hsc physics topic 3 from ideas to implementation

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Slide 1

keep it simple scienceKey Concepts in Colour

HSC Physics Topic 3From Ideas to Implementation

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keep it simple science


Slide 2 Usage & copying is permitted according to the Site Licence Conditions only

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keep it simple science From Ideas to ImplementationFirst, an introduction:

HSC Physics Topic 3

The History of Physicsis marked by a number of “landmark” discoveries that changed ourunderstanding 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, experimentsand 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 studyingsome concepts, theories and facts that are vital to your overallunderstanding of this subject.

In addition, as you learn both the history and some of the foundationideas of modern Physics, you will see that much of our moderntechnology is a direct result these discoveries...

When “Cathode Rays” were being studied between1850-1900, people said “interesting,

but what’s the use of it??”

Little did they know...

...the study ofCathode Rays led

directly to theinvention of the

TV set, sofamiliar today.

About the Same Time as Cathode Rays werebecoming understood, other scientists were studyingelectromagnetic radiation and obscure phenomena such as the

“Photoelectric Effect”.

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

The invention of thetransistor followed... the

basis of all modernelectronics and

computer systems.

No-one could haveguessed that this led to,not only the radio andmobile phone, but to

solar cells...

The Study ofCrystal

Structureled to the discovery of

Superconductors, the applications of which are

only just beginning to beimplemented.



Current & FutureApplications

Plank’sQuantumTheory

Conductors &Superconductors

Einstein’sNew Model

of Light

BandTheory for

Conductors

Television

Discovery of theElectron.

Thomson’sExperiment.

Valves,Transistors &

Microprocessors

Semi-Conductors

PhotoelectricEffect

AtomicStructure &

Lattices

CathodeRays

Hertz’s Discovery of Radio WavesBehaviour ofCharged Particles in

a Magnetic Field

FROM IDEAS TOIMPLEMENTATION

1. From CathodeRays to Television 2. From Radio to

Photocells.QUANTUM THEORY

3. From Atomsto Computers

4. From Crystalsto Superconductors

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The Discovery of Cathode RaysBy the 1850’s, scientists had developed the technologyto produce quite high voltages of electricity and tomake sealed glass tubes from which most of the air hadbeen removed using a vacuum pump.

It wasn’t long before these 2 things were combined, andsome mysterious phenomena were discovered.

You may have done some laboratory investigationswith “Discharge Tubes” as shown at right.

1. FROM CATHODE RAYS TO TELEVISIONEach tube contains a different pressure of gas.(All are very low pressure, but some lower thanothers.) High voltage from an induction coil is

applied to each tube in turn.

The result is that each tube shows glowingstreamers, 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 fromthe gas, but the glass tube glows at one end.

It was soon established that whatever was causingthese glows or “discharges” in the tubes was comingfrom the negative electrode, or “cathode”...so theseemissions were called “Cathode Rays”.

Over the following 20 years these mysterious “rays”were studied by many scientists. Sir William Crookesdevised so many clever variations on these CathodeRay Tubes (CRT’s) that they were known as “CrookesTubes”.

You will have seen, in the school laboratory, a numberof different CRT’s and repeated many of Crookes’sfamous experiments... next slide.

This tube is glowingand showing lightand dark bands, or

“striations”



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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 manydifferent 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 aMaltese Cross

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

A beam of Cathode Rayscan cause a fluorescent

screen to glow.

Wheel spins when cathode rays strike the paddles.

Fluorescence was knownto be caused by certainwaves, such as ultra-

violet (UV) rays

Experiments with CRTs

Tube With aRotating

Paddle-WheelThis shows that the

rays havemomentum, and

therefore have mass.

Tube With a Fluorescent Screen

The evidence from these variousexperiments was very inconsistent...some of the features of cathode rayssuggested they are particles, otherresults suggested they are waves.

CRT withfluorescentscreen

Beam ofcathode rayson screen

Electricplates oneither sideof beam(no voltageapplied yet)

-ve +ve

Tube ContainingElectric PlatesWhat does this prove?

Cathode Rays must be astream of charged

particles.

In fact, by consideringthe charge on the platesat left, it follows that the

particles must benegatively charged,because the beam is

deflected by repulsionfrom the negative plate,and attraction towards

the positive.When voltage is applied to the plates,

the beam deflects



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Unfortunately, when the earlyexperimenters tried experiments similarto those in the previous slide, they got avariety of confusing and conflictingresults.

Consequently they were confused aboutthe nature of the Cathode Rays.

Evidence that CR’s are WavesCathode 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 AboutCathode Rays

Evidence that CR’s were ParticlesCathode 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 involvedthe Cathode Ray Tube. This is a relatively simple

device that allows the manipulation of a stream of charged particles.



Activity 1The 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 tomake the early “discharge tubes”?

