kiss notes ideas to implementation

copyright 2005-2006 keep it simple scienceHSC Physics Topic 3

Coffs Harbour High School SL#703335

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but first, an introduction...

HSC Physics Topic 3

FROM IDEAS to IMPLEMENTATIONWhat 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 THEORY3. 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 thatchanged our understanding of the Universe...

Newtons Laws of Motion, and Gravitation, and Einsteins 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 theHistory of Physics, from about 1850 into the 20th century.

However, it is not just history. Along the way, you will bestudying some concepts, theories and facts that are vital toyour overall understanding of this subject.

In addition, as you learn both the history and some of thefoundation ideas of modern Physics, you will see that muchof our modern technology is a direct result thesediscoveries...

When Cathode Rayswere being studied

between 1850-1900,people said

interesting, but whatsthe use of it??

Little did they know...

...the study ofCathode Rays leddirectly to theinvention of theTV set, so familiartoday.

About the Same Timeas Cathode Rays were becoming understood, otherscientists were studying electromagnetic radiation andobscure phenomena such as the Photoelectric Effect.

and Meanwhile,the unravelling of atomicstructure and study ofelectrical conductivity inweird substances likeGermanium and Silicon,led to the discovery ofsemiconductors.The invention of thetransistor followed... thebasis of all modernelectronics and computer systems.

No-one could haveguessed that this ledto, not only the radioand mobile phone, butto solar cells...

Solar cells being used tomake electricity on aremote outback property

and the Study of Crystal Structureled 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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Cathode Rays.Discovery &Properties

F = QE&

E = V d

F = QvBsin

Behaviour of aCharged Particle in

a Magnetic FieldRevision of

Electric Fields

Conductivity inMetals.

Superconductivity

Failure to follow-uup

E = hfand

c = f

Current & PotentialApplications of

Superconductivity

The TV screen.Main parts and

their roleRevision ofBlack BodyRadiation

Particle-WWaveDualityof Light

PlanksQuantumTheory

Discovery of theElectron...

ThompsonsExperiment

Confirmation of EMR.Measurement of c

Differing viewson

Sciences placein society

EinsteinsContribution

Electrons & Holesin Conductivity

Doping.n-ttype & p-ttypeSemiconductors

Valves to Transistors toMicroprocessors...Impacts on Society

The Braggs &X-rray

Crystalography

Revision ofAtomic Structure

&Structures of Solid

Lattices

PhotoelectricEffect

&Applications: solar cells photocells

Band Theory ofConductors,Insulators &

Semiconductors

Hertzs Discovery of Radio Waves

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IIMMPPLLEEMMEENNTTAATTIIOONN

From CATHODE RAYSto TELEVISION From RADIO

to PHOTOCELLS:Quantum Theory

From ATOMSto COMPUTERS

From CRYSTALSto

SUPERCONDUCTORS

CONCEPT DIAGRAM (Mind Map) OF TOPICSome students find that memorizing the OUTLINE of a topic helps them learn and remember the concepts andimportant facts. As you proceed through the topic, come back to this page regularly to see how each bit fits thewhole. At the end of the notes you will find a blank version of this Mind Map to practise on.

Discovery of Cathode RaysBy the 1850s, scientists had developed the technology toproduce quite high voltages of electricity and to makesealed glass tubes from which most of the air had beenremoved using a vacuum pump.

It wasnt long before these 2 things were combined, andsome mysterious phenomena were discovered.

You may have done some laboratory investigations withDischarge Tubes as follows...

It was soon established that whatever was causing theseglows or discharges in the tubes was coming from thenegative electrode, or cathode...so these emissions werecalled Cathode Rays.

Over the following 20 years these mysterious rays werestudied by many scientists, most notably Sir WilliamCrookes. He devised so many clever variations on theseCathode Ray Tubes (CRTs) that they were known asCrookes Tubes.

You will have seen, in the school laboratory, a number ofdifferent CRTs and repeated many of Crookess famousexperiments...

Experiments with CRTs

Maltese Cross Tube

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

Furthermore, Crookes tried this experiment with manydifferent metals as his electrodes. The type of metal madeno difference... Cathode Rays are identical, regardless ofthe materials used.

Tube With a Fluorescent Screen

Tube With a Rotating Paddle-Wheel

This evidence from these various experiments was veryinconsistent... some of the features of cathode rayssuggested they are particles, other results suggested theyare waves.

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Copyright 2005-2006 keep it simple scienceHSC Physics Topic 3

1. FROM CATHODE RAYS TO TELEVISION

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

This tube isglowing andshowing light

and darkbands, or

striations

The result is that each tube shows glowingstreamers, or light and dark bands, or glows atthe end(s).

