Magnetic Efectic of Electric Current

MAGNETIC EFFECTS OF ELECTRIC CURRENT-NOTE Magnet and magnetism. The substances which have the property of attracting small pieces of iron, nickel, cobalt, etc. are called magnets and this property of attraction is called magnetism. Natural magnets. Natural magnets are piece of lodestone, which is a black iron ore (Fe3O4) called magnetite. Origin of the word magnetism Natural magnets called lodestones were found as early as the sixth century B.C. in the province of Magnesia in ancient Greece, from which the word magnetism derives its name. Magnetic poles. These regions of concentrated magnetic strength inside the magnet just near its ends are called magnetic poles. The end of a freely suspended magnet which points towards north is called the north seeking or North Pole while its end pointing towards south is called south seeking or South Pole. Law of magnetic poles. It states that like poles repel while unlike poles of magnets attract each other. Thus, two N-poles repels, two S-poles repel while N-pole attracts S-pole. Basic properties of magnets. Some basic properties of magnets are as follows: (i) Attractive property. A magnet attracts small pieces of iron, cobalt, nickel, etc. (ii) Directive property. A freely suspended magnet aligns itself nearly in the north-south direction. (iii) Law of magnetic poles. Like magnetic poles repel and unlike magnetic poles attract each other. (iv) Magnetic poles exist in pairs. If we break a magnet into two pieces, we always get two small dipole magnets. It is not possible to obtain an isolated N-pole or S-pole. Artificial magnets. Pieces of iron and other magnetic materials can be made to acquire the properties of natural magnets. Such magnets are called artificial magnets. Uses of magnets: (i) Magnets are used in radio and stereo speakers. (ii) They are used in almirah and refrigerator doors to snap them closed. (iii) They are used in video and audio cassette tapes, on the hard discs

and floppies for

computers. (iv) In children's toys. (v) In medicine, the magnetic resonance imaging (MRI) scanners expose the inner parts of the patient's body for detailed examination by doctors. Compass needle. It consists of a small and light magnetic needle pivoted at the centre of a small circular brass case provided with a glass top, as shown in Fig. The ends of the compass needle point approximately towards north and south directions. The end pointing towards north is called North Pole and that pointing towards south is called South Pole. The north pole of the needle is generally painted black or red. An activity to show that a wire carrying an electric current behaves like a magnet

Danish physicist H.C. Oersted was the first to demonstrate in 1820 that a current carrying conductor produces a magnetic field around it. ■ Take a straight thick copper wire and place it between the points X and Y in an electric circuit as shown in Fig. ■ Place a small compass near to this copper wire. See the position of its needle. ■ Pass the current through the circuit by inserting the key into the plug. ■ Observe the change in the position of the compass needle. ■ Why is the needle deflected? What do you conclude?

MAGNETIC EFFECTS OF ELECTRIC CURRENT-NOTE As we pass current though the copper wire XY, the compass needle gets deflected from its position of rest. Since a magnetic needle can be deflected only by a magnetic field, so the current carrying wire produces a magnetic field around it or it behaves like a magnet. A current carrying conductor produces a magnetic field around it. This effect is called magnetic effect of current. MAGNETIC FIELD AND FIELD LINES Magnetic field. The region around a magnet within which its influence can be experienced is called its magnetic field. Magnetic lines of force. A magnetic line of force may be defined as the curve the tangent to which at any point gives the direction of the magnetic field at that point. It may also be defined as the path along with a free north pole tends to move. Properties of lines of force: (i) These are closed curves which start in air from the N-pole and end at the S-pole and then return to the N- pole through the interior of the magnet. (ii) No two magnetic lines of force can intersect each other. If two magnetic lines of forces intersect, then there will be two tangents and hence two directions of the magnetic field at the point of intersection. This is not possible. (iii) They start from and end on the surface of the magnet normally. (iv) The lines of forces have a tendency to contract lengthwise and expand sidewise. This explains attraction between unlike poles and repulsion between like poles. (v) The relative closeness of the lines of force gives a measure of the strength of the magnetic field which is maximum at the poles. Methods of plotting lines of force. The following two methods are used for drawing lines of force of a bar magnet: (i) Ironfilings method. (ii) Compass needle method. IRON-FILINGS METHOD ■ Fix a sheet of white paper on a drawing board using some adhesive material.

