KISS Notes Ideas to Implementation

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HSC Physics Topic 3 1

but first, an introduction...

HSC Physics Topic 3

FROM IDEAS to IMPLEMENTATION

What is this topic about?

To keep it as simple as possible, (K.I.S.S.) this topic involves the study of:

1. FROM CATHODE RAYS to TELEVISION

2. FROM RADIO to PHOTOCELLS: QUANTUM THEORY

3. FROM ATOMS to COMPUTERS: SEMICONDUCTORS

4. FROM CRYSTALS to SUPERCONDUCTORS

...all in the context of how Physics has contributed to modern technology

The History of Physics

is marked by a number of “landmark” discoveries that changed our understanding of the Universe...

• Newton’s Laws of Motion, and Gravitation, and • Einstein’s Theory of Relativity

have already been studied. 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...

Solar cells being used to make electricity on a remote outback property

and the

Study of Crystal Structure

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

beginning to be implemented.

Photo: Peter Hamza

Photo: Oliver Ransom

Photo: John de Boer

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copyright © 2005-2006 keep it simple science HSC Physics Topic 3 2 Cathode Rays. Discovery & Properties F = QE & E = V d F = QvBsinθθ Behaviour of a Charged Particle in a Magnetic Field Revision of Electric Fields Conductivity in Metals. Superconductivity Failure to follow-uup E = hf and c = f λλ

Current & Potential Applications of Superconductivity

The TV screen. Main parts and

their role Revision of “Black Body Radiation” Particle-WWave Duality of Light Plank’s Quantum Theory Discovery of the Electron... Thompson’s Experiment Confirmation of EMR. Measurement of “c” Differing views on Science’s place in society Einstein’s Contribution

Electrons & “Holes” in Conductivity

“Doping”. n-ttype & p-ttype

Semiconductors

Valves to Transistors to Microprocessors... Impacts on Society

The Braggs & X-rray Crystalography Revision of Atomic Structure & Structures of Solid Lattices Photoelectric Effect & Applications: • solar cells • photocells “Band Theory” of Conductors, Insulators & Semiconductors Hertz’s Discovery of Radio Waves

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From CATHODE RAYS

to TELEVISION From RADIO

to PHOTOCELLS: Quantum Theory From ATOMS to COMPUTERS From CRYSTALS to SUPERCONDUCTORS

CONCEPT DIAGRAM (“Mind Map”) OF TOPIC

Some students find that memorizing the OUTLINE of a topic helps them learn and remember the concepts and important facts. As you proceed through the topic, come back to this page regularly to see how each bit fits the whole. At the end of the notes you will find a blank version of this “Mind Map” to practise on.

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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 follows...

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, most notably Sir William Crookes. He 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...

Experiments with CRT’s

Maltese Cross Tube

What does this prove?

Cathode Rays travel in straight lines, from the Cathode. Furthermore, 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.

Tube With a Fluorescent Screen

Tube With a Rotating Paddle-Wheel

This 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.

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.

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

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 gas in the tube, but the glass itself glows at one end of the tube.

CATHODE ( -vve) ANODE (+ve) in the shape of a Maltese Cross Shadow of the cross in the glow at the end

of the tube

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W

Whheeeell ssppiinnss wwhheenn ccaatthhooddee rraayyss ssttrriikkee tthhee ppaaddddlleess

TThhiiss sshhoowwss tthhaatt tthhee rraayyss hhaavvee mmoommeennttuumm,, aanndd tthheerreeffoorree hhaavvee mmaassss

Fluorescence was known to be caused by certain

waves, such as ultra-violet (UV) rays

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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 above, 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.

Early Confusion About Cathode Rays

Unfortunately, when the early experimenters tried something similar to the above, they did NOT detect a deflection of the beam. So, they concluded there was NO charge associated, and were confused about the nature of the Cathode Rays.

Evidence that CR’s were Waves:

Cathode Rays:

• Travel in straight lines like light waves. • Cause fluorescence, like ultra-violet waves. • Can “expose” photographic film, 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!)