2. Name the great English scientist of the 19th century who was famous for hisexperiments with cathode rays.

3. Why were they called “cathode” rays?

4. List 3 pieces of evidence which suggested, to early investigators, that themysterious 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 andelectric 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 perunit 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 particlewe deal with is the electron. The value of its charge is

Qe = (-)1.602 x 10-19C.

Get used to this very small value.

Example CalculationIn a CRT, a stream of electrons passes between 2electrically charge plates. The electric field strength is400NC-1. What is the force acting on each electron?

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

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

By definition,the direction ofthe field is theway a positivecharge wouldmove in the

field

Attraction

Repulsion

Electric Fields



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The field around and between point charges isirregular in direction, and varies in strength at everypoint.

The field between parallel charge plates, however, isuniform in strength and direction at every point(except at the edges). The direction of the field is theway a positive charge would move.

The strength of the field depends on the Voltageapplied to the plates, and the distance betweenthem:

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 CalculationTwo 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 FieldBetween Plates

Electric Field Between Parallel Charged Plates



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In the previous topic you learned that when an electriccurrent flows through a magnetic field, the wireexperiences a force... the “Motor Effect”.

Now you need to realise that the reason is that everyelectric charge, if moving through a magnetic field, willexperience a force.

You may have seen the following experiment with a CRTin the laboratory:CRT with fluorescentscreen. The beam ofcathode rays goes

straight across.

If a magnet is brought near, thebeam deflects.

A force is acting on the movingcharged particles.

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

SolutionF = 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) theforce is in the opposite

direction... back of hand side.

Check that the deflection in thephoto at left is correct.

S

Velocity vector, v

MagneticField B

Force, F

Force on a Moving Charge in a Magnetic FieldThe size of the force can be calculated as follows:

F = QvBsinθθ

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 movesat right angles to the field.

BMagneticField

θθ



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+ve

-ve

Cathode Rays

Fluorescent screen tomeasure deflection

EElleeccttrriicc FFiieelldd EEffffeecctt (charged plates)

Cathode Rays

E fielddown page

B into page

When the 2 forces cancel;

Force due to = Force due toElectric Field Magnetic Field

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

Charge to mass ratio = Q m

This established beyond doubt that cathode rays wereparticles, not waves.

Furthermore, he repeated the experiment with manydifferent cathode materials and always got the sameresult. This meant that the exact same cathode rayparticles were coming from every type of atom.

Other experimenters had already determined thecharge-mass ratio for the hydrogen atom (the smallestatom). It was apparent that the cathode ray particle wasmuch smaller than a hydrogen atom. The conclusionwas that all atoms must be made of smaller parts, oneof which was the “cathode ray particle”, soon re-namedthe “ELECTRON”.

This was a vital piece of knowledge for betterunderstanding of atoms and electricity, and thedevelopment of many new technologies.

Variable voltage

Discovery of the Electron... Thomson’s Experiment

MMaaggnneettiicc FFiieelldd EEffffeecctt (Adjustable Electromagnets)

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

In 1897, the confusion and debate about Cathode rayswas settled by one of the most famous, and criticallyimportant, experiments in the history of Science.

The British physicist Sir John Joseph Thomson setup an experiment in which cathode rays could bepassed through both an electric field, and through amagnetic field, at the same time.



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

Over the following 30 years, CRTtechnology evolved into the televisionscreen. By the middle of the 20th century,TV was developing to become the majorsystem for home entertainment and by the1980’s the same screens became the vitaldisplay units for computers.

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

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

The Electron Gunproduces the beam of cathode rays (electrons).

The electrons leave a cathode, and are acceleratedtowards a series of anodes by the high voltageelectric field between them, just like in the CRT’s ofCrookes or Thompson.

How a TV Screen WorksThe Deflection Platesare used to deflect the beam to create spotsof light at different points on the screen.

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

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

The Fluorescent Screenglows with light when the electron beam

strikes the fluorescent chemical coated on the insideof the glass.

The total image is built from many thousands of light-spots (“pixels” = picture elements). The illusion ofmovement is achieved by replacing each full-screenpicture many times per second.

To produce colour TV there are actually 3 electronguns, and 3 sets of deflection plates. Three separatebeams are steered onto separate spots of fluorescentchemicals which glow red, green or blue (RGB). Thefinal colour is a combination of these 3 colourscombined.





CRTs, Electrons & TVs Student Name .................................

1. The effect of a magnetic field on a moving, charged particle can be describedmathematically by the equation F = QvB sinθθ. State what is meant by each ofthese 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 differentmetals 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 HertzBy the 1880’s, the theory of electromagnetic radiation(EMR) had been around for 20 years, but no-one hadfound proof that these waves existed. Until, that is,the famous experiment of Heinrich Hertz in 1887.