The patterns change at different gas pressures.

At the very lowest pressure, there is no glowfrom gas in the tube, but the glass itself glows atone end of the tube.

CATHODE ( -vve)

ANODE (+ve)in the shapeof a Maltese

CrossShadow of thecross in the

glow at the endof the tube

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WWhheeeell ssppiinnss wwhheenn ccaatthhooddee rraayyss ssttrriikkee tthhee ppaaddddlleess

TThhiiss sshhoowwss tthhaatt tthhee rraayysshhaavvee mmoommeennttuumm,, aannddtthheerreeffoorree hhaavvee mmaassss

Fluorescence was knownto be caused by certainwaves, 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, itfollows that the particles must be negatively charged,because the beam is deflected by repulsion from thenegative plate, and attraction towards the positive.

Early Confusion About Cathode RaysUnfortunately, when the early experimenters triedsomething similar to the above, they did NOT detect adeflection of the beam. So, they concluded there was NOcharge associated, and were confused about the nature ofthe Cathode Rays.

Evidence that CRs 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 CRs were ParticlesCathode 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 involvedthe Cathode Ray Tube...

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

Revision of Electric FieldsIn 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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BBeeaamm ooffccaatthhooddee rraayyss oonnssccrreeeenn

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-ve +ve

This debate was finally settled by a famousexperiment you will study soon...

In 1897, J.J. Thomson showed that cathoderays 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 unitof charge experienced by a charge in the field...

E = F Q

However, in this topic we are more interested incalculating 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 Calculation:In the CRT shown at top left of this page, a stream ofelectrons passes between 2 electrically charge plates. Theelectric field strength is 400NC-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 will be in the opposite direction to the electric field.

+ -

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

+ -+ +

Attraction

Repulsion



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Electric Field Between Parallel Charged Plates

The field around and between point charges is irregular indirection, and varies in strength at every point. The fieldbetween parallel charge plates, however, is uniform instrength and direction at every point (except at the edges).The direction of the field is the way a positive charge wouldmove.

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 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 CRT inthe laboratory:

The strength of the field depends on the Voltage appliedto the plates, and the distance between them:

E = V d

E = Electric Field strength, in NC-1.V = Voltage applied to the plates, in volts (V).d = distance between the plates, in metres (m).

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

Solution: E = V / d= 12.0 / 0.0125= 960NC-1.

CRT with fluorescent screen.

Cathode Ray beam goesstraight across.

If a magnet isbrought near, thebeam deflects.

A force is actingon the movingcharged particles.

The size of the force can be calculated as follows:

F = QvBsin

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). = Angle between the velocity vector and magnetic

Field vector lines.

Since sin90o = 1,and sin0o = 0,

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

BMag.Field

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

Solution:F = QvBsin

= -1.602x10-19x2.50x106x0.0235xsin70o= -8.84 x 10-15N. (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 theopposite direction... back of hand side.Can you verify the upward deflection in thephoto above is consistent with theory?

Velocity vector, v

MagneticField B

Force, F

SS

Positively (+ve)charged plate +

-Negatively (-ve)charged plate Uniform FieldBetween Plates



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Discovery of the Electron...Thomsons Experiment

In 1897, the confusion and debate about Cathode rays wassettled 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 inwhich cathode rays could be passed through both anelectric field, and through a magnetic field, at the sametime.

How a TV Screen WorksThomson used a fluorescent screen at the end of his CRTto detect and measure the deflection of the cathode rays(electrons). Over the following 30 years, CRT technologyevolved into the television screen. By the middle of the20th century, TV was developing to become the majorsystem for home entertainment and by the 1980s the samescreens became the vital display units for computers.

A TV picture-tube is really just a more sophisticatedversion of Thomsons CRT. The image on the screen ismade up of thousands of spots of light, created as cathoderays strike a fluorescent 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 accelerated towardsa series of anodes by the high voltage electric field betweenthem, just like in the CRTs of Crookes or Thompson.

The Deflection Platesare used to deflect the beam to create spots of light atdifferent points on the screen.

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

Between them, the sets of plates can steer the beam ontoany point on the screen.

The Fluorescent Screenglows with light when the electron beam strikes thefluorescent 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 isachieved by replacing each full-screen picture many timesper second.

To produce colour TV there are actually 3 electron guns,and 3 sets of deflection plates. Three separate beams aresteered onto separate spots of fluorescent chemicals whichglow red, green or blue (RGB). The final colour is acombination of these 3 colours combined.

