HSC Physics Topic 3 From Ideas to Implementation

keep it simple science Key Concepts in Colour HSC Physics Topic 3

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Slide 1

HSC Physics Topic 3

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From Ideas to Implementation About the Same Time as Cathode Rays were

First, an introduction: The History of Physics is marked by a number of “landmark” discoveries that changed our understanding of the Universe, such as Newton’s Laws of Motion, and Gravitation, and Einstein’s Theory of Relativity. This topic covers a number of other great discoveries, experiments and scientists, so it is definitely a study of the History of Physics, from about 1850 into the 20th century. However, it is not just history. Along the way, you will be studying some concepts, theories and facts that are vital to your overall understanding of this subject. In addition, as you learn both the history and some of the foundation ideas of modern Physics, you will see that much of our modern technology is a direct result these discoveries...

becoming understood, other scientists were studying electromagnetic radiation and obscure phenomena such as the “Photoelectric Effect”.

No-one could have guessed that this led to, not only the radio and mobile phone, but to solar cells...

and Meanwhile, the unravelling of atomic structure and study of electrical conductivity in “weird” substances like Germanium and Silicon, led to the discovery of “semiconductors”.

The invention of the transistor followed... the basis of all modern electronics and computer systems.

When “Cathode Rays” were being studied between 1850-1900, people said “interesting, but what’s the use of it??” Little did they know... ...the study of Cathode Rays led directly to the invention of the TV set, so familiar today. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

The Study of Crystal Structure led to the discovery of Superconductors, the applications of which are only just beginning to be implemented.

Slide 2

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Cathode Rays

Behaviour of Charged Particles in a Magnetic Field

1. From Cathode Rays to Television

Hertz’s Discovery of Radio Waves Discovery of the Electron. Thomson’s Experiment.

Plank’s Quantum Theory

Television

2. From Radio to Photocells. QUANTUM THEORY

FROM IDEAS TO IMPLEMENTATION

Einstein’s New Model of Light

Photoelectric Effect

Atomic Structure & Lattices

3. From Atoms to Computers 4. From Crystals to Superconductors

Band Theory for Conductors Conductors & Superconductors

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Slide 3

Valves, Transistors & Microprocessors

SemiConductors

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1. FROM CATHODE RAYS TO TELEVISION

The Discovery of Cathode Rays By the 1850’s, scientists had developed the technology to produce quite high voltages of electricity and to make sealed glass tubes from which most of the air had been removed using a vacuum pump. It wasn’t long before these 2 things were combined, and some mysterious phenomena were discovered. You may have done some laboratory investigations with “Discharge Tubes” as shown at right.

Each tube contains a different pressure of gas. (All are very low pressure, but some lower than others.) High voltage from an induction coil is applied to each tube in turn.

This tube is glowing and showing light and dark bands, or “striations”

It was soon established that whatever was causing these glows or “discharges” in the tubes was coming from the negative electrode, or “cathode”...so these emissions were called “Cathode Rays”. Over the following 20 years these mysterious “rays” were studied by many scientists. Sir William Crookes devised so many clever variations on these Cathode Ray Tubes (CRT’s) that they were known as “Crookes Tubes”. You will have seen, in the school laboratory, a number of different CRT’s and repeated many of Crookes’s famous experiments... next slide. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 4

The result is that each tube shows glowing streamers, or light and dark bands, or glows at the end(s). The patterns change at different gas pressures. At the very lowest pressure, there is no glow from the gas, but the glass tube glows at one end. Usage & copying is permitted according to the Site Licence Conditions only

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Maltese Cross Tube CATHODE (-v ve)

Tube With a Fluorescent Screen

Experiments with CRTs

ANODE (+ve) in the shape of a Maltese Cross

A beam of Cathode Rays can cause a fluorescent screen to glow. Fluorescence was known to be caused by certain waves, such as ultraviolet (UV) rays

Wheel spins when cathode rays strike the paddles.

Shadow of the cross in the glow at the end of the tube

This shows that the rays have momentum, and therefore have mass.

What does this prove? Cathode Rays travel in straight lines, from the Cathode.

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Tube Containing Electric Plates

CRT with fluorescent screen

Crookes tried this experiment with many different metals as his electrodes. The type of metal made no difference... Cathode Rays are identical, regardless of the materials used. The evidence from these various experiments was very inconsistent... some of the features of cathode rays suggested they are particles, other results suggested they are waves.

Tube With a Rotating Paddle-Wheel

What does this prove? Cathode Rays must be a stream of charged particles.

Beam of cathode rays on screen Electric plates on either side of beam (no voltage applied yet)

-ve

+ve

When voltage is applied to the plates, the beam deflects Slide 5

In fact, by considering the charge on the plates at left, it follows that the particles must be negatively charged, because the beam is deflected by repulsion from the negative plate, and attraction towards the positive.

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Confusion About Cathode Rays

Unfortunately, when the early experimenters tried experiments similar to those in the previous slide, they got a variety of confusing and conflicting results. Consequently they were confused about the nature of the Cathode Rays.

Evidence that CR’s are Waves Cathode Rays: • Travel in straight lines like light waves. • Cause fluorescence, like ultra-violet. • Can “expose” photographic film, just as light does.

Evidence that CR’s were Particles Cathode Rays: • Carry kinetic energy and momentum, and therefore must have mass. • Carry negative electric charge. (but this vital clue was missed!) All these investigations and discoveries involved the Cathode Ray Tube. This is a relatively simple device that allows the manipulation of a stream of charged particles.

This debate was finally settled by a famous experiment you will study soon... In 1897, J.J. Thomson showed that cathode rays had both mass and negative charge. He had discovered the electron. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 6

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Activity 1

The following activity might be completed by class discussion, or your teacher may have paper copies for you to do.

Cathode Rays

Student Name .................................