■Place a bar magnet in the centre of it. ■ Sprinkle some iron filings uniformly around the bar magnet. A salt-sprinkler may be used for this purpose. ■ Now tap the board gently. ■ What do you observe? The iron filings arrange themselves in a pattern as shown in Fig. The magnet exerts a force on the iron filings of the surrounding region. This force arranges the iron filings along definite lines extending from one end of the magnet to the other. The region surrounding a magnet, in which the force of the magnet can be detected, is said to have a magnetic field. The lines along which the iron filings align themselves represent magnetic field lines.

COMPASS NEEDLE METHOD. ■ Take a small compass and a bar magnet. ■ Place the magnet on a sheet of white paper fixed on a drawing board, using some adhesive material. ■ Mark the boundary of the magnet. ■ Place the compass near the north pole of the magnet. How does it behave? The south pole of the needle points towards the north pole of the magnet. The north pole of the compass is directed away from the north pole of the magnet. ■ Mark the position of two ends of the needle. ■ Now move the needle to a new position such that its south pole occupies the position previously occupied by its north pole. ■ In this way, proceed step by step till you reach the south pole of the magnet as shown in the Fig ■ Join the points marked on the paper by a smooth curve. This curve represents a field line. ■ Repeat the above procedure and draw as many lines as you can. You will get a pattern shown in Fig. These lines represent the magnetic field around the magnet. These are known as magnetic field lines. ■ Observe the deflection in the compass needle as you move it along a field line. The deflection increases as the needle is moved towards the poles. Why? MAGNETIC EFFECTS OF ELECTRIC CURRENT-NOTE ■ What is the direction of the magnetic field both outside and inside the magnet? The deflection of the compass needle increases as it is moved towards the poles. This is because the magnetic field is stronger near the two poles and hence exerts larger force on the compass needle in such regions. Magnetic field is a vector qantity as it has both direction and magnitude. The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it. Therefore it is taken by convention that the field lines emerge from north pole and merge at the south pole (note the arrows marked on

the field lines in Fig. Inside the magnet, the direction of field lines is from its south pole to its north pole. Thus the magnetic field lines are closed curves.

Magnetic Field Due to a Current in a Conductor To know the direction of magnetic field we follow these rule (i) Right hand thumb rule. If the current carrying conductor is held in the right hand such that the thumb points in the direction of the current, then the direction of the curl of the fingers will give the direction of the magnetic field, as shown in Fig. (ii) Maxwell's cork screw rule. If a right handed screw be rotated along the wire so that it advances in the direction of current, then the direction in which the screw rotates gives the direction of the magnetic field as shown in Fig. Factors on which the magnetic field produced by a straight current carrying conductor depends: (i) If we increase the current in the conductor, the deflection of the compass needle increases. This shows that, the magnitude of the magnetic field produced at a given point is directly proportional to the current passing through the wire. That is,

B

I

(ii) For a given current, if we move the compass needle away from the wire, its deflection decreases. This shows that the magnitude of the magnetic field produced

by a given current in the wire is

inversely proportional to the distance from the wire. That

is,

Magnetic Field due to a Current Carrying Circular Loop Magnetic field due to a current through a circular loop.

The lines of force near the wire are almost concentric circles. As we move towards the centre of loop, the concentric circles become larger and larger. Near the centre of the loop, the arcs of these big circles appear as parallel straight lines. Thus the magnetic field is almost uniform at the centre of the loop. By applying right hand rule. We can see that the magnetic field

lines due to all sections of the wire are in the same direction within the loop. The magnetic field produced at the centre of circular coil carrying coil depends on following factors: (i) It is inversely proportional to the radius of the coil. That is,

(ii) It is directly proportional to the number of turn’s n of the coil. As

the direction of current in

each circular turn is same, the fields due to the various turns get added

up. That is,

MAGNETIC EFFECTS OF ELECTRIC CURRENT-NOTE

(iii) It is directly proportional to the strength of current passing the coil. That is, Clock rule to determine the polarity of any face of a circular current loop.