Using the familiar “induction coil” to produce sparksacross a gap, Hertz showed that some invisible waveswere being produced...

Hertz had discovered radio waves.

2. FROM RADIO to PHOTOCELLS: QUANTUM THEORY

High-vvoltageInduction coil

Wire loop acts as a receivingantenna. The radio waves inducecurrents in the wire, and sparks

in the gap.

Sparks produced in smallgap in receiving loop

HOW DID HERTZ MEASURE SPEED OF THERADIO WAVES?

He reflected the radio waves (from metal sheets) sothat they set up interference patterns. By moving

his “receiving loop” around the lab. he couldmeasure exactly where the peaks of interferenceoccurred (where the waves added in amplitude).

From this, the wavelengths of the waves were calculated.

The frequency could be determined from thesettings of his wave transmitter.

Then the wave equation was used: V = λλ.fHe found the radio waves travelled at the

speed of light.

ssppaarrkkggaapp

Radio wavesemitted from spark

This was powerful evidence supporting the theory thatlight was just one of a whole spectrum ofElectromagnetic waves that had been predicted earlier.

In recognition of Hertz’s contribution to our knowledgeof waves, the unit of wave frequency (Hz) is named inhis honour.

Within another 20 years, radio was being used forlong-distance communications using morse code.Within 100 years the world was blanketed with radiotransmissions for communication and entertainment.

Hertz went on to experiment with these invisiblewaves and showed that they could be reflected,refracted, polarised and diffracted just like lightwaves. The clincher was when he measured theirvelocity and got an answer of 3x108ms-1...the waves were travelling at the speed of light!


Slide 15


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Investigating Radio WavesYou may have done some simple studies in thelaboratory, such as:

By adding a “tapping key” switch to the transmittercircuit, it is easy to send messages to the receiver inthe form of “dots-and-dashes” of static noise.

What Hertz Failed to InvestigateIn one of his many experiments with the new waveshe had discovered, Hertz found that his “receivingloop” became more sensitive and sparked more if itwas exposed to other radiations coming from histransmitter.

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 hadproduced the “Photoelectric Effect”:

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

This Photoelectric Effect was exploited in the 20thcentury to develop the technology of photocells andsolar cells.

Wire of receiving loop. Spark gap

Ultra-vviolet rays give their This can eject an energy to electrons on the electron from the surfacemetal surface. so sparks are more likely.

SolarCells

Induction coil& Power Pack

Array of wire connected to inductioncoil acts as a transmitting antenna

Radio receiver picks up loudbursts of noise, from some

distance away

The induction coil’s high-vvoltagesparking produces all sorts of

EMR, including radio, light, UV &even X-rrays.



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In a previous Preliminary topic(“Cosmic Engine”) you learned aboutthe way that energy is radiated from

hot objects. A “perfect” emitter ofradiation 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 thewavelength of the peak of the radiationbecame shorter, and frequency became

higher.

The problem was that the standardPhysics theories of the time could not

explain the shape of these graphs, whichwere obtained from experiment.

shorter longerWavelength of Radiation

very hotobject

hot object“peak”wavelength

“peak”wavelength

shorter

Amou

nt o

f Ene

rgy

Radi

ated

HOT BODY RADIATION

CURVES

warmobject

“peak” wavelengthlonger

Black Body Radiation

The explanation for the “Black-BodyRadiation” required a totally new idea.



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Problems with Classical PhysicsAt the same time that Plank was proposing his Quantum Theory to explainthe Black Body radiation details, the “Photoelectric Effect” (that Hertz hadobserved but failed to study) was being investigated by others.Experiments on the photoelectric effect were producing results that couldNOT be explained by the existing theory of light. For a century or more, light had been accepted as a wave. This explained itsreflection, refraction, interference, and many other phenomena. However, the photoelectric effect experiments were giving resultsthat suggested light was best explained as a stream of particles... this could turn Science on its ear! Enter Albert Einstein...

E = h.fE = 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) = 3.00x108ms-1.λλ = wavelength, in metres (m).f = frequency, in hertz (Hz)

Example CalculationA ray of red light has a wavelength of 6.50x10-7m.

a) What is its frequency?b) How much energy is carried by one quantum of this light?

Solutiona) 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) someelectrons 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 toexplain the black body radiation. He found that the onlyway to explain the exact details coming from theexperiments, was that the energy was quantised: emittedor absorbed in “little packets” called “quanta”.(singular “quantum”)

The existing theories of “classical” Physics assumedthat the amount of energy carried by a light wavecould have any value, on a continuous scale. Plank’stheory was that the energy could only take certainvalues, 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 canhave 2 atoms, 3 atoms and so on, BUT you cannothave 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 ofenergy carried by a “quantum” of light is related tothe frequency of the light.

Plank’s Quantum Theory

b) E = h.f= 6.63x10-34 x 4.62x1014

= 3.06x10-19 J.