1. Which 2 technologies, both available from about 1850, were combined to make the early “discharge tubes”? 2. Name the great English scientist of the 19th century who was famous for his experiments with cathode rays. 3. Why were they called “cathode” rays? 4. List 3 pieces of evidence which suggested, to early investigators, that the mysterious rays were a type of wave radiation.

5. a) What did the experiments with a “paddle-wheel” CRT suggest about the rays? b) What did the experiments with a CRT fitted with a fluorecent screen and electric deflection plates suggest about the rays?

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Slide 7

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Electric Fields

The strength of the field is defined as the force per unit of charge experienced by a charge in the field...

E= F Q

In a Preliminary Course topic you learned that: • Electric Charges exert force on each other... ...like charges REPEL each other. ...opposite charges ATTRACT each other. • Charges act as if surrounded by a “Force Field”. FIELDS AROUND “POINT” CHARGES By definition, the direction of the field is the way a positive charge would move in the field

However, in this topic we are more interested in calculating forces, so

F = Q.E

is more useful.

F = Force, in newtons (N), experience by the charge. Q = Electric charge in coulombs (C). E = Electric field strength, in newtons per coulomb (NC-1) Note: In this topic the most common charged particle we deal with is the electron. The value of its charge is

Qe = (-)1.602 x 10-19C. Get used to this very small value.

FIELDS BETWEEN “POINT” CHARGES Repulsion

Example Calculation In a CRT, a stream of electrons passes between 2 electrically charge plates. The electric field strength is 400NC-1. What is the force acting on each electron?

Solution

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F = Q.E = -1.602x10-19 x 400 = -6.41x10-17N.

The negative sign simply means that the direction of the force is in the opposite direction to the electric field.

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Electric Field Between Parallel Charged Plates

The field around and between point charges is irregular in direction, and varies in strength at every point. The field between parallel charge plates, however, is uniform in strength and direction at every point (except at the edges). The direction of the field is the way a positive charge would move. The strength of the field depends on the Voltage applied to the plates, and the distance between them:

E= V d

Slide 9

+

Negatively (-v ve) charged plate

Uniform Field Between Plates

Example Calculation Two parallel plates are 1.25cm apart. (convert to metres) A voltage of 12.0V is applied across the plates. What is the magnitude of the field between the plates?

Solution

E = Electric Field strength, in NC-1. V = Voltage applied to the plates, in volts (V). d = distance between the plates, in metres (m). KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Positively (+ve) charged plate

E=V/d = 12.0 / 0.0125 = 960NC-1. Usage & copying is permitted according to the Site Licence Conditions only

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Force on a Moving Charge in a Magnetic Field

In the previous topic you learned that when an electric current flows through a magnetic field, the wire experiences a force... the “Motor Effect”. Now you need to realise that the reason is that every electric charge, if moving through a magnetic field, will experience a force. You may have seen the following experiment with a CRT in the laboratory:

The size of the force can be calculated as follows:

θ F = QvBsinθ F = Force acting, in newtons (N). Q = Electric charge, in coulombs (C). v = velocity of the charged particle, in ms-1. B = Magnetic Field strength, in Tesla (T). θ = Angle between the velocity vector and the magnetic field vector lines. θ B

Since sin90o = 1, and sin0o = 0,

CRT with fluorescent screen. The beam of cathode rays goes straight across.

Magnetic Field

then maximum force occurs when the charge moves at right angles to the field.

Example Calculation

S

If a magnet is brought near, the beam deflects. A force is acting on the moving charged particles. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

In the CRT at left, the cathode rays (electrons; Qe=-1.602x10-19C) are moving at a velocity of 2.50x106ms-1. The magnet provides a field of 0.0235T. Held as shown, the field lines are at an angle of 70o to the beam. What force acts on each electron?

Solution

θ F = QvBsinθ = -1.602x10-19x2.50x106x0.0235xsin70o = -8.84 x 10-15N. (negative sign simply refers to direction) Slide 10

Direction of the force? Remember the Right-Hand Palm Rule? Velocity vector, v

Magnetic Field B Force, F However, this applies to positive (+ve) charges. For negative charges ( -ve) the force is in the opposite direction... back of hand side. Check that the deflection in the photo at left is correct.

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Discovery of the Electron... Thomson’s Experiment

In 1897, the confusion and debate about Cathode rays was settled by one of the most famous, and critically important, experiments in the history of Science. The British physicist Sir John Joseph Thomson set up an experiment in which cathode rays could be passed through both an electric field, and through a magnetic field, at the same time.

Electric Field Effect

(charged plates)

-ve

The strengths of the fields could be calculated from the currents and voltages applied to the plates and electromagnets, so Thomson was able to calculate the ratio between the charge and mass of the cathode rays.

This established beyond doubt that cathode rays were particles, not waves.

E field down page

Variable voltage

Magnetic Field Effect

Fluorescent screen to measure deflection (Adjustable Electromagnets)

Cathode Rays

B into page

Thomson was able to adjust the strengths of the 2 fields so that their opposite effects exactly cancelled out, and the beam went straight through to the centre of the screen.

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Force due to = Force due to Electric Field Magnetic Field

Charge to mass ratio = Q m

+ve

Cathode Rays

When the 2 forces cancel;

Slide 11

Furthermore, he repeated the experiment with many different cathode materials and always got the same result. This meant that the exact same cathode ray particles were coming from every type of atom. Other experimenters had already determined the charge-mass ratio for the hydrogen atom (the smallest atom). It was apparent that the cathode ray particle was much smaller than a hydrogen atom. The conclusion was that all atoms must be made of smaller parts, one of which was the “cathode ray particle”, soon re-named the “ELECTRON”. This was a vital piece of knowledge for better understanding of atoms and electricity, and the development of many new technologies. Usage & copying is permitted according to the Site Licence Conditions only

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How a TV Screen Works

Thomson used a fluorescent screen at the end of his CRT to detect and measure the deflection of the cathode rays (electrons).

The Deflection Plates

Over the following 30 years, CRT technology evolved into the television screen. By the middle of the 20th century, TV was developing to become the major system for home entertainment and by the 1980’s the same screens became the vital display units for computers.