Clock rule. The polarity of any face of circular current loop can be determined by using clock rule, as shown in Fig. If the current round any face of the loop flows in anticlockwise direction, it behaves like a north pole. If the current flows in the clockwise direction, the face behaves like a south pole.

Solenoid. ■ A long cylindrical coil of insulated copper wire of large number of circular turns is called a solenoid. ■ When an electric current is passed through a solenoid, it produces a magnetic field around it. Its magnetic

field pattern is shown in Fig.

Magnetic field of a straight solenoid.

Magnetic field of a bar magnet.

► When a current is passed through the solenoid, the current in each circular loop has the same direction, their

magnetic effects get added up producing a strong magnetic field. ► Inside the solenoid, the magnetic field is almost uniform and parallel to the axis of the solenoid. ►The magnetic field produced by a solenoid is very much similar to that of a bar magnet. Like a bar magnet,

one end of the solenoid has N-polarity while the other end has S-polarity. ►The polarity of any end (face) of the coil can be determined by using clock rule.

For all practical purposes, the magnetic field of a solenoid and that of a bar magnet can be taken identical.

Factors on which the strength of the magnetic field produced by a current carrying solenoid depends: (i) Number of turns in the solenoid. The larger the number of turns in the solenoid, stronger is the magnetic field produced. (ii) Strength of the current. The larger the current passed through the solenoid, stronger is the magnetic field produced. (iii) Nature of the core material. By winding the coil over a soft iron cylinder, called core, the magnetic field can be increased several thousands times.

Electromagnet. A soft iron core placed inside a solenoid behaves like a powerful magnet when a current is passed through the solenoid. This device is called an electromagnet. When the current is switched off, the iron core loses its magnetism and so it is no longer an electromagnet. Thus, electromagnets are temporary magnets.

Factors on which the strength of an electromagnet depends: (i) Number of turns in the coil. The larger the number of turns in the coil, greater is the strength of the electromagnet. (ii) Strength of the current. The larger the current passed through the solenoid, more powerful is the electromagnet. MAGNETIC EFFECTS OF ELECTRIC CURRENT-NOTE

(iii) Nature of the core material. The core of the magnetic material like soft iron increases the strength of the electromagnet. Uses

Cranes and lifts use electromagnets to separate and lift large quantities of iron scrap and steel •

We find them in electrical devices like electric bells, telegraphs, telephones, loud speakers, electric trains, electric motors and so on

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Doctors use weak electromagnets to remove steel splinters from the eye

Differences between an electromagnet and a permanent magnet Electromagnet

Permanent Magnet

1. It is a temporary magnet. It shows magnetism only as long as the current is

1. It retains magnetism for a long time even after the through its

removal of the magnetizing field (or current).

coil. 2. It can produce very strong magnetic field.

2. It produces a much weaker field than an electromagnet.

3. The strength of an electromagnet can be easily

3. Its strength cannot be changed,

varied by changing the strength of current or number of turns in the coil. 4. The polarity of an electromagnet can be reversed

The polarity of a permanent magnet cannot be

by sending the current in reverse direction.

changed.

Advantages of electromagnets over permanent magnets (i) An electromagnet can produce a very strong magnetic field. (ii) The strength of the magnetic field of an electromagnet can be increased/decreased by increasing/decreasing the strength of current or the number of turns in the solenoid. (iii) The polarity of an electromagnet can be reversed by sending the current in the reverse direction.