+ve

-ve

Cathode Rays

Fluorescent screen tomeasure deflection

Electric Field Effect (charged plates)

Magnetic Field Effect (Adjustable Electromagnets)

Cathode Rays

E down page

B into page

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

At this point, Force due to = Force due toElectric Field Magnetic Field

Since the strengths of the fields could be calculated fromthe currents and voltages applied to the plates andelectromagnets, Thomson was able to calculate the ratiobetween the charge and mass of the cathode rays.Charge to mass ratio = Q

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

Furthermore, he repeated the experiment with manydifferent cathode materials and always got the same result.This meant that the exact same cathode ray particles werecoming from every type of atom.

Other experimenters had already determined the charge-mass ratio for the hydrogen atom (the smallest atom). Itwas apparent that the cathode ray particle was much smallerthan a hydrogen atom. The conclusion was that all atomsmust be made of smaller parts, one of which was thecathode ray particle, soon re-named ELECTRON.

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

Variable voltage

Photo: Peter Hamza

Worksheet 1

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

The discovery of a).............................. Rays wasmade with simple b)................................... tubesfrom which most of the air was removed with ac)....................................... pump. When highd)................................ was applied to electrodes ateach end of the tube, it would produce a varietyof e)........................, .................................... and.............................. The exact pattern changed asthe f)..................................... in the tube wasaltered. It was discovered that the effects weredue to mysterious emissions coming from thecathode (or g)............................. electrode).

About the 1870s, Sir William h)............................and others, built special CRTs to study thecathode rays. The famous i)...................................cross tube showed that the rays travelled instraight lines. Tubes with j)....................................screens showed that the rays caused fluorescence,and tubes equipped with a paddle-wheelproved that the rays carried both k).........................energy and l).....................................................

Unfortunately, attempts to detect deflection byapplying an m)......................................... field wereunsuccessful, so for many years there wasconfusion over whether CRs weren)................................. or ............................................

Evidence they were waves: CRs travel in o)...................................... like light. They cause p).......................................... like UVrays. 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 aroundanything with electric charge. The direction ofthe field is defined as v).......................................................................................................Any charge within a field will experience aw)............................ The field between 2x)........................... .................................. plates isuniform in both y)..................................... and..........................................., and is determined bythe z)................................... applied to the platesand the aa).................................. between them.

Electric charges also experience a force if theyare ab)....................................... through aac)................................... field. This is easilyobserved by bringing a ad).............................. neara CRT with a fluorescent screen; the magnetcauses the beam to ae)...........................................The direction of the force and the deflection ofthe CR beam is easily determined by theaf)..................................................... Rule.

In 1897, J.J. ag)........................................... used thedeflection of a CR beam by bothah)..................................... and .................................fields to measure the ratio ofai)...................................................... of a cathoderay. This established, beyond doubt, that CRs areaj)............................. and are a small part containedwithin all ak)........................... Thomson haddiscovered the al).............................................. Thesimple CRT was later used as the basis to inventthe am).......................................... screen.

The main parts of the picture tube are: The an).................................. Gun, whichproduces a beam of ao)........................... from aap).................................... and accelerates themtowards a series of aq).................................. The ar)..................................... plates, which useelectric fields to as).................................... the beamonto the screen. The at)..................................... screen, whichforms the image when fluorescent chemicalsau)............................ with spots of light whenstruck by av)...........................................

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COMPLETED WORKSHEETSBECOME SECTION SUMMARIES

Part B Practice Problems

Field Between Charged Plates & Force on a Charge

1. Two parallel plates are 4.00cm apart in avacuum tube. A voltage of 50.0V is appliedacross the plates.An alpha particle with charge of (+)3.20x10-19Cpasses between the plates.a) What is the size of the electric field betweenthe plates?b) What force will act on the alpha particle?c) Describe the direction of the

i) fieldii) force

relative to the +ve and -ve plates.

2. An electron (Q=-1.60x10-19C) experiences aforce of -7.82x10-15N within an electric fieldcreated 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 electriccharge, experiences a force of 2.29x10-12N in afield produced by 2 plates 5.00cm apart. A 200Vpotential difference is applied across the plates.a) Find the strength of the field between theplates.b) What charge does the speck of dust carry?c) The static charge was created when someelectrons 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 movetoward the negative plate. Did the speck lose orgain electrons?

4. Two parallel plates have a 40.0V potentialdifference between them. An electron betweenthem experiences a force of (-)5.88x10-17N.How far apart are the plates?

5. In an inkjet printer, small droplets of ink aregiven an electric charge, then steered onto thepaper by accelerating them in electric fields toachieve the desired velocities and directions.

What force would be experienced by a dropletwith charge of (+)9.75x10-10C, which is betweenparallel plates with potential difference of 100V,and separated by 5.00mm?