One set of charged plates are arranged so the field can deflect the beam up or down. Another set are arranged at right angles to cause deflection left or right.

A TV “picture-tube” is really just a more sophisticated version of Thomson’s CRT. The image on the screen is made up of thousands of spots of light, created as cathode rays strike a fluorescent screen on the inside of the glass.

The Fluorescent Screen

are used to deflect the beam to create spots of light at different points on the screen.

Between them, the sets of plates can “steer” the beam onto any point on the screen.

The 3 main parts of a TV picture-tube are:

The Electron Gun produces the beam of cathode rays (electrons). The electrons leave a cathode, and are accelerated towards a series of anodes by the high voltage electric field between them, just like in the CRT’s of Crookes or Thompson. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 12

glows with light when the electron beam strikes the fluorescent chemical coated on the inside of the glass. The total image is built from many thousands of lightspots (“pixels” = picture elements). The illusion of movement is achieved by replacing each full-screen picture many times per second. To produce colour TV there are actually 3 electron guns, and 3 sets of deflection plates. Three separate beams are steered onto separate spots of fluorescent chemicals which glow red, green or blue (RGB). The final colour is a combination of these 3 colours combined. Usage & copying is permitted according to the Site Licence Conditions only

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Activity 2

The following activity might be completed by class discussion, or your teacher may have paper copies for you to do.

CRTs, Electrons & TVs

Student Name .................................

1. The effect of a magnetic field on a moving, charged particle can be described θ. State what is meant by each of mathematically by the equation F = QvB sinθ these symbols.

2. a) Outline the famous experiment done by JJ Thomson in 1897. b) What did he actually measure as his final result? c) He repeated the experiment with a variety of cathodes made from different metals and got the same result each time. What was the conclusion from this? 3. Outline the function of these main parts of a TV picture tube. a) Electron gun. b) Deflection plates. c) Fluorescent screen. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 13

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2. FROM RADIO to PHOTOCELLS: QUANTUM THEORY

The Radio Experiments of Hertz By the 1880’s, the theory of electromagnetic radiation (EMR) had been around for 20 years, but no-one had found proof that these waves existed. Until, that is, the famous experiment of Heinrich Hertz in 1887. Using the familiar “induction coil” to produce sparks across a gap, Hertz showed that some invisible waves were being produced...

Hertz had discovered radio waves. Radio waves emitted from spark spark gap

High-v voltage Induction coil

Sparks produced in small gap in receiving loop

Wire loop acts as a receiving antenna. The radio waves induce currents in the wire, and sparks in the gap.

Hertz went on to experiment with these invisible waves and showed that they could be reflected, refracted, polarised and diffracted just like light waves. The clincher was when he measured their velocity and got an answer of 3x108ms-1... the waves were travelling at the speed of light! KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 14

This was powerful evidence supporting the theory that light was just one of a whole spectrum of Electromagnetic waves that had been predicted earlier. In recognition of Hertz’s contribution to our knowledge of waves, the unit of wave frequency (Hz) is named in his honour. Within another 20 years, radio was being used for long-distance communications using morse code. Within 100 years the world was blanketed with radio transmissions for communication and entertainment.

HOW DID HERTZ MEASURE SPEED OF THE RADIO WAVES? He reflected the radio waves (from metal sheets) so that they set up interference patterns. By moving his “receiving loop” around the lab. he could measure exactly where the peaks of interference occurred (where the waves added in amplitude). From this, the wavelengths of the waves were calculated. The frequency could be determined from the settings of his wave transmitter. Then the wave equation was used: V = λ.f He found the radio waves travelled at the speed of light. Usage & copying is permitted according to the Site Licence Conditions only

What Hertz Failed to Investigate ®

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Investigating Radio Waves You may have done some simple studies in the laboratory, such as: Array of wire connected to induction coil acts as a transmitting antenna The induction coil’s high-v voltage sparking produces all sorts of EMR, including radio, light, UV & even X-rrays.

In one of his many experiments with the new waves he had discovered, Hertz found that his “receiving loop” became more sensitive and sparked more if it was exposed to other radiations coming from his transmitter. He didn’t realise the significance of this observation, and failed to follow up on it. We now know (with perfect hind-sight) that he had produced the “Photoelectric Effect”: Ultra-v violet rays give their energy to electrons on the metal surface. Wire of receiving loop.

This can eject an electron from the surface so sparks are more likely. Spark gap

Later, this phenomenon was used by Einstein as proof of the new “Quantum Theory”... read on.

Induction coil & Power Pack

Radio receiver picks up loud bursts of noise, from some distance away

By adding a “tapping key” switch to the transmitter circuit, it is easy to send messages to the receiver in the form of “dots-and-dashes” of static noise. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

This Photoelectric Effect was exploited in the 20th century to develop the technology of photocells and solar cells. Solar Cells

Slide 15

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Black Body Radiation

In a previous Preliminary topic (“Cosmic Engine”) you learned about the way that energy is radiated from hot objects. A “perfect” emitter of radiation had become known as a “black-body”... It was well known that as a “black body” became hotter, it not only emitted more energy as radiation, but that the wavelength of the peak of the radiation became shorter, and frequency became higher. The problem was that the standard Physics theories of the time could not explain the shape of these graphs, which were obtained from experiment. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 16

Amount of Energy Radiated

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“peak” wavelength shorter

HOT BODY RADIATION CURVES very hot object

“peak” wavelength

hot object

warm “peak” wavelength longer shorter

object

longer Wavelength of Radiation

The explanation for the “Black-Body Radiation” required a totally new idea. Usage & copying is permitted according to the Site Licence Conditions only

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Plank’s Quantum Theory

In 1900, Max Plank proposed a radical new theory to explain the black body radiation. He found that the only way to explain the exact details coming from the experiments, was that the energy was quantised: emitted or absorbed in “little packets” called “quanta”. (singular “quantum”) The existing theories of “classical” Physics assumed that the amount of energy carried by a light wave could have any value, on a continuous scale. Plank’s theory was that the energy could only take certain values, based on “units” or quanta of energy. It’s the same as with matter: The smallest amount of (say) carbon you can have is 1 atom. Then you can have 2 atoms, 3 atoms and so on, BUT you cannot have 1/2 atoms of carbon... the matter is quantised, with whole atoms as the minimum “quantum”. Well, says Plank, energy is the same! Plank’s Quantum Theory proposed that the amount of energy carried by a “quantum” of light is related to the frequency of the light.