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It was Albert Einstein who came to the rescue andneatly combined Plank’s Quantum Theory with theclassical wave theory of light, in a way that solved allthe apparent conflicts, and explained thePhotoelectric 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 givenby E = h.f, according to Plank’s Quantum Theory.

• When a photon interacts with matter, it can eithertransfer all its energy, or none of it... it cannot transfer part of its quantised energy.

Light is NOTa stream of particles

Light is NOTa 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 TheoryEinstein’s model for light involves a “duality”... lightmust have a dual nature. Many of its properties arewave related; e.g. ability to reflect, refract and showinterference patterns. In other cases, especially whenenergy transfers are occurring, the light photons arelike little particles.

This explained the Black Body Radiation curves, andthe weird features of the Photoelectric Effect.

Confirmation of Einstein’s ModelEinstein’s idea is very neat, but is it correct?

Einstein was able to make certain mathematicalpredictions regarding further features of thePhotoelectric Effect. (The exact details arecomplicated, and not required learning.)

In 1916, the experiments were done to test Einstein’spredictions, and the results agreed with his predictionsprecisely!

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 in1921, for his contribution to understanding thePhotoelectric Effect.



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Solar CellsSolar Cells (or “photovoltaic cells”) are devices which produceelectricity directly from light energy. They are very familiar in thepopular garden lights which need no wiring or battery replacements.

During the day, the solar cell(s) charge up a small re-chargablebattery. At night, the battery provides electricity to a low-powergarden lamp.

More importantly, solar cells hold the promise of cheap, efficient,environmentally-friendly electricity production. Solar-poweredhomes are becoming more and more common as the technologybecomes more affordable and more people are concerned by theenvironmental 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 electronsin a sandwich of semiconductor material, called a “p-n junction”. The energygained by electrons causes them to be emitted so that they travel throughthe semiconductor structure and create a potential difference across it. Thisvoltage causes a current to flow in the electrical circuit.

Small array of solar cells poweringa small electric motor and fan

Applications of the Photoelectric Effect

PhotocellsA 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 energycauses emission of an electron, which is collected on a nearby anode. A sensitive electric circuit is able to measurethe level of electron emission, and this gives a measure of the amount of light being received.





Quantum Theory & Photoelectric EffectStudent 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 syllabusrequires you to assess Einstein’s contribution to the Quantum Theory inrelation to Black Body Radiation.

To begin with, you might note that Einstein did NOT think up the QuantumTheory... Max Plank did that in 1900. However, it seems that Plank inventedthe quantum idea purely as a mathematical “trick” to explain the BlackBody Radiation curves. Plank never proposed that the quanta might givelight 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 combinedPlank’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 ofmodern 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


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keep it simple science Is Science Research Removed from Social & Political Forces?In the 1930’s Einstein was forced to flee Nazi Germanybecause he was of Jewish descent. In America, he warnedthe President about the possible development of an atomicbomb by the Nazis. This caused the Americans to begin theresearch which led to the first atomic bomb, developeddirectly from Einstein’s theories. He was not involved in theresearch, but was appalled when the atomic bomb was usedagainst Japan in 1945.

Einstein believed that Science is a process that should workfor peace and the good of all people, and not be involved inthe political & social forces that come and go.

Who was right? There is no correct, nor simple, answer tothat. You must form your own opinion... just be sure youhave an informed opinion.

In World Wars I & II, Science and scientistsplayed a major role in research and developmentof new weapons and war technologies. Someexamples 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 themorality of weapons research, and the two greatscientists of this section of the topic epitomisethese different views.

Max Plank was a patriotic German who believedthat it was his duty to help his country fight awar. He gladly contributed to weapons researchin WW I, and leading up to WW II he was thedirector of the main Scientific Institute in NaziGermany. Plank’s outlook seems to have beenthat Science is part of the political & socialstructure, and must take an active role in it.

Albert Einstein was German-born, but became aSwiss citizen, and later American. In WW I he(and only 3 others) signed an anti-wardeclaration. He spent the war in neutralSwitzerland, lobbying for peace and an end towar.

Atom-bbomb damageHiroshima, Japan



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Revision of Atomic StructureAfter Thomson identified the electron as a particlepresent in all atoms, it didn’t take long for scientists tofigure out the details of atomic structure. You arereminded of the basic model of a typical atom:

3. FROM ATOMS to COMPUTERS: SEMICONDUCTORS

CChheemmiiccaallBBoonnddss

Migratingelectron

In aconductor,electrons

can “jump”from oneatom tothe next

Electrons in orbit at different“Energy Levels”

Electrons are quite easy toremove from some atoms...

this leads to electricalconductivity, the Photoelectric

Effect, etcElectrical ConductivityWhen millions and billions of atoms form alattice structure (most strong solids are likethis) they do so by forming chemical bonds witheach other in a regular array.