Force on a Moving Charge in a Magnetic Field

6.An electron (Q=-1.60x10-19C) is travelling northat 3.00x107ms-1 in a cathode ray tube when itenters a magnetic field of strength 4.96x10-2T.The field is directed vertically upwards throughthe CRT.Find the magnitude and direction of the forceexperienced by the electron.

7.In a nuclear accelerator, a charged ion has beenaccelerated up to a velocity of 2.90x108ms-1. As itenters a magnetic field of strength 8.05T (field isperpendicular to ions velocity vector) itexperiences a force of magnitude 3.75x10-9N.What is the magnitude of the charge on the ion?

8.A particle of the solar wind with charge of(+)1.60x10-19C (it is in fact a proton) encountersthe Earths magnetic field at an angle of 25o tothe field lines. At this point the field has astrength of 5.48x10-4T. The proton experiences aforce of 7.40x10-15N.Find the velocity of the proton.

9.In an experiment similar to Thomsons, a streamof electrons in a CRT are each experiencing aforce of magnitude 4.06x10-15N when travellingthrough a perpendicular magnetic field at avelocity of 7.80x106ms-1.a) What is the strength of the magnetic field?The force on the electrons is exactly counteractedby an electric field produced by charged plateswhich are 8.00mm apart.b) What is the strength of the electric field?c) What is the voltage being applied across theplates?

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FULLY WORKED SOLUTIONSIN THE ANSWERS SECTION

Remember that for full marksin calculations, you need to show

FORMULA, NUMERICAL SUBSTITUTION,APPROPRIATE PRECISION and UNITS

The Radio Experiments of HertzBy the 1880s, the theory of electromagnetic radiation(EMR) had been around for 20 years, but no-one hadfound proof that these waves existed. Until, that is, thefamous experiment of Heinrich Hertz in 1887.

Using the familiar induction coil to produce sparksacross a gap, Hertz showed that some invisible waves werebeing produced... he had discovered radio waves.

Hertz went on to experiment with these invisible waves andshowed that they could be reflected, refracted, polarizedand diffracted just like light waves. The clincher was whenhe measured their velocity and got an answer of 3x108ms-1...the speed of light!

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

In recognition of Hertzs contribution to our knowledge ofwaves, the unit of wave frequency (Hz) is named in hishonour.

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

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 in theform of dots-and-dashes of static noise.

9



2. FROM RADIO to PHOTOCELLS: QUANTUM THEORY

High-voltageInduction coil

RRaaddiioo wwaavveessEEmmiitttteedd ffrroommssppaarrkk

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

Sparks produced insmall gap in receivingloop

How did Hertz measure the speed of the radio waves?

He reflected the radio waves (from metal sheets) sothat they set up interference patterns. By moving hisreceiving loop around the lab. he could measureexactly where the peaks of interference occurred(where the waves added in amplitude). From this, thewavelengths of the waves were calculated.

The frequency could be determined from the settingsof his wave transmitter.

Then the wave equation was used: V = .f

He found the radio waves travelled at the speed of light.

What Hertz Failed to InvestigateIn one of his many experiments with the new waves he haddiscovered, Hertz found that his receiving loop becamemore sensitive and sparked more if it was exposed to otherradiations coming from his transmitter.

He didnt realize the significance of this observation, andfailed 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 as proof ofthe new Quantum Theory... read on.

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

sparkgap

Wire of receiving loop. Spark gap

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

SolarCells

IInndduuccttiioonn ccooiill && PPoowweerr PPaacckk

Array of wire connected toinduction coil acts as atransmitting antenna

RRaaddiioo rreecceeiivveerr ppiicckkss uupplloouudd bbuurrssttss ooff nnooiissee,,ffrroomm ssoommee ddiissttaannccee aawwaayy

The induction coilshigh-voltagesparking producesall sorts of EMR,including radio,light, UV & even

X-rays



10

Black Body RadiationIn a previous Preliminary topic (Cosmic Engine) youlearned about the way that energy is radiated from hotobjects. A perfect emitter of radiation had becomeknown as a black-body...

It was well known that as a black body became hotter, itnot only emitted more energy as radiation, but that thewavelength of the peak of the radiation became shorter,and frequency became higher.

The problem was that the standard Physics theories of thetime could not explain the shape of these graphs, whichwere obtained from experiment.

Planks Quantum TheoryIn 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: emitted orabsorbed in little packets called quanta (singularquantum).

The existing theories of classical Physics assumed thatthe amount of energy carried (say) by a light wave couldhave any value, on a continuous scale. Planks theory wasthat the energy could only take certain values, based onunits or quanta of energy.