Problems with Classical Physics

E = h.f E = energy of a quantum, in joules ( J) h = “Plank’s constant”, with a value of 6.63x10-34 f = frequency of the wave, in hertz (Hz) You are reminded also, of the wave equation:

V = λ.f

λ.f (or, for light) c =λ

c = velocity of light (in vacuum) = 3.00x108ms-1. λ = wavelength, in metres (m). f = frequency, in hertz (Hz)

Example Calculation

A ray of red light has a wavelength of 6.50x10-7m. a) What is its frequency? b) How much energy is carried by one quantum of this light?

Solution

λ.f a) c =λ 3.00x108 = 6.50x10-7x f ∴ f = 3.00x108/6.50x10-7 = 4.62x1014Hz. b) E = h.f = 6.63x10-34 x 4.62x1014 = 3.06x10-19 J. What IS the Photoelectric Effect? When metal surfaces are exposed to light waves (especially high frequency light or ultra-violet) some electrons are found to be ejected from the metal surface, as long as a certain critical energy level is exceeded.

At the same time that Plank was proposing his Quantum Theory to explain the Black Body radiation details, the “Photoelectric Effect” (that Hertz had observed but failed to study) was being investigated by others. Experiments on the photoelectric effect were producing results that could NOT be explained by the existing theory of light. For a century or more, light had been accepted as a wave. This explained its reflection, refraction, interference, and many other phenomena. However, the photoelectric effect experiments were giving results that suggested light was best explained as a stream of particles... this could turn Science on its ear! Enter Albert Einstein... KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

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Einstein and Quantum Theory

It was Albert Einstein who came to the rescue and neatly combined Plank’s Quantum Theory with the classical wave theory of light, in a way that solved all the apparent conflicts, and explained the Photoelectric Effect as well! To keep it as simple as possible, (K.I.S.S. Principle) Einstein proposed that: • Light is a wave, but • the energy of the wave is concentrated in little “packets” or “bundles” of wave energy, now called “Photons”. • Each photon of light has an amount of energy given by E = h.f, according to Plank’s Quantum Theory. • When a photon interacts with matter, it can either transfer all its energy, or none of it... it cannot transfer part of its quantised energy. Light is NOT a stream of particles Light is NOT a wave

Light is a stream of “wave packets”... “PHOTONS”. They have wave properties... refraction, interference, etc. They can also behave like a particle sometimes. Each photon is a Quantum of light energy. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 18

Einstein’s model for light involves a “duality”... light must have a dual nature. Many of its properties are wave related; e.g. ability to reflect, refract and show interference patterns. In other cases, especially when energy transfers are occurring, the light photons are like little particles. This explained the Black Body Radiation curves, and the weird features of the Photoelectric Effect.

Confirmation of Einstein’s Model Einstein’s idea is very neat, but is it correct? Einstein was able to make certain mathematical predictions regarding further features of the Photoelectric Effect. (The exact details are complicated, and not required learning.) In 1916, the experiments were done to test Einstein’s predictions, and the results agreed with his predictions precisely! This was confirmation that the photon theory of light, and the quantum theory of energy were both correct. Einstein was awarded the Nobel Prize for Physics in 1921, for his contribution to understanding the Photoelectric Effect. Usage & copying is permitted according to the Site Licence Conditions only

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Applications of the Photoelectric Effect

Solar Cells Solar Cells (or “photovoltaic cells”) are devices which produce electricity directly from light energy. They are very familiar in the popular garden lights which need no wiring or battery replacements. During the day, the solar cell(s) charge up a small re-chargable battery. At night, the battery provides electricity to a low-power garden lamp. More importantly, solar cells hold the promise of cheap, efficient, environmentally-friendly electricity production. Solar-powered homes are becoming more and more common as the technology becomes more affordable and more people are concerned by the environmental problems of conventional electricity production.

Small array of solar cells powering a small electric motor and fan

Solar cells produce electricity from the Photoelectric Effect: Light photons falling on the cell give up their quantum of energy to electrons in a sandwich of semiconductor material, called a “p-n junction”. The energy gained by electrons causes them to be emitted so that they travel through the semiconductor structure and create a potential difference across it. This voltage causes a current to flow in the electrical circuit.

Photocells A photocell is a device which can detect and measure light. Photocells are used in light meters (photography), “electric-eyes” and a variety of light-measuring scientific equipment, such as photometers. Once again, the photoelectric effect is involved. When a photon of light strikes the receiving surface, its energy causes emission of an electron, which is collected on a nearby anode. A sensitive electric circuit is able to measure the level of electron emission, and this gives a measure of the amount of light being received. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

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Activity 3

The following activity might be completed by class discussion, or your teacher may have paper copies for you to do.

Quantum Theory & Photoelectric Effect 1. What did Heinrich Hertz discover in 1887?

Student Name .................................

2. What was Max Plank attempting to explain when he proposed his theory of “energy quanta” in 1900? 3. What is the “Photoelectric Effect”?

4. What did Einstein suggest about the nature of light waves in 1905?

5. List 2 technologies which are applications of the Photoelectric Effect. For each, describe an important use of the technology.

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Assessment of Einstein’s Contribution to Quantum Theory

“Assess” means to measure or judge the value of something. The syllabus requires you to assess Einstein’s contribution to the Quantum Theory in relation to Black Body Radiation.