Structureof an ATOM

Atomic Nucleusof protons & neutrons

In a metal atom, the outer (“valence”) electrons are very looselyheld by the atomic nucleus. They “feel” the force of attractionfrom other, surrounding atoms just as strongly as the attractionfrom their “own” atom. The result is that these outer electrons caneasily 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... anelectric current is flowing, and we say the metal is a goodConductor.

In other solids such as plastic or glass, the outer valenceelectrons are more strongly attracted to their own atom, andcannot easily escape from it, to move from atom to atom. We saythese things are poor conductors, or good Insulators.

ATOMS in a SOLID ARRAYElectrical Conduction occurs when electrons can

“migrate” freely from one atom to the next



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The explanation given in the previous slide forconductors and insulators is OK, until you find outabout “Semiconductors”. Elements such as Siliconand Germanium have a number of “strange”properties including being rather poor conductors ofelectricity until given a little jolt of energy. Then,suddenly they become quite good conductors.

To understand semiconductivity, you need to learnabout Band StructuresWe have known since the early 20th century that theelectrons around an atom can occupy different“orbits” or energy levels surrounding the nucleus.These energy levels are “quantised” (QuantumTheory applies) so there may be “forbidden energyzones” between them. An electron cannot exist inthis “fobidden zone” because the energy level theredoes NOT correspond to a whole quantum.

This ability, called “Semiconductivity”, allowsthese 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 thedifferent bands as they gain or lose energy. To jumpup over a “forbidden zone” they must have enoughenergy to achieve the quantum energy level requiredto occupy the next band.

In any atom in its “rest state”, the highest bandoccupied by electrons is the “Valence Band”. If anelectron has enough energy to get to the unoccupiedlevels above there, the electron is effectively free to“wander off”. If an electric field is applied, theelectron becomes part of a flowing current, and thesubstance is conducting electricity.

That’s why any energy band above the valence bandis called a “Conduction Band”.

Band Structure TheoryThe unoccupied band

above the valence band, is called the

“conduction band”.

The highest energy levelthat has electrons in it, iscalled the “valence band”.

“Forbiddenenergy gap”.

Electronscannot exist

here.

Electrons inquantised

“energy bands”.

Some bandsoverlap each

other.



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In terms of “Band Theory”, the difference inconductivity between different substances is simplythe relationship between the Valence Band and theConduction Band.

In metals, electrons can move into the conduction band atany time, so the solid array of atoms is a good conductorat all times.

In an insulator, such as plastic, the electrons can neverachieve the conduction band unless they are given ahuge boost of energy. At normal temperatures andvoltage levels, the substance will not carry a current.

Conduction Band

These bandsoverlap

Valence Band

Valence Band

Valence Band

ForbiddenEnergy gap Atoms of Semiconductor substance

e.g. Silicon, normally have 4 valence electronsEach

chemicalbond is

formed byatoms

sharing 2electrons.

Theseelectronsare in the valenceenergyband.

Atomwith 5valence

electronsused to“Dope”

thelattice.

extravalenceelectron

DOPING increases the conductivity of the lattice.

Conductors, Insulators & SemiconductorsA semiconductor, like Silicon, will not normally carrycurrent, because electrons lack the energy to jump the“forbidden energy gap”. However, if the temperature isincreased, and a voltage applied, there comes a pointwhen electrons jump the gap in great numbers, and thesubstance suddenly conducts very well indeed.

This effect does not occur at room temperature unlessthe semiconductor substance is “Doped”.

Doping a Semiconductor“Doping” means to add a very small quantity of adifferent type of atom to an otherwise pure solidlattice of semiconductor atoms.

Conduction Band

Conduction Band

In Conductorsthese bandsoverlap each

other.

In Insulators thesebands are separatedby a wide “forbidden

energy gap”.

In Semiconductorsthere is only a

narrow gapbetween bands.



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Normally we imagine that an electric current is composedof a flow of negative electrons. However, in asemiconductor, when an electron jumps out of the valenceband and flows off somewhere, it leaves behind a “hole”in the valence band. This hole, is a space that an electronfrom elsewhere can jump into.

Imagine a line of atoms in a semiconductor lattice:

Now imagine a sequence of movements in which the nextelectron in the valence band has enough energy to jumpinto 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 nodifference whether the current really is negativeelectrons going one way, or “holes” going the otherway... either way, it constitutes an electric current.The holes are considered as positively chargedspaces (relative to the electrons) and so the flow ofpositive 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 ofatoms with an “extra” valence electron. The other wayto do it is to use atoms with only 3 valence electrons,creating extra “holes” in the lattice.

If you can imagine this sequence like thepictures making a motion cartoon, you canimagine that an electron flows to the right

and the hole flows to the left.