Its the same as with matter: The smallest amount of (say)carbon you can have is 1 atom. Then you can have 2 atoms,3 atoms and so on, BUT you cannot have 1/2 atoms ofcarbon... the matter is quantised, with whole atoms as theminimum quantum. Well, says Plank, energy is the same!

Planks Quantum Theory proposed that the amount ofenergy carried by a quantum of light is related to thefrequency of the light:

Problems with Classical PhysicsAt the same time that Plank was proposing his QuantumTheory to explain the Black Body radiation details, thePhotoelectric Effect (that Hertz had observed but failedto study) was being investigated by others.

Experiments on the photoelectric effect were producingresults that could NOT be explained by the existing theoryof light. For a century or more, light had been accepted asa wave. This explained its reflection, refraction,interference, and many other phenomena.

However, the photoelectric effect experiments were givingresults that suggested light was best explained as a streamof particles... this could turn Science on its ear!

Enter Albert Einstein...

E = h.f

E = energy of a quantum, in joules ( J)h = Planks constant, which has a value of 6.63x10-34f = frequency of the wave, in hertz (Hz)

You are reminded also, of the wave equation:

V = .f (or, for light) c = .f

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

Example Calculation:

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

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

Solution:a) c = .f

3.00x108 = 6.50x10-7x f f = 3.00x108/6.50x10-7

= 4.62x1014Hz.b) E = h.f

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

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) someelectrons 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 hotobject

hotobjectpeakwavelength

peak wavelengthlonger

peakwavelengthshorter

Amou

nt o

f Ene

rgy

Radi

ated

warmobject

BLACK BODYRADIATIONCURVES



11

Einstein and Quantum TheoryIt was Albert Einstein who came to the rescue and neatlycombined Planks Quantum Theory with the classical wavetheory of light, in a way that solved all the apparentconflicts, and explained the Photoelectric Effect as well!

To keep it as simple as possible, (K.I.S.S. Principle) Einsteinproposed that:

Light is a wave, but the energy of the wave is concentrated in little packetsor bundles of wave energy, now called Photons. Each photon of light has an amount of energy given by

E = h.f, according to Planks Quantum Theory. When a photon interacts with matter, it can either transfer

all its energy, or none of it... it cannot transfer part ofits quantised energy.

Einsteins model for light involves a duality... light musthave 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 areoccurring, the light photons are like little particles. Thisexplained the Black Body Radiation curves, and the weirdfeatures of the Photoelectric Effect.

Confirmation of the Einstein ModelEinsteins idea is very neat, but is it correct?

Einstein was able to make certain mathematical predictionsregarding further features of the Photoelectric Effect.(The exact details are complicated, and not requiredlearning)

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

This was confirmation that the photon theory of light, andthe quantum theory of energy were both correct. Einsteinwas awarded the Nobel Prize for Physics in 1921, for hiscontribution to understanding the Photoelectric Effect.

Applications of the Photoelectric Effect

Solar CellsSolar Cells (or photovoltaic cells) are devices whichproduce electricity directly from light energy. They are veryfamiliar in the popular garden lights which need no wiringor 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-powergarden 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 frontpage) and in special situations, such as power for orbitingsatellites.

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

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 aphoton of light strikes the receiving surface, its energy causesemission of an electron, which is collected on a nearbyanode.

A sensitive electric circuit is able to measure the level ofelectron emission, and this gives a measure of the amount oflight being received.

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aa ssttrreeaamm ooffppaarrttiicclleess

LLiigghhtt iiss NNOOTT

aa wwaavvee

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 cellspowering a small electric

motor and fan



12

Assessment of Einsteins Contribution to Quantum TheoryAssess means to measure or judge the value of something. The syllabus requires you to assess

Einsteins 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 didthat in 1900. However, it seems that Plank invented the quantum idea purely as a mathematical trickto 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 Planks Quantum Theorywith the classical idea that light is a wave. This totally new way to look at things was one of the turningpoints 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 Einsteins 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) signedan anti-war declaration. He spent the war in neutralSwitzerland, lobbying for peace and an end to war. In the1930s he was forced to flee Nazi Germany because he wasof Jewish descent. In America, he fought against thedevelopment of the atomic bomb (developed directly fromhis own theories) and was appalled when it was used againstJapan in 1945.

Einstein believed that Science is a process that should workfor peace and the good of all people, and not be involvedin the 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 scientists played a majorrole in research and development of new weapons and wartechnologies. 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 ofweapons research, and the two great scientists of thissection of the topic epitomise these different views.

Max Plank was a patriotic German who believed that it washis duty to help his country fight a war. He gladlycontributed to weapons research in WW I, and leading upto WW II he was the director of the main ScientificInstitute in Nazi Germany. Planks outlook seems to havebeen that Science is part of the political & social structure,and must take an active role in it.

