HSC Physics Topic 3 From Ideas to Implementation

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KCiC Physics 7 Ideas to Implementation

Slide 1

keep it simple science

Key Concepts in Colour

HSC Physics Topic 3

From Ideas to Implementation

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keep it simple science

From Ideas to Implementation

First, an introduction:

HSC Physics Topic 3

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...

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.

About the Same Time

as Cathode Rays were

becoming understood, other scientists were studying electromagnetic radiation and obscure phenomena such as the

“Photoelectric Effect”.

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. No-one could have

guessed that this led to, not only the radio and

mobile phone, but to solar cells...

The Study of

Crystal

Structure

led to the discovery of Superconductors, the applications of which are

only just beginning to be implemented.

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Current & Future Applications Plank’s Quantum Theory Conductors & Superconductors Einstein’s New Model of Light Band Theory for Conductors Television Discovery of the Electron. Thomson’s Experiment. Valves, Transistors & Microprocessors Semi-Conductors Photoelectric Effect Atomic Structure & Lattices Cathode Rays

Hertz’s Discovery of Radio Waves Behaviour of Charged Particles in a Magnetic Field

FROM IDEAS TO

IMPLEMENTATION

1. From Cathode

Rays to Television

2. From Radio to

Photocells.

QUANTUM THEORY

3. From Atoms

to Computers

4. From Crystals

to Superconductors

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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.

1. FROM CATHODE RAYS TO TELEVISION

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.

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.

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.

This tube is glowing and showing light and dark bands, or

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Maltese Cross Tube

What does this prove?

Cathode Rays travel in straight lines, from the Cathode.

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.

CATHODE

(-vve) ANODE (+ve) inthe shape of a Maltese Cross

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

A beam of Cathode Rays can cause a fluorescent

screen to glow.

Wheel spins when cathode rays strike the paddles.

Fluorescence was known to be caused by certain

waves, such as ultra-violet (UV) rays

Experiments with CRTs

Tube With a

Rotating

Paddle-Wheel

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

Tube With a

Fluorescent Screen

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.

CRT with fluorescent screen Beam of cathode rays on screen Electric plates on either side of beam (no voltage applied yet) -ve +ve

Tube Containing

Electric Plates

What does this prove?

Cathode Rays must be a stream of charged

particles.

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. When voltage is applied to the plates,

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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.

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.

Confusion About

Cathode Rays

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.

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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 8

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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

FIELDS BETWEEN “POINT” CHARGES

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

E = F

Q

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

Q

_e

= (-)1.602 x 10

-19

C.

Get used to this very small value.

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

F = Q.E

= -1.602x10-19x 400 = -6.41x10-17_N.

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

By definition, the direction of

the field is the way a positive charge would move in the field Attraction Repulsion

Electric Fields

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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

E = Electric Field strength, in NC-1.

V = Voltage applied to the plates, in volts (V). d = distance between the plates, in metres (m).

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 = V / d = 12.0 / 0.0125 = 960NC-1_. Positively (+ve) charged plate

+

Negatively (-vve) charged plate Uniform Field Between Plates

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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:

CRT with fluorescent screen. The beam of cathode rays goes

straight across.

If a magnet is brought near, the beam deflects.

A force is acting on the moving charged particles.

Example Calculation

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

F = QvBsinθθ

= -1.602x10-19x2.50x106x0.0235xsin70o = -8.84 x 10-15N.

(negative sign simply refers to direction)

Direction of the force?

Remember the Right-Hand Palm Rule?

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.

S

Velocity vector, v Magnetic

Field B

Force, F

Force on a Moving Charge in a Magnetic Field

The size of the force can be calculated as follows:

F = QvBsin

θθ

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).

θθ

= Angle between the velocity vector and the magnetic field vector lines.