Atomwith only3 valenceelectronsused to“Dope”

thelattice.

extra holein thelattice


Slide 27


n-Type Semiconductors are dopedwith atoms with 5 valence electrons, such as arsenicor antimony. This adds extra valence electrons to thelattice. Electrical current is carried mainly by this flowof negative charges (hence “n”-type).

Thermionic Valves: Cathode Ray Tubes“Thermionic” refers to the way these CRT’s would emitmany electrons from the cathode (and thereby carry a

current) when the cathode became hot. Once “warmed up” thevalve can act as an electronic “switch” in a circuit, when the

voltage to the anode is varied.

CharacteristicsRelatively large & expensive.

Consume relatively large amounts of electricity

Produce large amounts of “waste” heat.

Although faster than mechanicalswitches, 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, andmaintenance needs are high.

10-22

0 cm

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

A Little History: Electronics & ComputersThe concept of a machine to carry out high speedcalculations and “logical” operations has been aroundfor centuries. Prior to the 20th century, any suchdevice had to be mechanical, using “clockwork” gearsand so on. There were some notable successes withcontrol devices for weaving looms, and mechanical“adding machines”, but applications were very limited.

During World War II the first electronic computers werebuilt (in tight secrecy) to help decode enemy radiomessages. Instead of gears and dials, the “Collosus”computer used thermionic valves to electronicallyswitch circuits on and off, to store and manipulatedata. These valves are described at the right.

p-Type & n-Type SemiconductorsThe two different ways to “dope” the lattice result in two different types of semiconductor material:

p-Type Semiconductors are dopedwith atoms with 3 valence electrons, such asaluminium or gallium. This adds extra “holes” to thelattice. Electrical current is carried mainly by thisflow of positive holes (hence “p”-type).

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Thermionic valves had been widely used in radios forsome years and were vital components of the newindustry of television.

Valves were also important in the switching ofconnections in telephone exchanges, where thegrowing communication demands required automaticdialing and connection technology. (The originalsystem involved human “operators” manuallyplugging wires into sockets to connect phone calls.)

However, the valve-based technology was provingtoo slow, too unreliable and too expensive for thebooming telephone industry. The major U.S. phonecompany “Bell Telephone” set its scientists the taskof researching new materials and processes toreplace the valves.

In 1947, 3 scientists at Bell Laboratories, invented thetransistor, using a “sandwich” of p-type and n-typedoped semiconductor material.

2 cm

The comparison is a “no-brainer”...

The transistor replaced Thermionic Valvesas 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 thesemiconductor (conductivity that can be

switched on and off) transistors can do thesame 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”.

• is highly reliable, and rarely needs maintenance.



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To make semiconductor material with the desiredconductivity properties, it is necessary to firstly prepareextremely pure samples, then add tiny amounts of the“doping” chemical, and finally grow crystals of thesemiconductor from the molten material in a furnace.

The original transistors were made from Germaniumbecause the technology to produce crystals of the pureelement was already known. However, Germanium is a rareelement, whereas its close “sister element” Silicon, is oneof the most abundant elements on Earth.

By the 1960’s, the technology to obtain pure crystals ofSilicon had been developed, and because Silicon is soabundant and therefore cheaper, it quickly replacedGermanium. Silicon’s electrical properties turned out to bebetter too. For example, it held its semiconductiveproperties constant over a wider range of temperatures.

Also in the 1960’s, the technology of the computer began toemerge for financial and communication uses. The “solid-state” transistor technology allowed a computer to be builtto fit a table-top, rather than fill a room. Every teenager hada brick-size “transistor radio”, in the same way that in thisdecade everyone has a mobile phone the size of amatchbox.

A Little More History... Silicon v GermaniumThe miniature “integrated circuitboard” led tothe technology of the “silicon chip” where thousands,and now millions of

transistor-equivalents can be printedmicroscopically in the space of a postagestamp... a “microchip”.

Twenty years later, these notes are beingcomposed with an even cheaper PC which canprocess 2x109 bytes, (2GB). The computershave become a million times more powerful!

In the 1980’s thefirst cheap PC’s

(personalcomputers) could

process amagnificent 2x103

“bytes” ofinformation.

Computer “motherboard”





Semiconductors Student Name .................................

1. In terms of “Band Theory”, how are conductors, insulators andsemiconductors 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 siliconcrystal to make an n-type semiconductor?

c) What type of atoms (and give specific example) are used to dope a siliconcrystal to make a p-type semiconductor?

4. Name the type of CRT used in the first electronic computers and name thefirst semiconductor devices which replaced them.

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Slide 31

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Site Licence Conditions only

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It could be argued that the invention of thetransistor was one of the most profoundtechnological developments in history. Itranks right up there beside the developmentssuch as:

Fire: 500,000 years ago.Fire transformed human society because ofits power to warm people, cook food andprotect from predators.