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 anb)..................................... coil which produced c).....................across a gap. The sparking produced radio waves which hedetected with a d)...................................................... in which asmall gap also sparked. He was able to show that the newradiations showed typical wave properties such ase).................................. and ................................................Hertz was also able to measure the f)....................................of the waves, and show it was equal to the speed ofg)...................................... He also produced evidence of theh)................................................. Effect, but failed toinvestigate it further.

Meanwhile, other researchers had studied the way energy isemitted from hot objects. The i).............................................Radiation curves showed a shape that could not beexplained by the accepted theories. In 1900, j)........................proposed the k)....................................... Theory toaccount for the problem. The basic idea of his theory isthat the energy of light (or other EMR) is l)........................the same way that matter is. The minimum quantity ofmatter is one m)............................., and fractions cannotoccur. Plank proposed that the energy of EMR is the same,and that the amount of energy carried by onen)................................ is related to the o)................................of the wave.

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

In 1905, Einstein used Planks s)........................... Theory toexplain all the difficulties. His idea was: Light is a wave, but the energy is concentrated inbundles called t).................................... Each bundle carries a u)............................ of energy, asdescribed by Planks theory. When a photon interacts with matter, it can eithertransfer v)............... of its energy, or w)....................... of it,but cannot transfer x)............................................................

This idea allows light to have its wave properties such asy).........................................., ................................................. and............................................., but to also sometimes showz)..............................-like properties when it transfers energy.Based on his theory, Einstein made certain mathematicalaa)................................. regarding the ab)..................................Effect. These were confirmed by ac).................................. in1916. This confirmed Planks ad)............................. Theory,and explained all the problems with ae).................................................... radiation & the af).................................. Effect.

Part B Practice Problems Quantum Theory(Planks Constant = 6.63x10-34)( c = 3.00x108 ms-1)

1.A light wave has a wavelength of 4.25x10-7m.a) What is its frequency?b) How much energy is carried by one photon?

2.Compare the amount of quantum energy carried by aphoton of

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

3.A photon of radiation is carrying 8.75x10-14J of energy.Calculatea) its frequencyb) its wavelength

4.To cause emission of an electron from the surface of acertain metal requires the electron to gain a minimum of2.38x10-20J of energy.a) Find the frequency and wavelength of the photon ofEMR which carries this threshold amount of energy.b) What happens if the electron is struck by a photon witha longer wavelength than this?c) What will happen if the electron was struck by a photonof higher frequency than calculated in (a)?

5.An electron was emitted from a metal surface after beingstruck by a photon of EMR.The electron left the surface with energy of 6.22x10-17J. Itfirstly had to use 9.28x10-19J of energy to escape themetal surface. All of this energy was gained by interactionwith a single photon.Find the frequency and wavelength of the photon.

13



FULLY WORKED SOLUTIONSIN THE ANSWERS SECTION



Revision of Atomic StructureAfter Thomson identified the electron as a particle presentin all atoms, it didnt take long for scientists to figure outthe details of atomic structure. You are reminded of thebasic model of a typical atom:

Electrical ConductivityWhen millions and billions of atoms form a latticestructure (most strong solids are like this) they do so byforming chemical bonds with each other in a regular array.

In a metal atom, the outer (valence) electrons are veryloosely held by the atomic nucleus. They feel the force ofattraction from other, surrounding atoms just as strongly asthe attraction from their own atom. The result is thatthese outer electrons can easily move from atom to atom.

If an electric field is present (due to a voltage beingapplied) billions of electrons begin moving in the samedirection... an electric current is flowing, and we say themetal is a good Conductor.

In other solids such as plastic or glass, the outer valenceelectrons are more strongly attracted to their own atom,and cannot easily escape from it, to move from atom toatom. We say these things are poor conductors, or goodInsulators.

Band Structure TheoryThe explanation just given for conductors and insulators isOK, until you find out about Semiconductors.Elements such as Silicon and Germanium have a numberof strange properties including being rather poorconductors of electricity until given a little jolt of energy.Then, suddenly they become quite good conductors.

To understand semiconductivity, you need to learn aboutBand Structures.We have known since the early 20th century that theelectrons around an atom can occupy different orbits orenergy levels surrounding the nucleus. These energy levelsare quantised (Quantum Theory applies) so there may beforbidden energy zones between them. An electroncannot exist in this fobidden zone because the energylevel there does NOT correspond to a whole quantum.

14



3. FROM ATOMS to COMPUTERS: SEMICONDUCTORS

ChemicalBonds

ATOMS in a SOLID ARRAYElectrical Conduction occurs when electrons can migrate

freely from one atom to the next

Migratingelectron

In a conductor, electronscan jump from one atomto the next

This ability, called Semiconductivity, allowsthese materials to act as electrical switches,turning electrical currents on and off, according totheir energy state.