Note that all these investigations and discoveries involved the Cathode Ray Tube...

a relatively simple device that allows the

manipulation of a stream of charged particles.

Revision of Electric Fields

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”.

TRY THE WORKSHEET at the end of this section.

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-ve

+ve

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.

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

Example Calculation:

In the CRT shown at top left of this page, 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

= -1.602x10-19_{x 400} = -6.41x10-17_N.

The negative sign simply means that the direction of the force will be 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

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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.

TRY THE WORKSHEET at the end of the section.

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 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).

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_.

CRT with fluorescent screen. Cathode Ray beam goes straight across.

If a magnet is brought near, the beam deflects. A force is acting on the moving charged particles.

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).

θ = Angle between the velocity vector and 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 Mag. Field θ

In the CRT above, the cathode rays (electrons; Qe=-1.602x10-19_C)

are moving at a velocity of 2.50x106_ms-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

= -1.602x10-19_x2.50x106_{x0.0235xsin70}o = -8.84 x 10-15_{N. (negative sign}

simply refers to direction)

How do you know the 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.

Can you verify the upward deflection in the photo above is consistent with theory?

Velocity vector, v Magnetic Field B Force,F SS Positively (+ve) charged plate

+

-Negatively (-ve)

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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 J.J. 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.

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). 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.

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.

+ve

-ve

Cathode Rays

Fluorescent screen to measure deflection Electric Field Effect (charged plates)

Magnetic Field Effect (Adjustable Electromagnets)

Cathode Rays

E down page

Binto 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. At this point, Force due to = Force due to

Electric Field Magnetic Field

Since the strengths of the fields could be calculated from the currents and voltages applied to the plates and electromagnets, 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 “ELECTRON”. This was a vital piece of knowledge for better understanding of atoms and electricity, and the development of many new technologies.

Variable voltage

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Worksheet 1

Part A Fill in the blanks.

Check answers at the back.

The discovery of a)... Rays was

made with simple “b)... tubes”

from which most of the air was removed with a

c)... pump. When high

d)... was applied to electrodes at

each end of the tube, it would produce a variety

of e)..., ... and

... The exact pattern changed as

the f)... in the tube was

altered. It was discovered that the effects were

due to mysterious emissions coming from the

cathode (or g)... electrode).

About the 1870’s, Sir William h)...

and others, built special CRT’s to study the

cathode rays. The famous “i)...

cross” tube showed that the rays travelled in

straight lines. Tubes with j)...

screens showed that the rays caused fluorescence,

and tubes equipped with a “paddle-wheel”

proved that the rays carried both k)...

energy and l)...

Unfortunately, attempts to detect deflection by

applying an m)... field were

unsuccessful, so for many years there was

confusion over whether CR’s were

n)... or ...

Evidence they were waves:

• CR’s travel in o)... like light.

• They cause p)... like UV

rays.

• They can expose q)...

Evidence they were particles:

• Carry r)... and ...

and therefore must have s)...

• Carry t)... electric charge

An electric u)... is created around

anything with electric charge. The direction of

the field is defined as v)...

...

Any charge within a field will experience a

w)... The field between 2

x)... ... plates is

uniform in both y)... and

..., and is determined by

the z)... applied to the plates

and the aa)... between them.

Electric charges also experience a force if they

are ab)...

through a

ac)... field. This is easily

observed by bringing a ad)... near

a CRT with a fluorescent screen; the magnet

causes the beam to ae)...

The direction of the force and the deflection of

the CR beam is easily determined by the

“af)... Rule.

In 1897, J.J. ag)... used the

deflection of

a CR beam by both

ah)... and ...

fields to measure the ratio of

ai)... of a cathode

ray. This established, beyond doubt, that CR’s are

aj)... and are a small part contained

within all ak)... Thomson had

discovered the al)... The

simple CRT was later used as the basis to invent

the am)... screen.

The main parts of the “picture tube” are:

• The an)... Gun, which

produces a beam of ao)... from a

ap)... and accelerates them

towards a series of aq)...