Since sin90o _{= 1,}

and sin0o = 0,

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

B

Magnetic Field

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+ve

-ve

Cathode Rays

Fluorescent screen to measure deflection

EElleeccttrriicc FFiieelldd EEffffeecctt

(charged plates)

Cathode Rays

E field down page

B into page

When the 2 forces cancel;

Force due to = Force due to

Electric Field

Magnetic Field

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.

Charge to mass ratio = Q

m

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

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.

Variable voltage

Discovery of the Electron...

Thomson’s Experiment

M

Maaggnneettiicc FFiieelldd EEffffeecctt

(Adjustable Electromagnets)

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.

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.

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

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 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.

How a TV Screen Works

The Deflection Plates

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

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

The Fluorescent Screen

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 light-spots (“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.

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Activity 2

CRTs, Electrons & TVs

Student Name ...

1. The effect of a magnetic field on a moving, charged particle can be described

mathematically by the equation F = QvB sinθθ. State what is meant by each of

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.

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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 Hertzin 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.

2. FROM RADIO to PHOTOCELLS: QUANTUM THEORY

High-vvoltage Induction coil

Wire loop acts as a receiving antenna. The radio waves induce

currents in the wire, and sparks in the gap.

Sparks produced in small gap in receiving loop

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 = λλ.f He found the radio waves travelled at the

speed of light.

ssppaarrkk ggaapp

Radio waves emitted from spark

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.

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...

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Slide 15

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Investigating Radio Waves

You may have done some simple studies in the laboratory, such as:

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.

What Hertz Failed to Investigate

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”:

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

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

Wire of receiving loop. Spark gap

Ultra-vviolet rays give their This can eject an

energy to electrons on the electron from the surface metal surface. so sparks are more likely.

Solar Cells

Induction coil & Power Pack

Array of wire connected to induction coil acts as a transmitting antenna

Radio receiver picks up loud bursts of noise, from some

distance away

The induction coil’s high-vvoltage sparking produces all sorts of EMR, including radio, light, UV &

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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.

shorter longer Wavelength of Radiation very hot object hot object “peak” wavelength “peak” wavelength shorter Am ou nt o f E ne rg y Ra di at ed HOT BODY RADIATION CURVES warm object “peak” wavelength longer

Black Body Radiation

The explanation for the “Black-Body

Radiation” required a totally new idea.

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Problems with Classical Physics

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...

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 =

λλ.f

(or, for light) c =

λλ.f

c = velocity of light (in vacuum) = 3.00x108_ms-1_.

λλ = wavelength, in metres (m).

f = frequency, in hertz (Hz)

Example Calculation

A ray of red light has a wavelength of 6.50x10-7_m.

a) What is its frequency?

b) How much energy is carried by one quantum of this light?

Solution

a) c =λλ.f 3.00x108= 6.50x10-7x f ∴ ∴ f = 3.00x108_/6.50x10-7 = 4.62x1014Hz.

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.

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.

Plank’s

Quantum Theory

b) E = h.f

= 6.63x10-34x 4.62x1014 = 3.06x10-19J.

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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.

Einstein and Quantum Theory

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.

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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.

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.

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

Applications of the Photoelectric Effect

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.

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Activity 3

Quantum Theory & Photoelectric Effect

Student Name ...

1. What did Heinrich Hertz discover in 1887?

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.

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!

Einstein, 1905

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Slide 22

Is Science Research Removed from Social & Political Forces?

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.

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.

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.

Atom-bbomb damage Hiroshima, Japan

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Revision of Atomic Structure

After Thomson identified the electron as a particle 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:

3. FROM ATOMS to COMPUTERS: SEMICONDUCTORS

CChheemmiiccaall BBoonnddss Migrating electron In a conductor, electrons can “jump” from one atom to the next

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

Electrical Conductivity

When millions and billions of atoms form a lattice structure (most strong solids are like this) they do so by forming chemical bonds with each other in a regular array.