Agriculture: 10,000 years ago.This transformed society from nomadichunting-gathering to settled communitiesthat invented law, commerce, governmentand “civilization”.

Metallurgy & the Industrial Revolution,

which led to new tools, machinery, massproduction, urbanisation, and mass transportsystems.

Assessment of Impacts of the Transistor on SocietyThe transistor helped create the “Information &

Communication Revolution”, which is still developing today. Electronic circuits, usingmicrochips, are the basis of all the computers which allow:

• instant access to (virtually) all the information on the planetvia 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 massconsumerism which keeps thing cheap through massproduction & distribution.

Computers keep trackof the billions of

business transactionsthat feed us, clothe us,entertain us, transportus and service all our

needs.Like it or hate it, (some people think weshould have stayed in the trees) the

modern world could not exist without theinvention of the transistor!



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Investigating Crystal Structures... Bragg and Son

The regular shapes of crystals (such as salt) had longbeen assumed to be due to a regular arrangement of theatoms or ions in a lattice-like structure. However, until theearly 20th century, there was no way to prove or confirmthis idea.

The discovery of high frequency EMR in the form of X-rays opened up a new line of investigation. Sir WilliamBragg and his son Lawrence, beamed X-rays throughcrystals and studied the diffraction patterns which wereformed as the crystal lattice scattered the X-rays.

4. FROM CRYSTALS TO SUPERCONDUCTORS

Crystalx-rraybeam

X-rrays diffracted by the crystallattice & form Interference

patterns which are capturedon the film.

Photographic filmsensitive to x-rrays

The Braggs were able to analyse the interferencepattern in order to deduce the arrangement of theatoms within the crystal. For this, they were jointlyawarded the Nobel Prize for Physics in 1915.

This opened up a whole new investigative technique,allowing scientists to probe the structure of matter asnever before. It was X-ray diffraction crystallography,for example, that allowed the structure of DNA to bedetermined in the 1950’s.

Crystal StructuresThanks to scientists like the Braggs, we nowunderstand the atomic-level structure of mostsubstances. You learned previously how a substancelike the semiconductor Silicon is a lattice of atomschemically bonded together:

Eachchemicalbond is

formed byatoms

sharing 2electronswith eachneighbour

atom.



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Unlike silicon, salt and other crystals, metal atoms are notchemically bonded to each other by the sharing or exchanging ofelectrons.

You will remember that the outer “valence” electrons in metals areweakly held, and can access the “conduction band” at any time.The result is that the valence electrons on each atom are NOTconfined to that atom, but freely wander around from atom to atom.

Each metal atom is, therefore, ionised because its valenceelectron(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 ispresent, the electronswill all flow in the samedirection as an electriccurrent. That’s whymetals are all goodconductors. Superconductivity!

Crystal Structure of MetalsResistance in Metals

So why is there resistance in a metalwire? Although the electrons can flow

quite easily, their movement is nottotally free.

Any impurities in the metal distort theshape of the lattice and impede the

electron flow. Also, as the ions vibratedue to thermal energy, the vibration

causes more collisions amongelectrons, so their flow is resisted. Astemperature increases, the vibrations

increase too, and that’s why resistancein metals increases with temperature.

Logically, if you re-read the previousparagraph and think backwards, you

might infer that if you had a really puremetal, and cooled it right down so that

all lattice vibrations stopped, then itwould become a perfect conductor.



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In 1911, a Dutch physicist managed to cool mercury down toabout 4oK (-269oC) and found that its electrical resistancedropped to zero.

Over the following years, various other metals were found tobecome superconducting at very low temperatures. The potentialto build electrical generators and equipment with zero resistancewas a very attractive idea, but the temperatures involved (nohigher than about 20oK) were so low that there seemed nopractical way to take advantage.

Then in 1986, Swiss scientists discovered some ceramicmaterials containing rare elements like Yttrium and Lanthanum,which became superconductors at much higher temperatures.Still cold by human standards, but 100o higher than the metalsuperconductors, these ceramics had zero resistance attemperatures as high as 130oK (around -150oC). This is atemperature that is much more practical to achieve.

The syllabus requires that you identify some of thesuperconducting metals and compounds. Here is a very short list...

TemperatureSuperconductor of Transition (oK)Metals to SuperconductivityMercury 4Lead 9AlloyNiobium-Germanium 23CeramicsYttrium-Barium-Copper oxide 92Thallium-Barium-Calcium-Copper oxide 125 (-148oC)

The Meissner EffectYou may have seen a practical

demonstration of a superconductor inaction, in class. The “Meissner Effect” is

named after the scientist who discovered it.

If a disk of superconductor ceramic ischilled below its “transition temperature”,a small magnet placed close above it will“levitate”; spinning freely if prodded, butheld up against gravity by unseen forces.