• The ar)... plates, which use

electric fields to as)... the beam

onto the screen.

• The at)... screen, which

forms the image when fluorescent chemicals

au)... with spots of light when

struck by av)...

COMPLETED WORKSHEETS BECOME SECTION SUMMARIES

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Part B Practice Problems

Field Between Charged Plates

& Force on a Charge

1. Two parallel plates are 4.00cm apart in a

vacuum tube. A voltage of 50.0V is applied

across the plates.

An alpha particle with charge of (+)3.20x10

-19

_C

passes between the plates.

a) What is the size of the electric field between

the plates?

b) What force will act on the alpha particle?

c) Describe the direction of the

i) field

ii) force

relative to the +ve and -ve plates.

2. An electron (Q=-1.60x10

-19

_{C) experiences a}

force of -7.82x10

-15

_{N within an electric field}

created by parallel plates which are 2.50mm apart.

a) Find the size of the electric field.

b) Find the voltage applied to the plates.

3. A speck of dust carrying a static electric

charge, experiences a force of 2.29x10

-12

_{N in a}

field produced by 2 plates 5.00cm apart. A 200V

potential difference is applied across the plates.

a) Find the strength of the field between the

plates.

b) What charge does the speck of dust carry?

c) The static charge was created when some

electrons were either removed from, or added to,

the speck of dust.

How many electrons were added or removed?

d) The speck of dust was observed to move

toward the negative plate. Did the speck lose or

gain electrons?

4. Two parallel plates have a 40.0V potential

difference between them. An electron between

them experiences a force of (-)5.88x10

-17

_N.

How far apart are the plates?

5. In an inkjet printer, small droplets of ink are

given an electric charge, then “steered” onto the

paper by accelerating them in electric fields to

achieve the desired velocities and directions.

What force would be experienced by a droplet

with charge of (+)9.75x10

-10

_{C, which is between}

parallel plates with potential difference of 100V,

and separated by 5.00mm?

Force on a Moving Charge

in a Magnetic Field

6. An electron (Q=-1.60x10

-19

_{C) is travelling north}

at 3.00x10

7

_ms

-1

_{in a cathode ray tube when it}

enters a magnetic field of strength 4.96x10

-2

_T.

The field is directed vertically upwards through

the CRT.

Find the magnitude and direction of the force

experienced by the electron.

7. In a nuclear accelerator, a charged ion has been

accelerated up to a velocity of 2.90x10

8

_ms

-1

_{. As it}

enters a magnetic field of strength 8.05T (field is

perpendicular to ion’s velocity vector) it

experiences a force of magnitude 3.75x10

-9

_N.

What is the magnitude of the charge on the ion?

8. A particle of the solar wind with charge of

(+)1.60x10

-19

_{C (it is in fact a proton) encounters}

the Earth’s magnetic field at an angle of 25

o

_to

the field lines. At this point the field has a

strength of 5.48x10

-4

_{T. The proton experiences a}

force of 7.40x10

-15

_N.

Find the velocity of the proton.

9. In an experiment similar to Thomson’s, a stream

of electrons in a CRT are each experiencing a

force of magnitude 4.06x10

-15

_{N when travelling}

through a perpendicular magnetic field at a

velocity of 7.80x10

6

_ms

-1

_.

a) What is the strength of the magnetic field?

The force on the electrons is exactly counteracted

by an electric field produced by charged plates

which are 8.00mm apart.

b) What is the strength of the electric field?

c) What is the voltage being applied across the

plates?

FULLY WORKED SOLUTIONS IN THE ANSWERS SECTION

Remember that for full marks in calculations, you need to show FORMULA, NUMERICAL SUBSTITUTION,

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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 Hertz in 1887.

Using the familiar “induction coil” to produce sparks across a gap, Hertz showed that some invisible waves were being produced... he had discovered radio waves.

Hertz went on to experiment with these invisible waves and showed that they could be reflected, refracted, polarized and diffracted just like light waves. The clincher was when he measured their velocity and got an answer of 3x108_ms-1_...

the speed of light!

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.

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.