FORCE ON A CURRENT-CARRYING CONDUCTOR IN A MAGNETIC FIELD We know that an electric circuit flowing through a conductor produces a magnetic field. This field exerts a force on a magnet placed near the conductor. In accordance with Newton's Third law, the magnet must also exert an equal and opposite force on the current-carrying conductor. Thus a magnetic field exerts a force on a circuit-carrying conductor. Such a force was first suggested and demonstrated experimentally by French Scientist Andre Marie Ampere is 1820. Factors does the force experienced by a current carrying conductor placed in a uniform magnetic field depend

If a current I is flowing along the wire of length L which is placed perpendicular to the direction of the magnetic field B, then the force F experienced by the wire perpendicular to the current and the magnetic field (as given by Fleming's left hand rule) is expressed as :

F = BIL Thus, F depends on current I, length L and strength of fieldB. Inference A current carrying conductor experiences a force when placed in a magnetic field. The direction of force is reversed when the direction of current in the conductor is reversed.

The force acting on the current-carrying conductor can be changed by changing the direction of the magnetic field.

Fleming's left Hand Rule Fleming's left hand rule helps us to predict the movement of a current carrying conductor placed in a magnetic field. MAGNETIC EFFECTS OF ELECTRIC CURRENT-NOTE

According to this rule, extend the thumb, forefinger, and the middle finger of the left hand in such a way that all the three are mutually perpendicular to each another. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, then, the thumb points in the direction of the force exerted on the conductor. Devices that use current carrying conductors and magnetic fields include electric motors, generators, loudspeakers and microphones.

Q. When is the force exerted on a current-carrying conductor (i) maximum and (ii) minimum? Ans. (i) When the current-carrying conductor is held perpendicular to the direction of the magnetic field, the force exerted on it is maximum.(ii) When the current-carrying conductor is held parallel to the direction of the magnetic field, the force exerted on it is minimum or zero.

Q.A current carrying straight conductor is placed in east-west direction. What will be the direction of the force experienced by this conductor due to earth's magnetic field? How will this force get affected on?(i) Reversing the direction of flow of current ? (ii) Doubling the magnitude of current? Ans. The direction of earth's magnetic field is from geographical south to geographical north. According to Fleming's left hand rule, the current carrying straight conductor placed in east-west direction will be deflected downwards. (i) On reversing the direction, the conductor is deflected in the upward direction. (ii) If the magnitude of current is doubled, it will result in doubling the magnitude of the force. Q. On what factors does the force experienced by a current carrying conductor placed in a uniform magnetic field depend? Ans. Factors on which the force experienced by a current carrying conductor placed in a magnetic field depends. If a current / is flowing along the wire of length L which is placed perpendicular to the direction of the magnetic field B, then the force F experienced by the wire perpendicular to the current and the magnetic field (as given by Fleming's left hand rule) is expressed as: F = BIL, Thus, F depends on current /, length L and strength of field B. Q. An electron enters a magnetic field at right angles to it as shown in Fig. The direction of force acting on the electron will be

ELECTRIC MOTOR Electric motor. An electric motor is a rotating device which converts electric energy into mechanical energy. Principle. An electric motor works on the principle that a current carrying conductor placed in a magnetic field experiences a force, the direction of force is given by Fleming's left hand rule. Construction. I. Field magnet. It is a strong horse shoe type magnet with concave poles. II. Armature. It is a rectangular coil ABCD having a large number of turns of thin insulated copper wire wound over a soft iron core. The armature is placed between the poles the field magnet and it can be rotated about an axis perpendicular to the magnetic field les.

III. Split ring commutator. It consists of a cylindrical metal ring split into two halves S 1 and S2. The two ends A and D of the armature coil are connected to the split rings S1 and S 2 respectively. As the coil rotates, the split rings also rotate about the same axis of rotation. The function of the split

MAGNETIC EFFECTS OF ELECTRIC CURRENT-NOTE

ring commutator is to reverse the direction of current in the coil after every half rotation. IV. Brushes. Two graphite or flexible metal rods maintain a sliding contact with split rings S1 and S2, alternately. V. Battery. A battery of few cells is connected to the brushes. The current from the battery flows to the armature coil through the brushes and the split rings. Working of a DC Motor When the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn towards the right, causing rotation. When the coil turns through 900, the brushes lose contact with the commutator and the current stops flowing through the coil. However the coil keeps turning because of its own momentum. Now when the coil turns through 180 0, the sides get interchanged. As a result the commutator ring C1 is now in contact with brush B 2 and commutator ring C 2 is in contact with brush B 1. Therefore, the current continues to flow in the same direction.

