places this variation is easterly; other places it is westerly. This variation is shown on sectional and IFR charls.

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Lines of variation

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Magnetic lnclination (Dip) The lines of force in the earth's magnetic f ield pass through the centre of the earth, exit at both magnetic poles, and bend around to re-enter at the opposite pole. Near the Equator, these lines become almost parallel to the surface of the earth. However, as they near the poles, they tilt toward the earth until in the immediate area of the magnetic poles they dip rather sharply into the earth. Because the poles of a compass tend to align themselves with the magnet lines of force, the magnet within the compass tends to tilt or dip toward the earth in the same manner as the lines of force. This angle of inclination (or 'dip') can be measured with a specially constructed compass.

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Theories of Magnetism Weber's Theory A popular theory of magnetism considers the molecular alignment of the material. This is known as weber's theory. This theory assumes that all magnetic Jubstances are composed of tiny molecular magnets.

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The, tiny magnet, composition of Weber,s Theory

Any unmagnified material has the magnetic forces of its molecular magnets neutralized by adjacent molecular magnets, thereby eliminating any magnetic effect. magnetized material will have most of its molecula.r magnets lined up so ifrat tfre north-pole of ea-ch molecule points in one direction, and the south-pole faces the opposite direction. A material with its molecules thus aligned will then have one_effective north-pole, and one effective south-pole. An illustration of webers Theory is shown in figure 10.7, where a steel bar is magnetized by sirot r

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When bar magnets are stored, the same principle must be remembered. Therefore, bar magnets should always be stored in pairs with a nodh-pole and a south-pole placed together, ideally also with keepers. This provides a complete path for the magnetic flux without any flux leakage. Figure 10.21 - Bar magnets - stored end{o-end with keepers

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Electromagnetism During a lecture demonstration in 1820 the Danish scientist Hans Christian Oersted (17701851) noticed that a compass needle, placed near to a current-carrying wire, was deflected from its normal North-South position. This may not sound a very remarkable discovery, but Oersted realized that it was evidence of a fundamental and far reaching fact. A magnetic field is established around any conductor when current is passing through it. The lines of force which depict such a field take the form of concentric circles disposed around the surface of the conductor. The relationship between direction of current through the conductor and the direction of flux produced around the conductor is the same as that of the forward movement and rotation of a screw with a right-hand thread or the familiar corkscrew.

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The same relationship can be described with the Right Hand Clasp rule. Here, the fingers are imagined to be clasped around the conductor, with the thumb pointing in the direction of conventional current flow, and the fingers point in the direction of magnetic flow around the conductor.

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- The Right Hand Clasp Rule

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Consider the circumstances when two conductors carrying current lie parallel with each other. Each conductor is surrounded with a magnetic field, the lines of force being of elastic nature and because they cannot intersect each other, their form is modified to constitute a resultant field as shown.

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Figure 10.24 - Attraction and Repulsion of Two Conductors Note how, with current flowing in the same direction in both conductors, the lines of force tend to encircle the two conductors and so produce mutual attraction between them. With current flowing in opposite directions in the conductors, then the mutual repulsion between the two individual fields tends to drive the conductors apart.

Force on a Conductor in a Magnetic Field lf a current carrying conductor is introduced into a maqnetic field at right angles to it, the conductor will experience a force directed at right angies to both the direction of the lines of flux and the direction of current. The rule for remembering these directions is called Fleming's Left Hand (Motor) Rule. To apply the rule, set the thumb, first finger and second finger of the Ieft hand at right angles to each other as shown. The thuMb indicates the direction of Motion when the First finger is in the direction of the magnetic lines of Flux, and the seCond finger is in the direction of the conventional Current flow in the wire.

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"?,gilTgl, Figure 10.25 - Fleming's Left Hand Rule The magnitude of the force on a current carrying conductor at right angles to a magnetic field depends on three factors.

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The magnetic flux density (B). The magnitude of the current flowing in the conductor (I). The length of the conductor in the magnetic field . metres)

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Electromagnets lf a current carrying conductor is formed into a single loop, the lines of force encircling the conductor will all pass through the loop in the same direction. A coil or solenoid is simply a conductor formed into a number of loops and the lines of force travels the coil lengthwise and complete themselves through the surrounding medium. The form of the field of a solenoid is thus similar to that of a simple bar magnet. The polarity of a solenoid is found by using the Right Hand Grasp Rule; imagine the solenoid grasped by the right hand with the fingers pointing in the direction of the conventional current, then the outstretched thumb will point towards the North-pole of the solenoid.

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Figure 10.27 - Right Hand Grasp Rule When current is llowing in a solenoid it produces a Magneto Motive Force (MMF) and its value is a product of the current and the number of turns on the coil, NI or Ampere Turns (AT). The magnetic field sirength (Symbol H) of a solenoid is defined in terms of Magneto Motive Force per unit length (1 Metre) and is therefore measured ln Ampere-Turns per metre.

H_N

Ampere-Turns per Metre.

Compared with a permanent magnet, a solenoid carrying current produces remarkably little magnetic flux, but the output can be increased enormously by inserting an iron core into the coil. This is because iron has a permeability several thousand times that of air. Permeability (the relative ease with which lines of force pervade a material) is defined by the ratio of flux density B to magnetic field strength F1 at any point in free space and is called the permeability of free space. lt is represented by the symbol o. Thus in free space

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Pernreance

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ln electromagnetic theory, permeance is the inverse of reluctance. Permeance is a measure of the quantity of flux for a number of current-turns in magnetic circuit. A magnetic circuit almost acts as though the flux is 'conducted', therefore permeance is larger for large cross sections of a material and smaller for longer lengths. This concept is analogous to that of electrical conductance.

-

It differs from permeability in that it includes the dimensions of the magnetic medium, whereas permeability does not. This is in the same way that, in electrical terms, resistance differs from resistivity.

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The equation for permeance is:

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NI

A

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