2. FROM RADIO to PHOTOCELLS: QUANTUM THEORY

High-voltage Induction coil

RRaaddiioo wwaavveess EEmmiitttteedd ffrroomm ssppaarrkk

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 the 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.

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 realize 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.

spark gap

Wire of receiving loop. Spark gap

Ultra-violet 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

IInndduuccttiioonn ccooiill && PPoowweerr PPaacckk Array of wire connected to induction coil acts as a transmitting antenna

RRaaddiioo rreecceeiivveerr ppiicckkss uupp lloouudd bbuurrssttss ooff nnooiissee,, ffrroomm ssoommee ddiissttaannccee aawwaayy The induction coil’s high-voltage sparking produces all sorts of EMR, including radio, light, UV & even

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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.

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 (say) 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 1/2atoms 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

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”, which has 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_. λ = 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}-7_{x f} ∴ f = 3.00x108_/6.50x10-7 = 4.62x1014_Hz. b) E = h.f = 6.63x10-34_{x 4.62x10}14 = 3.06x10-19_J.

TRY THE WORKSHEET at the end of this section

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.

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

BLACK BODY

RADIATION

CURVES

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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.

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 the Einstein 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.

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. Already they are used in remote areas (see photo on front page) and in special situations, such as power for orbiting satellites.

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.

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LLiigghhtt iiss aa ssttrreeaamm ooff ““wwaavvee ppaacckkeettss””... ““PPHHOOTTOONNSS””.. TThheeyy hhaavvee wwaavvee pprrooppeerrttiieess... rreeffrraaccttiioonn,, iinntteerrffeerreennccee,, eettcc.. TThheeyy ccaann aallssoo bbeehhaavvee lliikkee aa ppaarrttiiccllee ssoommeettiimmeess.. EEaacchh pphhoottoonn iiss aa QQuuaannttuumm ooff lliigghhtt eenneerrggyy..

Small array of solar cells powering a small electric

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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!

“A-bomb Dome”, Hiroshima, Japan

by Kathy de la Cruz

Is Science Research Removed from Social & Political Forces?

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. In the 1930’s he was forced to flee Nazi Germany because he was of Jewish descent. In America, he fought against the development of the atomic bomb (developed directly from his own theories) and was appalled when it 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 systems.

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.

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Worksheet 2

Part A Fill in the blanks. Check answers at the back.

In 1887, Heinrich Hertz discovered a)... waves. His experiment involved high voltage from an b)... coil which produced c)... across a gap. The sparking produced radio waves which he detected with a d)... in which a small gap also sparked. He was able to show that the new radiations showed typical wave properties such as e)... and ... Hertz was also able to measure the f)... of the waves, and show it was equal to the speed of g)... He also produced evidence of the h)... Effect, but failed to investigate it further.

Meanwhile, other researchers had studied the way energy is emitted from hot objects. The “i)... Radiation” curves showed a shape that could not be explained by the accepted theories. In 1900, j)... proposed the “k)... Theory” to account for the problem. The basic idea of his theory is that the energy of light (or other EMR) is “l)...” the same way that matter is. The minimum quantity of matter is one m)..., and fractions cannot occur. Plank proposed that the energy of EMR is the same, and that the amount of energy carried by one “n)...” is related to the o)... of the wave.

The “Photoelectric Effect” occurs when p)... is absorbed at a metal surface. The energy is transferred to an q)... which may then be r)... from the surface. Experiments with this effect were producing results that could not be explained.

In 1905, Einstein used Plank’s s)... Theory to explain all the difficulties. His idea was:

• Light is a wave, but the energy is concentrated in “bundles” called “t)...”

• Each bundle carries a u)... of energy, as described by Plank’s theory.

• When a photon interacts with matter, it can either transfer v)... of its energy, or w)... of it, but cannot transfer x)... This idea allows light to have its “wave properties” such as y)..., ... and ..., but to also sometimes show z)...-like properties when it transfers energy. Based on his theory, Einstein made certain mathematical aa)... regarding the ab)... Effect. These were confirmed by ac)... in 1916. This confirmed Plank’s ad)... Theory, and explained all the “problems” with ae)... ... radiation & the af)... Effect.