Einstein, 1905

To begin with, you might note that Einstein did NOT think up the Quantum Theory... Max Plank did that in 1900. However, it seems that Plank invented the quantum idea purely as a mathematical “trick” to explain the Black Body Radiation curves. Plank never proposed that the quanta might give light a particle-like nature. Plank never suggested that the old ideas of “classical” Physics might need changing. It was Einstein who did that! His “particle-wave” (photon) idea combined Plank’s Quantum Theory with the classical idea that light is a wave. This totally new way to look at things was one of the turning points of modern Physics, and set other scientists off into new and innovative directions of research. It should be noted that the other major turning point for Physics was Einstein’s Theory of Relativity, which he proposed in the same year (1905).

No wonder we credit him as being one of the greatest! KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 21

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Is Science Research Removed from Social & Political Forces?

In World Wars I & II, Science and scientists played a major role in research and development of new weapons and war technologies. Some examples include: • radio communications and Radar. • nuclear weapons. • rockets. • new aircraft designs and jet engines. • chemical weapons such as poison gas. There are two contrasting views about the morality of weapons research, and the two great scientists of this section of the topic epitomise these different views. Max Plank was a patriotic German who believed that it was his duty to help his country fight a war. He gladly contributed to weapons research in WW I, and leading up to WW II he was the director of the main Scientific Institute in Nazi Germany. Plank’s outlook seems to have been that Science is part of the political & social structure, and must take an active role in it.

In the 1930’s Einstein was forced to flee Nazi Germany because he was of Jewish descent. In America, he warned the President about the possible development of an atomic bomb by the Nazis. This caused the Americans to begin the research which led to the first atomic bomb, developed directly from Einstein’s theories. He was not involved in the research, but was appalled when the atomic bomb was used against Japan in 1945. Einstein believed that Science is a process that should work for peace and the good of all people, and not be involved in the political & social forces that come and go. Who was right? There is no correct, nor simple, answer to that. You must form your own opinion... just be sure you have an informed opinion. Atom-b bomb damage Hiroshima, Japan

Albert Einstein was German-born, but became a Swiss citizen, and later American. In WW I he (and only 3 others) signed an anti-war declaration. He spent the war in neutral Switzerland, lobbying for peace and an end to war. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

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3. FROM ATOMS to COMPUTERS: SEMICONDUCTORS

Structure of an After Thomson identified the electron as a particle ATOM present in all atoms, it didn’t take long for scientists to figure out the details of atomic structure. You are reminded of the basic model of a typical atom:

Revision of Atomic Structure

Electrical Conductivity

Electrons in orbit at different “Energy Levels”

Electrons are quite easy to remove from some atoms... this leads to electrical conductivity, the Photoelectric Effect, etc

Atomic Nucleus When millions and billions of atoms form a of protons & neutrons lattice structure (most strong solids are like this) they do so by forming chemical bonds with each other in a regular array. In a metal atom, the outer (“valence”) electrons are very loosely held by the atomic nucleus. They “feel” the force of attraction ATOMS in a SOLID ARRAY from other, surrounding atoms just as strongly as the attraction Electrical Conduction occurs when electrons can from their “own” atom. The result is that these outer electrons can “migrate” freely from one atom to the next easily move from atom to atom. Migrating electron

Chemical Bonds

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In a conductor, electrons can “jump” from one atom to the next

If an electric field is present (due to a voltage being applied) billions of electrons begin moving in the same direction... an electric current is flowing, and we say the metal is a good Conductor. In other solids such as plastic or glass, the outer valence electrons are more strongly attracted to their own atom, and cannot easily escape from it, to move from atom to atom. We say these things are poor conductors, or good Insulators. Slide 23

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Band Structure Theory

The explanation given in the previous slide for conductors and insulators is OK, until you find out about “Semiconductors”. Elements such as Silicon and Germanium have a number of “strange” properties including being rather poor conductors of electricity until given a little jolt of energy. Then, suddenly they become quite good conductors. This ability, called “Semiconductivity”, allows these materials to act as electrical switches, turning electrical currents on and off, according to their energy state. This is the basis of all modern electronics & computer systems To understand semiconductivity, you need to learn about Band Structures We have known since the early 20th century that the electrons around an atom can occupy different “orbits” or energy levels surrounding the nucleus. These energy levels are “quantised” (Quantum Theory applies) so there may be “forbidden energy zones” between them. An electron cannot exist in this “fobidden zone” because the energy level there does NOT correspond to a whole quantum. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 24

The unoccupied band above the valence band, is called the “conduction band”.

“Forbidden energy gap”. Electrons cannot exist here.

The highest energy level that has electrons in it, is called the “valence band”.

Electrons in quantised “energy bands”. Some bands overlap each other.

Nucleus

Electrons can “jump” up and down through the different bands as they gain or lose energy. To jump up over a “forbidden zone” they must have enough energy to achieve the quantum energy level required to occupy the next band. In any atom in its “rest state”, the highest band occupied by electrons is the “Valence Band”. If an electron has enough energy to get to the unoccupied levels above there, the electron is effectively free to “wander off”. If an electric field is applied, the electron becomes part of a flowing current, and the substance is conducting electricity. That’s why any energy band above the valence band is called a “Conduction Band”. Usage & copying is permitted according to the Site Licence Conditions only

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Conductors, Insulators & Semiconductors

In terms of “Band Theory”, the difference in conductivity between different substances is simply the relationship between the Valence Band and the Conduction Band. In Conductors these bands overlap each other.

In Insulators these bands are separated by a wide “forbidden energy gap”.

Conduction Band

Conduction Band

These bands overlap

Forbidden Energy gap

Valence Band

Valence Band

In Semiconductors there is only a narrow gap between bands. Conduction Band

Valence Band

In metals, electrons can move into the conduction band at any time, so the solid array of atoms is a good conductor at all times. In an insulator, such as plastic, the electrons can never achieve the conduction band unless they are given a huge boost of energy. At normal temperatures and voltage levels, the substance will not carry a current.