The Efficiency of the DC Motor Increases by: •

Increasing the number of turns in the coil

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Increasing the strength of the current

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Increasing the area of cross-section of the coil

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Increasing the strength of the radial magnetic field

An electric motor brings about rotational motion in domestic appliances such as electric fans, washing machines, refrigerators, mixers, grinders, blenders, computers, MP3 players, etc.

Electromagnetic induction. Whenever the

magnetic lines of force passing through a

closed circuit change, a voltage and hence a

current is induced in it. This phenomenon is

called electromagnetic induction. The voltage so produced is called induced

electromotive force (e.m.f.) and the current is

called induced current. This phenomenon

was discovered in 1831 by Michael Faraday

in England. ►Whether the magnet is moved

towards/away from the coil or the coil is moved

towards/away from the magnet, the magnetic lines of force passing through the closed coil get changed. This produces an induced potential difference in the coil which, in turn, sets up an induced current in the circuit. The current is indicated by the deflection in the galvanometer needle. When the coil and the magnet are both stationary, there is no deflection in the galvanometer. This indicates that when there is no change in the magnetic lines of force passing through the coil, no potential difference is induced in it.

MAGNETIC EFFECTS OF ELECTRIC CURRENT-NOTE

Galvanometer. A galvanometer is an instrument that can detect the presence of a current in a circuit. The pointer remains at zero (the centre of the scale) for zero current flowing through it. It can deflect either to the left or to the right of the zero mark depending on the direction of current.

Rules for Determining the Direction of Induced Current The direction of induced current can be determined by using Fleming's Right Hand Rule.

Stretch the forefinger, the middle finger and the thumb of the right hand, such that they are mutually perpendicular to each other. If forefinger indicates the direction of the magnetic field, the thumb indicates the direction of motion of the conductor, then, middle finger indicates the direction of induced current in the conductor. The electric generator works on the above explained phenomenon.

Current is induced in a coil when the current in the neighboring coil changes. We can conclude that a potential difference is produced in the coil-2 whenever the electric current through the coil-1 is changing (starting or stopping). Coil-1 is called the primarly coil and coil-2 is called the secondary coil. As the current in the first coil changes, the magnetic field associated with it also changes. Thus the magnetic field lines around the secondary coil also change. Hence the change in magnetic field lines associated with the secondary coil is the cause of induced electric current in it. This process, by which a changing magnetic field in a conductor induces a current in another conductor, is called electromagnetic induction.

A.C. Generator It is a device which converts mechanical energy into alternating form of electrical energy. Principle. It works on the principle of electromagnetic induction. When a closed coil is rotated in a uniform magnetic field with its axis perpendicular to the magnetic field, the magnetic field lines passing through the coil change and an induced emf and hence a current is set-up in it. Construction. 1. Field magnet. It is a strong horse shoe-type permanent magnet with concave poles. 2. Armature. ABCD is a rectangular armature coil. It consists of a large number of turns of insulated copper wire wound on a soft iron cylindrical core. It can be rotated about

an axis perpendicular to the magnetic field of the field magnet. 3. Slip rings. These are two brass rings S 1 and S2 rigidly connected to the two ends of the armature coil. As the coil rotates, slip rings also rotate about the same axis of rotation. 4. Brushes. These are two graphite rods B1 and B 2 which are kept pressed against the slip rings S1 and S 2. Through these brushes, the current induced in the armature coil is sent to the external circuit.