Part B Practice Problems Quantum Theory

(Plank’s Constant = 6.63x10-34₎ ( c = 3.00x108_ms-1₎

1.

A light wave has a wavelength of 4.25x10-7_m. a) What is its frequency?

b) How much energy is carried by one photon?

2.

Compare the amount of quantum energy carried by a photon of

i) infra-red (heat) radiation (λ = 5.45x10-6_m) and ii) UV radiation (λ = 5.45x10-9_m)

3.

A photon of radiation is carrying 8.75x10-14_{J of energy.} Calculate

a) its frequency b) its wavelength

4.

To cause emission of an electron from the surface of a certain metal requires the electron to gain a minimum of 2.38x10-20_{J of energy.}

a) Find the frequency and wavelength of the photon of EMR which carries this “threshold” amount of energy. b) What happens if the electron is struck by a photon with a longer wavelength than this?

c) What will happen if the electron was struck by a photon of higher frequency than calculated in (a)?

5.

An electron was emitted from a metal surface after being struck by a photon of EMR.

The electron left the surface with energy of 6.22x10-17_{J. It} firstly had to “use” 9.28x10-19_{J of energy to escape the} metal surface. All of this energy was gained by interaction with a single photon.

Find the frequency and wavelength of the photon.

FULLY WORKED SOLUTIONS IN THE ANSWERS SECTION

Remember that for full marks in calculations, you need to show FORMULA, NUMERICAL SUBSTITUTION,

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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:

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.

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.

Band Structure Theory

The explanation just given 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 levelssurrounding 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.

3. FROM ATOMS to COMPUTERS: SEMICONDUCTORS

Chemical Bonds

ATOMS in a SOLID ARRAY

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

Migrating electron

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

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”.

“Forbidden energy gap”. Electrons cannot exist there. Electrons in quantised “energy bands” Some bands overlap

The unoccupied band above the valence band, is called the “ccoonndduuccttiioonn bbaanndd” The highest energy level that has electrons in it, is called the “vvaalleennccee bbaanndd” Structure of an ATOM

-Electrons in orbit at different “Energy Levels” Electrons are relatively easy to remove from some atoms... this leads to electrical conductivity, Photoelectric Effect, etc Atomic Nucleus

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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, In Insulators, In Semiconductors,

these bands the bands are there is a small gap

overlap. separated by a between the bands.

wide “forbidden energy gap”.

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.

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:

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...

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 on the left 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. Conduction Band Valence Band Conduction Band Forbidden Energy gap Valence Band Conduction Band Valence Band

AAttoommss ooff SSeemmiiccoonndduuccttoorr ssuubbssttaannccee 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 Atom with 3 valence electrons used to “Dope” the lattice. extra hole in the lattice

DOPING increases the conductivity of the lattice

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

hole

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p-Type and n-Type Semiconductors

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

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).

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).

Some 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.

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.

Because of the properties of the semiconductor (conductivity that can be switched on and off) the transistor can do the same job as the thermionic valve, but

• is only a fraction of the size and 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.

Thermionic Valves are 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. Despite these limitations, “Collosus” was very import-ant in helping to win the war.

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

10

c

m

Photo by Don Jolley

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Silicon v Germanium

To make semiconductor material with the desired conductivity properties, it is necessary to firstly prepare extremely pure samples, then add minute 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 an MP3 and a mobile phone the size of a matchbox.

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.} Twenty years later, these notes are being typed on an even cheaper PC which can process 2x109 _{bytes, (2 GB). The} computers have become a million times more powerful!

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, by ancient humans around 500,000 years ago. Fire transformed human society because of its power to warm people, cook food and protect from predators. • agriculture, about 10,000 years ago.

This transformed society from nomadic hunting-gathering to settled communities that invented law, commerce, government and “civilization”.

• metallurgy and the Industrial Revolution, which led to new tools, machinery, mass production, urbanization, and mass transport systems.