A semiconductor, like Silicon, will not normally carry current, because electrons lack the energy to jump the “forbidden energy gap”. However, if the temperature is increased, and a voltage applied, there comes a point when electrons jump the gap in great numbers, and the substance suddenly conducts very well indeed. This effect does not occur at room temperature unless the semiconductor substance is “Doped”.

Doping a Semiconductor “Doping” means to add a very small quantity of a different type of atom to an otherwise pure solid lattice of semiconductor atoms. Atoms of Semiconductor substance e.g. Silicon, normally have 4 valence electrons Each chemical bond is formed by atoms sharing 2 electrons. These electrons are in the valence energy band.

extra valence electron

Atom with 5 valence electrons used to “Dope” the lattice.

DOPING increases the conductivity of the lattice. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 25

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Conduction of Electrons & Holes

Normally we imagine that an electric current is composed of a flow of negative electrons. However, in a semiconductor, when an electron jumps out of the valence band and flows off somewhere, it leaves behind a “hole” in the valence band. This hole, is a space that an electron from elsewhere can jump into. Imagine a line of atoms in a semiconductor lattice: Electron has enough energy to conduct away, leaving a hole behind. hole

Now imagine a sequence of movements in which the next electron in the valence band has enough energy to jump into the hole, leaving its own hole behind... 1. 2.

If you can imagine this sequence like the pictures making a motion cartoon, you can imagine that an electron flows to the right and the hole flows to the left. In fact, in terms of electrical energy, it makes no difference whether the current really is negative electrons going one way, or “holes” going the other way... either way, it constitutes an electric current. The holes are considered as positively charged spaces (relative to the electrons) and so the flow of positive holes may be thought of as genuine “Conventional Current”. So, there is another way to “Dope” a semiconductor. The diagram in the previous slide shows the use of atoms with an “extra” valence electron. The other way to do it is to use atoms with only 3 valence electrons, creating extra “holes” in the lattice.

Electrons are jumping to the right extra hole in the lattice

3.

4. ...and the hole is jumping left.

Atom with only 3 valence electrons used to “Dope” the lattice.

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p-Type & n-Type Semiconductors

The two different ways to “dope” the lattice result in two different types of semiconductor material:

p-Type Semiconductors are doped with atoms with 3 valence electrons, such as aluminium or gallium. This adds extra “holes” to the lattice. Electrical current is carried mainly by this flow of positive holes (hence “p”-type).

n-Type Semiconductors

are doped with atoms with 5 valence electrons, such as arsenic or antimony. This adds extra valence electrons to the lattice. Electrical current is carried mainly by this flow of negative charges (hence “n”-type). ®

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A Little History: Electronics & Computers

“Thermionic” refers to the way these CRT’s would emit many electrons from the cathode (and thereby carry a current) when the cathode became hot. Once “warmed up” the valve can act as an electronic “switch” in a circuit, when the voltage to the anode is varied.

Characteristics

The concept of a machine to carry out high speed calculations and “logical” operations has been around for centuries. Prior to the 20th century, any such device had to be mechanical, using “clockwork” gears and so on. There were some notable successes with control devices for weaving looms, and mechanical “adding machines”, but applications were very limited.

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Slide 27

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Relatively large & expensive. Consume relatively large amounts of electricity Produce large amounts of “waste” heat. 10-2 20 cm

During World War II the first electronic computers were built (in tight secrecy) to help decode enemy radio messages. Instead of gears and dials, the “Collosus” computer used thermionic valves to electronically switch circuits on and off, to store and manipulate data. These valves are described at the right.

Thermionic Valves: Cathode Ray Tubes

Although faster than mechanical switches, valves are slow-acting by modern standards. Require time to “warm up”. Have a limited lifetime, and can “burn out” like a light bulb. Therefore their reliability is low, and maintenance needs are high.

Despite these limitations, “Collosus” was very important in helping to win the war.

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A Little History Continued... Invention of the Transistor

Thermionic valves had been widely used in radios for some years and were vital components of the new industry of television. Valves were also important in the switching of connections in telephone exchanges, where the growing communication demands required automatic dialing and connection technology. (The original system involved human “operators” manually plugging wires into sockets to connect phone calls.) However, the valve-based technology was proving too slow, too unreliable and too expensive for the booming telephone industry. The major U.S. phone company “Bell Telephone” set its scientists the task of researching new materials and processes to replace the valves. In 1947, 3 scientists at Bell Laboratories, invented the transistor, using a “sandwich” of p-type and n-type doped semiconductor material. Transistors

But a transistor: • is only a fraction of the size. • costs much less to make. • consumes only tiny amounts of electricical power. • produces virtually no waste heat. • operates much faster than a valve. • does not need to “warm-up”. • is highly reliable, and rarely needs maintenance. The comparison is a “no-brainer”... The transistor replaced Thermionic Valves as rapidly as electronics industries could redesign their products, and begin mass production

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Because of the properties of the semiconductor (conductivity that can be switched on and off) transistors can do the same job as thermionic valves.

Slide 28

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A Little More History... Silicon v Germanium

To make semiconductor material with the desired conductivity properties, it is necessary to firstly prepare extremely pure samples, then add tiny amounts of the “doping” chemical, and finally grow crystals of the semiconductor from the molten material in a furnace. The original transistors were made from Germanium because the technology to produce crystals of the pure element was already known. However, Germanium is a rare element, whereas its close “sister element” Silicon, is one of the most abundant elements on Earth.

The miniature “integrated circuit board” led to the technology of the “silicon chip” where thousands, and now millions of transistor-equivalents can be printed microscopically in the space of a postage stamp... a “microchip”. In the 1980’s the first cheap PC’s (personal computers) could process a magnificent 2x103 “bytes” of information.