ExplanationAs the magnet is brought near, its

magnetic field induces currents in theceramic. 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 anexternal magnetic field to penetrate.

dish

LiquidNitrogen

Disk of Superconducting

Ceramic

Small Levitatingmagnet

Superconductivity in Metals and Ceramics



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How do we explain the phenomenon ofsuperconductivity?

The accepted explanation is known as “BCS Theory”,where “BCS” are the initials of the 3 scientists whodeveloped the theory in the 1950’s.

Imagine part of the solid lattice of positive ions in aconducting metal or ceramic. As an electron (part of anelectric current) approaches, it attracts the positiveions and distorts the crystal structure slightly:

This distortion concentrates the positive charge in thispart of the lattice, and attracts other electrons.

In a normal conductor, this distortion leads tocollisions and loss of energy by the flowing electronswhich repel each other... this is the normal electricalresistance within the conductor.

Approachingelectron

How Superconductivity Occurs... BCS TheoryBut in a superconductor below its “transitiontemperature”, something very strange occurs; due toQuantum 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 scopeof this Course... KISS Principle) each electron of theCooper Pair helps the other to pass through the latticewithout any loss of energy. This means there is ZEROresistance.

However, at a temperature above the “transition”, thethermal vibrations in the lattice keep breaking up theCooper Pairs as fast as they can form. This destroysthe superconductivity, and the normal electricalresistance of the substance returns.

Cooper-PPairof electrons forms

Electrons in aCooper-Pairare linked to

each other by“QuantumEffects”.


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AdvantagesSuperconductor technology offers

• High efficiency in any electrical situation, becausethere is no energy loss due to resistance.

• The ability to generate extremely strong magneticfields from superconducting electromagnets.

• Faster operation of computers, since superconductingswitching devices could be

10 times faster than a semiconductor transistor.

Limitations• Superconducting metals must be chilled with liquid

helium. This is impractical and expensive.

• New, superconducting ceramics can be chilled withliquid nitrogen, which is cheaper and much more

practical, BUT these ceramics:• are fragile, brittle and difficult to make into wires.

• can be chemically unstable and have a limited life span.

Possible Future ApplicationsCurrent computer technology is based on

semiconductor microchips. Although these becomefaster and more powerful every year, there is a limitto how far they can go. A superconductor computer

could open a whole new level of enhancedperformance due the possible high speed switching

of circuits.

Electricity generation & distributioncould be made much more efficient with

superconductor technology.

A lot of energy is lost due to resistance heating intransmission lines. This could be eliminated if

power lines were superconductors.

Generators lose energy by resistance heating in thecoils needed to produce magnetic fields, and are

limited in the strength of the fields they canproduce. Superconducting coils would allow

generators to be much more powerful and efficient.

Greater efficiency generally in electrical technologywould reduce associated environmental problems,

such as Greenhouse gas emissions.

Using Superconductor Technology



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The Maglev TrainThe idea of using superconducting electromagnets to“levitate” a train above a magnetic guide-rail hasbeen around for many years and experiments havebeen going on for decades.The guiderail(s) under the train contain conventionalelectromagnets. On board, helium-chilled super-conducting electromagnets produce powerfulmagnetic fields.

The fields in the rail and the train repel each other sothat the entire train is levitated 1-2cm above the track.

Propulsion and braking is also done magnetically, by thefields in front and behind the train attracting and repellingit. The actual motive power is supplied from the rail, notfrom onboard the train.

The big advantage is the high speed possible withoutany rail friction, and the low maintenance and lownoise that goes with this. A disadvantage is the veryhigh cost of building the guide rail track.

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

MAGLEV = MAGnetic LEVitation

ShanghaiMaglevTrain

Using Superconductor Technology cont.

Although the practical, everyday uses ofsuperconductors are very limited so far, Science hasbeen using superconductors for decades.

The major use is to generate hugely powerfulmagnetic fields to accelerate particles for research.

Using superconducting electromagnets,chilled with liquid helium to near -270oC,

powerful magnetic fields can be generated. These areused to accelerate particles to close to the speed oflight, then collide them together to study the structureof matter. This research is aimed at understanding notonly matter itself, but the origins of the Universe.

Scientific Research Uses Superconductors





Superconductivity Student Name .................................1. What technique was used by the father and son team of Braggs to study thestructure of crystals?

2. Explain why metals are generally excellent conductors of electricity.

3.a) Why is there some electrical resistance in a metal at normal temperatures?

b) Why does resistance increase with temperature?

4. What is the “Meissner Effect” and why does it occur?

5. What does “BCS Theory” attempt to explain. Outline the main principle.

6. What is “Maglev” short for?

7. What are some limitations of the high-temp. superconducting ceramics?

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hsc physics topic 3 from ideas to implementation

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