The transistor ushered in 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) any town or city on Earth.

• 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 invention of the transistor!

Photo by John de Boer

Photo pipp

Photo: Martin Boulanger

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Worksheet 3

Fill in the blanks. Check your answers at the

back.

a)... orbit around the nucleus of

atoms at various b)... levels.

Basically, a substance will be an electrical

conductor if c)... can move from

d)... freely. If

electrons cannot do this at all, the substance is an

e)...

A “semiconductor” is a substance which has very

low f)... until its

electrons are given just a little energy. Then, at a

certain point,

it suddenly becomes

g)... This allows electrical

circuits to be h)... on and off,

and is the basis of modern i)... and

j)...

The best explanation of semiconductivity

involves “k)...

Theory”, summarized as follows:

• the highest energy level in an atom that has

electrons in it, is called the l)...

band.

• any further (unoccupied) levels above this are

called m)... bands.

• If an electron has enough energy to get to a

m)... band, then it is free to

flow,

and form an electric

n)...

However, between the bands there may be

“forbidden” o)...

The energy levels are quantised, so a “forbidden”

level is where the energy is not equal to a whole

p)...

In a conductor, the q)... band and

r)... bands s)... each

other. This means electrons can freely enter the

conduction band and t)...

can flow through the substance.

In an u)..., these bands are

separated by a wide v)...

so that electrons can never reach the

w)... band.

In a semiconductor, the valence and conduction

bands are separately by a x)... gap.

In the “rest” state, electrons cannot get across,

and the substance does not

y)... However, it only

requires a slight increase in energy and suddenly

many electrons z)... the gap and the

substance begins aa)...

The semiconductor material can be made more

sensitive and conductive if ab)...

quantities of other elements are added to the

atomic lattice. This is called “ac...”

the semiconductor.

Semiconductors can carry electricity in 2 ways: by

the flow of ad)... which have

reached the conduction band, or by the flow of

“ae)...” left behind by departing

electrons.

If a af)... is doped with

atoms with 5 valence electrons, this results in

ag)... in the lattice to carry

the current. This is an “ah)...-Type”

semiconductor.

If it is ai)... with atoms with

only aj)... valence electrons, this creates

extra ak)... in the lattice to

carry current. This is a “al)...-Type

semiconductor.

Before semiconductors, electronic switching was

done by am)... valves. These

were an)... tubes. The

ao)... was invented to

replace these valves. Compared to a valve, a

transistor is

• ap) ... (size) and aq)... (cost)

• consumes ar)... electricity and

produces almost no as)...

• operates at a at)... rate

• does not need to au)...

• is highly av)...

The early transistors were made from

aw)..., but this was later replaced

by ax)... because it is more

ay)... and a lot az)...

Miniaturization of electronics on “silicon

ba)...” has led to the development of

“bb)...” which are at the heart

of all modern computers.

COMPLETED WORKSHEETS BECOME SECTION SUMMARIES

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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.

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:

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, ionized 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.

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 would become a perfect conductor.

4. FROM CRYSTALS TO SUPERCONDUCTORS

Crystal x-ray

beam

X-rays diffracted by the crystal lattice, form Interference patterns which are captured on the film.

Photographic film sensitive to x-rays

SSii SSii SSii SSii

SSii SSii

SSii

SSii SSii SSii SSii

SSii Each chemical bond is formed by atoms sharing 2 electrons with each neighbour + + + + + + + + + + + + + + + + + +

Superconductivity!

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Superconductivity in Metals and Ceramics

In 1911, a Dutch physicist managed to cool mercury down to about 4o_{K (-269}o_{C) 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 20o_{K) 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 130o_K (around -150o_{C). 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 (-148o_C)

How Superconductivity Occurs...

BCS Theory

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.

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.

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 them.

+ + ++ ++ + + ++ + + ++ ++ ++ ++ Approaching electron + + ++ ++ + + ++ + + ++ ++ ++ ++ CCooooppeerr PPaaiirr of electrons forms dish Liquid Nitrogen Disk of Superconducting Ceramic Small Levitating magnet