By the 1960’s, the technology to obtain pure crystals of Silicon had been developed, and because Silicon is so abundant and therefore cheaper, it quickly replaced Germanium. Silicon’s electrical properties turned out to be better too. For example, it held its semiconductive properties constant over a wider range of temperatures. Also in the 1960’s, the technology of the computer began to emerge for financial and communication uses. The “solidstate” transistor technology allowed a computer to be built to fit a table-top, rather than fill a room. Every teenager had a brick-size “transistor radio”, in the same way that in this decade everyone has a mobile phone the size of a matchbox. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 29

Computer “motherboard”

Twenty years later, these notes are being composed with an even cheaper PC which can process 2x109 bytes, (2GB). The computers have become a million times more powerful! Usage & copying is permitted according to the Site Licence Conditions only

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Activity 4

The following activity might be completed by class discussion, or your teacher may have paper copies for you to do.

Semiconductors

Student Name .................................

1. In terms of “Band Theory”, how are conductors, insulators and semiconductors different to each other? 2. a) Differentiate between a current carried by electrons and one carried by holes. b) Differentiate between an “n-type” and “p-type” semiconductor. 3. a) What is “doping” in the making of a semiconductor? b) What type of atoms (and give specific example) are used to dope a silicon crystal to make an n-type semiconductor? c) What type of atoms (and give specific example) are used to dope a silicon crystal to make a p-type semiconductor? 4. Name the type of CRT used in the first electronic computers and name the first semiconductor devices which replaced them. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 30

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Assessment of Impacts of the Transistor on Society

It could be argued that the invention of the transistor was one of the most profound technological developments in history. It ranks right up there beside the developments such as:

Fire: 500,000 years ago. Fire transformed human society because of its power to warm people, cook food and protect from predators.

Agriculture: 10,000 years ago. This transformed society from nomadic hunting-gathering to settled communities that invented law, commerce, government and “civilization”.

Metallurgy & the Industrial Revolution, which led to new tools, machinery, mass production, urbanisation, and mass transport systems. Like it or hate it, (some people think we should have stayed in the trees) the modern world could not exist without the invention of the transistor! KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

The transistor helped create the

“Information & Communication Revolution”, which is still developing today. Electronic circuits, using microchips, are the basis of all the computers which allow: • instant access to (virtually) all the information on the planet via the internet. • instant access to money from your bank account from (virtually) anywhere in the world. • instant communication via your mobile phone to and from (virtually) anywhere. Computers are the key to the global economy and mass consumerism which keeps thing cheap through mass production & distribution. Computers keep track of the billions of business transactions that feed us, clothe us, entertain us, transport us and service all our needs. Usage & copying is permitted according to the Site Licence Conditions only

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4. FROM CRYSTALS TO SUPERCONDUCTORS

Investigating Crystal Structures... Bragg and Son The regular shapes of crystals (such as salt) had long been assumed to be due to a regular arrangement of the atoms or ions in a lattice-like structure. However, until the early 20th century, there was no way to prove or confirm this idea. The discovery of high frequency EMR in the form of Xrays opened up a new line of investigation. Sir William Bragg and his son Lawrence, beamed X-rays through crystals and studied the diffraction patterns which were formed as the crystal lattice scattered the X-rays. Photographic film sensitive to x-rrays

x-rray beam

Crystal

X-rrays diffracted by the crystal lattice & form Interference patterns which are captured on the film. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 32

The Braggs were able to analyse the interference pattern in order to deduce the arrangement of the atoms within the crystal. For this, they were jointly awarded the Nobel Prize for Physics in 1915. This opened up a whole new investigative technique, allowing scientists to probe the structure of matter as never before. It was X-ray diffraction crystallography, for example, that allowed the structure of DNA to be determined in the 1950’s.

Crystal Structures Thanks to scientists like the Braggs, we now understand the atomic-level structure of most substances. You learned previously how a substance like the semiconductor Silicon is a lattice of atoms chemically bonded together: Each chemical bond is formed by atoms sharing 2 electrons with each neighbour atom. Usage & copying is permitted according to the Site Licence Conditions only

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Crystal Structure of Metals

Unlike silicon, salt and other crystals, metal atoms are not chemically bonded to each other by the sharing or exchanging of electrons. You will remember that the outer “valence” electrons in metals are weakly held, and can access the “conduction band” at any time. The result is that the valence electrons on each atom are NOT confined to that atom, but freely wander around from atom to atom. Each metal atom is, therefore, ionised because its valence electron(s) are on the loose. The metal lattice is often described as

“an array of ions, embedded in a sea of electrons”. This “sea of electrons” shifts and flows freely.

Any impurities in the metal distort the shape of the lattice and impede the electron flow. Also, as the ions vibrate due to thermal energy, the vibration causes more collisions among electrons, so their flow is resisted. As temperature increases, the vibrations increase too, and that’s why resistance in metals increases with temperature. Logically, if you re-read the previous paragraph and think backwards, you might infer that if you had a really pure metal, and cooled it right down so that all lattice vibrations stopped, then it would become a perfect conductor.

If an electric field is present, the electrons will all flow in the same direction as an electric current. That’s why metals are all good conductors. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Resistance in Metals So why is there resistance in a metal wire? Although the electrons can flow quite easily, their movement is not totally free.

Superconductivity! Slide 33

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Superconductivity in Metals and Ceramics

In 1911, a Dutch physicist managed to cool mercury down to about 4oK (-269oC) and found that its electrical resistance dropped to zero. Over the following years, various other metals were found to become superconducting at very low temperatures. The potential to build electrical generators and equipment with zero resistance was a very attractive idea, but the temperatures involved (no higher than about 20oK) were so low that there seemed no practical way to take advantage. Then in 1986, Swiss scientists discovered some ceramic materials containing rare elements like Yttrium and Lanthanum, which became superconductors at much higher temperatures. Still cold by human standards, but 100o higher than the metal superconductors, these ceramics had zero resistance at temperatures as high as 130oK (around -150oC). This is a temperature that is much more practical to achieve. The syllabus requires that you identify superconducting metals and compounds. Here is a very short list...