Structure of an ATOM

Atomic Nucleus of protons & neutrons

In a metal atom, the outer (“valence”) electrons are very loosely held by the atomic nucleus. They “feel” the force of attraction from other, surrounding atoms just as strongly as the attraction from their “own” atom. The result is that these outer electrons can easily move from atom to atom.

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.

ATOMS in a SOLID ARRAY

Electrical Conduction occurs when electrons can “migrate” freely from one atom to the next

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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.

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.

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

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”.

Band Structure Theory

The unoccupied band above the valence band,

is called the “conduction band”. The highest energy level that has electrons in it, is called the “valence band”. “Forbidden energy gap”. Electrons cannot exist here. Electrons in quantised “energy bands”. Some bands overlap each other.

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KCiC Physics 7 Ideas to Implementation

Slide 25 Usage & copying is permitted according to the ®

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 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.

Conduction Band

These bands overlap

Valence

Band Valence _Band Valence Band

Forbidden

Energy gap _{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. Atom with 5 valence electrons used to “Dope” the lattice. extra valence electron

DOPING increases the conductivity of the lattice.

Conductors, Insulators & Semiconductors

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.

Conduction

Band Conduction Band

In Conductors these bands overlap each

other.

In Insulators these bands are separated by a wide “forbidden energy gap”. In Semiconductors there is only a narrow gap between bands.

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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:

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...

Electron has enough energy to conduct away, leaving a hole behind.

hole

Electrons are jumping to the right

Conduction of Electrons & Holes

1.

2.

3.

4.

5.

...and the hole is jumping 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.

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.

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

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Slide 27

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).

Thermionic Valves: Cathode Ray Tubes

“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

Relatively large & expensive. Consume relatively large amounts

of electricity

Produce large amounts of “waste” heat.

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.

10

-22

0

cm

Despite these limitations, “Collosus” was very

A Little History:

Electronics & Computers

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. 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.

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

aluminiumor gallium.This adds extra “holes” to the lattice. Electrical current is carried mainly by this flow of positive holes (hence “p”-type).

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KCiC Physics 7 Ideas to Implementation _{Slide 28} Usage & copying is permitted according to the ®

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.

2

cm

The comparison is a “no-brainer”... The transistor replaced Thermionic Valves as rapidly as electronics industries could

re-design their products, and begin mass production

Transistors

A Little History Continued...

Invention of the Transistor

Because of the properties of the

semiconductor (conductivity that can be

switched on and off)

transistors can do the

same job as thermionic valves.

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”.

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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.

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 “solid-state” 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.

A Little More History... Silicon v Germanium

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”.

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!

In the 1980’s the first cheap PC’s (personal computers) could process a magnificent 2x103 “bytes” of information. Computer “motherboard”

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Activity 4

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.

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Slide 31

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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.

Assessment of Impacts of the Transistor on Society

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.

Like it or hate it, (some people think we should have stayed in the trees) the modern world could not exist without the

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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 X-rays 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.

4. FROM CRYSTALS TO SUPERCONDUCTORS

Crystal x-rray

beam

X-rrays diffracted by the crystal lattice & form Interference patterns which are captured

on the film. Photographic film sensitive to x-rrays

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.

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www.keepitsimplescience.com.au Slide 33

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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. 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.

Superconductivity!

Crystal Structure of Metals

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.

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

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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 some of the superconducting metals and compounds.

Here is a very short list...

Temperature

Superconductor

of Transition (

o

K)

Metals to Superconductivity Mercury 4 Lead 9 Alloy Niobium-Germanium 23 Ceramics Yttrium-Barium-Copper oxide 92 Thallium-Barium-Calcium-Copper oxide 125 (-148oC)

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.

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,

non-stop and generate a magnetic field that repels the approaching magnet. Superconductors will never allow an

external magnetic field to penetrate. dish Liquid Nitrogen Disk of Superconducting Ceramic Small Levitating magnet

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How do we explain the phenomenon of superconductivity?

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:

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.

Approaching electron

How Superconductivity Occurs... BCS Theory

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”)

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.

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.

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