This is the basis of all modern electronics & computer systems

Nucleus

Electrons can jump up and down through the differentbands as they gain or lose energy. To jump up over aforbidden zone they must have enough energy to achievethe quantum energy level required to occupy the next band.

In any atom in its rest state, the highest band occupied byelectrons is the Valence Band. If an electron has enoughenergy to get to the unoccupied levels above there, theelectron is effectively free to wander off . If an electricfield is applied, the electron becomes part of a flowingcurrent, and the substance is conducting electricity.

Thats why any energy band above the valence band iscalled a Conduction Band.

Forbiddenenergy gap.Electronscannot existthere.

Electrons inquantisedenergy bands

Some bandsoverlap

The unoccupied bandabove the valenceband, is called theccoonndduuccttiioonn bbaanndd

The highest energylevel that haselectrons in it, iscalled the vvaalleennccee bbaanndd

Structureof an ATOM -

Electrons in orbit at differentEnergy Levels

Electrons arerelatively easyto removefrom someatoms...this leads toelectricalconductivity,PhotoelectricEffect, etc

Atomic Nucleusof protons & neutrons



15

Conductors, Insulators & SemiconductorsIn terms of Band Theory, the difference in conductivitybetween different substances is simply the relationshipbetween the Valence Band and the Conduction Band.

In Conductors, In Insulators, In Semiconductors,these bands the bands are there is a small gapoverlap. separated by a between the bands.

wide forbiddenenergy gap.

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 a hugeboost of energy. At normal temperatures and voltagelevels, the substance will not carry a current.

A semiconductor, like Silicon, will not normally carrycurrent, because electrons lack the energy to jump theforbidden energy gap. However, if the temperature isincreased, and a voltage applied, there comes a point whenelectrons jump the gap in great numbers, and the substancesuddenly conducts very well indeed.

This effect does not occur at room temperature unless thesemiconductor substance is Doped.

Doping a SemiconductorDoping means to add a very small quantity of a differenttype of atom to an otherwise pure solid lattice ofsemiconductor atoms.

Conduction of Electrons & HolesNormally we imagine that an electric current is composed ofa flow of negative electrons. However, in a semiconductor,when an electron jumps out of the valence band and flows offsomewhere, it leaves behind a hole in the valence band. Thishole, 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 nextelectron in the valence band has enough energy to jump intothe hole, leaving its own hole behind...

If you can imagine this sequence like the pictures making amotion cartoon, you can imagine that an electron flows to theright and the hole flows to the left.

In fact, in terms of electrical energy, it makes no differencewhether the current really is negative electrons going one way,or holes going the other way... either way, it constitutes anelectric current. The holes are considered as positively chargedspaces (relative to the electrons) and so the flow of positiveholes 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 anextra valence electron. The other way to do it is to use atomswith only 3 valence electrons, creating extra holes in thelattice.

Conduction Band

Valence Band

Conduction Band

ForbiddenEnergy gap

Valence Band

Conduction Band

Valence Band

AAttoommss ooff SSeemmiiccoonndduuccttoorr ssuubbssttaanncceee.g. Silicon, normally have 4 valence electrons

Eachchemicalbond isformed byatoms

sharing 2electrons.

Theseelectronsare in the

valenceenergyband

Atom with 5 valence electronsused to Dope the lattice.

extra valenceelectron

Atom with3 valenceelectronsused toDope the

lattice.

extra hole inthe lattice

DOPING increases the conductivity of the lattice

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

hole

EElleeccttrroonnss aarree jjuummppiinngg ttoo tthhee rriigghhtt

......aanndd tthhee hhoollee iiss jjuummppiinngg lleefftt..



16

p-Type and n-Type SemiconductorsThe two different ways to dope the lattice result in twodifferent types of semiconductor material:

n-Type Semiconductorsare doped with atoms with 5 valence electrons, such asarsenic or antimony. This adds extra valence electrons tothe lattice. Electrical current is carried mainly by this flowof negative charges (hence n-type).

p-Type Semiconductorsare doped with atoms with 3 valence electrons, such asaluminium or gallium. This adds extra holes to the lattice.Electrical current is carried mainly by this flow of positiveholes (hence p-type).

Some History: Electronics & ComputersThe concept of a machine to carry out high speedcalculations and logical operations has been around forcenturies. Prior to the 20th century, any such device had tobe mechanical, using clockwork gears and so on. Therewere some notable successes with control devices forweaving looms, and mechanical adding machines, butapplications 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 Collosuscomputer used thermionic valves to electronically switchcircuits on and off, to store and manipulate data.