Superconductor

some

of

Temperature of Transition (oK)

Metals to Superconductivity Mercury 4 Lead 9 Alloy Niobium-Germanium 23 Ceramics Yttrium-Barium-Copper oxide 92 Thallium-Barium-Calcium-Copper oxide 125 (-148oC) KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 34

The Meissner Effect You may have seen a practical demonstration of a superconductor in action, in class. The “Meissner Effect” is named after the scientist who discovered it. If a disk of superconductor ceramic is chilled below its “transition temperature”, a small magnet placed close above it will “levitate”; spinning freely if prodded, but held up against gravity by unseen forces. Disk of Superconducting Ceramic

Small Levitating magnet

Liquid Nitrogen

dish

the

Explanation As the magnet is brought near, its magnetic field induces currents in the ceramic. Since there is NO electrical resistance, the currents flow freely, nonstop and generate a magnetic field that repels the approaching magnet. Superconductors will never allow an external magnetic field to penetrate. Usage & copying is permitted according to the Site Licence Conditions only

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How Superconductivity Occurs... BCS Theory

How do we explain superconductivity?

the

phenomenon

of

The accepted explanation is known as “BCS Theory”, where “BCS” are the initials of the 3 scientists who developed the theory in the 1950’s. Imagine part of the solid lattice of positive ions in a conducting metal or ceramic. As an electron (part of an electric current) approaches, it attracts the positive ions and distorts the crystal structure slightly:

Approaching electron

Cooper-P Pair of electrons forms

Electrons in a Cooper-Pair are linked to each other by “Quantum Effects”.

Due to quantum effects (which are beyond the scope of this Course... KISS Principle) each electron of the Cooper Pair helps the other to pass through the lattice without any loss of energy. This means there is ZERO resistance.

This distortion concentrates the positive charge in this part of the lattice, and attracts other electrons. In a normal conductor, this distortion leads to collisions and loss of energy by the flowing electrons which repel each other... this is the normal electrical resistance within the conductor. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

But in a superconductor below its “transition temperature”, something very strange occurs; due to Quantum Energy Effects, 2 nearby electrons “pair up” to form what is called a “Cooper Pair”: (Cooper is the “C” in “BCS Theory”)

Slide 35

However, at a temperature above the “transition”, the thermal vibrations in the lattice keep breaking up the Cooper Pairs as fast as they can form. This destroys the superconductivity, and the normal electrical resistance of the substance returns. Usage & copying is permitted according to the Site Licence Conditions only

Using Superconductor Technology

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Possible Future Applications Advantages Superconductor technology offers • High efficiency in any electrical situation, because there is no energy loss due to resistance. • The ability to generate extremely strong magnetic fields from superconducting electromagnets. • Faster operation of computers, since superconducting switching devices could be 10 times faster than a semiconductor transistor.

Limitations • Superconducting metals must be chilled with liquid helium. This is impractical and expensive. • New, superconducting ceramics can be chilled with liquid nitrogen, which is cheaper and much more practical, BUT these ceramics: • are fragile, brittle and difficult to make into wires. • can be chemically unstable and have a limited life span. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 36

Current computer technology is based on semiconductor microchips. Although these become faster and more powerful every year, there is a limit to how far they can go. A superconductor computer could open a whole new level of enhanced performance due the possible high speed switching of circuits. Electricity generation & distribution could be made much more efficient with superconductor technology. A lot of energy is lost due to resistance heating in transmission lines. This could be eliminated if power lines were superconductors. Generators lose energy by resistance heating in the coils needed to produce magnetic fields, and are limited in the strength of the fields they can produce. Superconducting coils would allow generators to be much more powerful and efficient. Greater efficiency generally in electrical technology would reduce associated environmental problems, such as Greenhouse gas emissions. Usage & copying is permitted according to the Site Licence Conditions only

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Using Superconductor Technology cont.

The Maglev Train

MAGLEV = MAGnetic LEVitation

The idea of using superconducting electromagnets to “levitate” a train above a magnetic guide-rail has been around for many years and experiments have been going on for decades. The guiderail(s) under the train contain conventional electromagnets. On board, helium-chilled superconducting electromagnets produce powerful magnetic fields. The fields in the rail and the train repel each other so that the entire train is levitated 1-2cm above the track. Propulsion and braking is also done magnetically, by the fields in front and behind the train attracting and repelling it. The actual motive power is supplied from the rail, not from onboard the train. The big advantage is the high speed possible without any rail friction, and the low maintenance and low noise that goes with this. A disadvantage is the very high cost of building the guide rail track.

Shanghai Maglev Train Experiments have been going on for years in Germany and in Japan. The first truly operational Maglev now connects the city of Shanghai in China, with its airport 30km away. German built, it cost US$1.2 billion, and reaches speeds around 400km/hr.

Scientific Research Uses Superconductors Although the practical, everyday uses of superconductors are very limited so far, Science has been using superconductors for decades. The major use is to generate hugely powerful magnetic fields to accelerate particles for research. KCiC Physics 7 Ideas to Implementation copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 37

Using superconducting electromagnets, chilled with liquid helium to near -270oC, powerful magnetic fields can be generated. These are used to accelerate particles to close to the speed of light, then collide them together to study the structure of matter. This research is aimed at understanding not only matter itself, but the origins of the Universe. Usage & copying is permitted according to the Site Licence Conditions only

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Activity 5

The following activity might be completed by class discussion, or your teacher may have paper copies for you to do.

Superconductivity

Student Name ................................. 1. What technique was used by the father and son team of Braggs to study the structure of crystals? 2. Explain why metals are generally excellent conductors of electricity. 3. a) Why is there some electrical resistance in a metal at normal temperatures? b) Why does resistance increase with temperature? 4. What is the “Meissner Effect” and why does it occur? 5. What does “BCS Theory” attempt to explain. Outline the main principle. 6. What is “Maglev” short for? 7. What are some limitations of the high-temp. superconducting ceramics?

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Slide 38

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