MAGNETIC EFFECTS OF ELECTRIC CURRENT-NOTE

Working. As shown in Fig, suppose the armature coil ABCD is in the horizontal position. Now the coil is rotated clockwise. The coil cuts the magnetic lines of force. The arm AB moves upwards while the arm CD moves downwards. According to Fleming's right hand rule, the induced current flows from A to B in arm AB and C to D in arm CD i.e., the induced current flows along ABCD. The induced current flows in the circuit through brush B2 to B v After half the rotation of the armature, the arm CD moves upwards and AS moves downwards. The induced current now flows in the reverse direction i.e., along DCBA. The current flows from Bx to B2. Thus the direction of current in the external circuit changes after every half rotation. Such a current which changes its direction after equal intervals of time is called alternating current. This device is called A.C. Generator.

Differences between electric motor and generator. Electric motor

Generator

1. It converts electrical energy into mechanical

1. It converts mechanical energy into electrical energy.

energy. 2. It is based on magnetic effect of current.

2. It is based on electromagnetic induction.

3. Current is supplied to the coil placed in magnetic field

3. The coil is rotated in a magnetic field by an external

by an external source of electrical energy. As a result of it,

arrangement. As a result, an electric current is induced in

coil starts rotating.

the coil.

Direct current. A direct current is that current which flows with constant magnitude in the same direction. Alternating current. An alternating current is that current whose magnitude changes continuously with time and whose direction reverses after equal intervals of time. Advantage of AC over DC. Only alternating voltage can be stepped up or stepped down by using a transformer. This makes AC more suitable than DC for transmission for electric power over long distances without much loss of energy. Frequency of a.c. mains in India In India, the direction of A.C. changes after every 1/100 second, i.e., the frequency of A.C. is 50 Hz.

Domestic Electric Circuits Domestic Wiring •

The electric power line enters our house through three wires- namely the live wire, the neutral wire and the earth wire. To avoid confusion we follow a colour code for insulating these wires. The red wire is the live wire, and the black wire is neutral. The earth wire is given green plastic insulation.

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The live wire has a high potential of 220 volts whereas the neutral wire has zero potential. Thus the potential difference between the live wire and the neutral wire is 220-0 = 220 volts.

MAGNETIC EFFECTS OF ELECTRIC CURRENT-NOTE

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The earth wire is much thicker in size and is made of copper. One end of it is connected to a copper plate buried deep under the earth. The earth connection is made to the electric meter and then to the main switch.

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In our homes, we receive supply of electric power through a main supply (mains), either supported through overhead electric poles or by underground cables.

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The live wire and neutral wire, coming from the electric pole, enter a box fitted just outside our house which has a main fuse F1. The fuse is connected in series with the live wire. This is done so because it is only the live wire which has a high potential of 220 volts unlike the neutral wire which carries zero potential. The fuse F1 has a high rating of about 50 amperes. Thus it prevents any damage such as fire to the entire electrical wiring entering the house due to short-circuit or overloading.

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The two wires then enter the electricity meter which records the electrical power consumed by us in kilowatt-hour (kWh). This meter is installed by the electric supply Department of our city.

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These two wires coming out of the meter are then connected to a main switch which is placed in a distribution box. Another fuse F2 is placed in series with the live wire in this box for the sake of consumer safety.

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There are two separate circuits in a house namely lighting circuit and power circuit. The lighting circuit with a 5 A fuse is used for running electric bulbs, fan, radio, TV, tube lights etc. and the power circuit with a 15 A fuse is used for running electric heater, electric iron, geyser, refrigerator etc as it draws more current.

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The distribution circuits are always connected in parallel combination. In a parallel circuit even if there is a fault or short-circuiting in any one line, the corresponding fuse blows off leaving the other circuits and appliances intact and prevents damage to the entire house.

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In case short-circuit occurs in the power circuit, then the power-fuse will blow off but our lights will continue to burn as the lighting circuit remains unaffected.

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A constant voltage of the main line is available for all other electrical appliances.

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Along with the two wires, a third wire called the earth wire also enters our house as shown in the fig. The earth connection is first made to the electric meter and then to the main switch. This wire then goes into the rooms along with the live and neutral wires.