Invention of the TransistorThermionic valves had been widely used in radios for someyears and were vital components of the new industry oftelevision.

Valves were also important in the switching of connectionsin telephone exchanges, where the growing communicationdemands required automatic dialing and connectiontechnology. (The original system involved humanoperators manually plugging wires into sockets toconnect phone calls.)

However, the valve-based technology was proving too slow,too unreliable and too expensive for the boomingtelephone industry. The major U.S. phone company BellTelephone set its scientists the task of researching newmaterials and processes to replace the valves.

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

Because of the properties of the semiconductor(conductivity that can be switched on and off) the transistorcan 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 TubesThermionic refers to the way these CRTs would emitmany electrons from the cathode (and thereby carry acurrent) when the cathode became hot. Once warmedup the valve can act as an electronic switch in acircuit, when the voltage to the anode is varied.

Characteristics: relatively large & expensive

consume relatively largeamounts of electricity

produce large amounts ofwaste heat

although faster thanmechanical switches, valvesare slow-acting by modernstandards

require time to warm up

have a limited lifetime, andcan burn out like a lightbulb. Therefore theirreliability is low, andmaintenance needs are high.

Despite these limitations,Collosus was very import-ant in helping to win thewar.

2 cm

The comparison is a no-brainer...

The transistor replaced Thermionic valves

as rapidly as electronics industries could re-designtheir products, and begin mass production

Transistors

10 c

m

Photo by Don Jolley

Photo by Ben Merghart



17

Silicon v GermaniumTo make semiconductor material with the desiredconductivity properties, it is necessary to firstly prepareextremely pure samples, then add minute amounts of thedoping 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 one ofthe most abundant elements on Earth.

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

Also in the 1960s, the technology of the computer beganto emerge for financial and communication uses. Thesolid-state transistor technology allowed a computer tobe built to fit a table-top, rather than fill a room. Everyteenager had a brick-size transistor radio, in the sameway that in this decade everyone has an MP3 and a mobilephone the size of a matchbox.

The miniature integrated circuit board led to thetechnology of the silicon chip where thousands, and nowmillions of transistor-equivalents can be printedmicroscopically in the space of a postage stamp...a microchip.

In the 1980s the first cheap PCs (personal computers)could process a magnificent 2x103 bytes of information.Twenty years later, these notes are being typed on an evencheaper PC which can process 2x109 bytes, (2 GB). Thecomputers have become a million times more powerful!

Assessment ofImpacts of the Transistor on Society

It could be argued that the invention of the transistor wasone of the most profound technological developments inhistory. It ranks right up there beside the developmentssuch as:

fire, by ancient humans around 500,000 years ago.Fire transformed human society because of its power towarm people, cook food and protect from predators. agriculture, about 10,000 years ago.This transformed society from nomadic hunting-gatheringto settled communities that invented law, commerce,government and civilization. metallurgy and the Industrial Revolution, which led tonew tools, machinery, mass production, urbanization, andmass transport systems.

The transistor ushered in the Information &Communication Revolution, which is still developingtoday. Electronic circuits, using microchips, are the basis ofall the computers which allow:

instant access to (virtually) all the information on theplanet 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 andfrom (virtually) anywhere.

Computers are the key to the global economy and massconsumerism which keeps thing cheap through massproduction & distribution. Computers keep track of thebillions 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 stayedin the trees) the modern world could not exist without theinvention of the transistor!

Phot

o by

John

de

Boer

Photopipp

Photo: Martin Boulanger

Silicon Chip

Worksheet 3

Fill in the blanks. Check your answers at theback.

a)........................... orbit around the nucleus ofatoms at various b)............................... levels.Basically, a substance will be an electricalconductor if c)................................ can move fromd)............................................................ freely. Ifelectrons cannot do this at all, the substance is ane)..................................................

A semiconductor is a substance which has verylow f)............................................... until itselectrons are given just a little energy. Then, at acertain point, it suddenly becomesg)......................................... This allows electricalcircuits to be h)................................. on and off,and is the basis of modern i)........................... andj).................................

The best explanation of semiconductivityinvolves k).......................................................Theory, summarized as follows: the highest energy level in an atom that haselectrons in it, is called the l)..................................band. any further (unoccupied) levels above this arecalled m).......................................... bands. If an electron has enough energy to get to am)...................................... band, then it is free toflow, and form an electricn).......................................

However, between the bands there may beforbidden o).....................................................The energy levels are quantised, so a forbiddenlevel is where the energy is not equal to a wholep)...............................

In a conductor, the q)...................... band andr)...................... bands s)................................... eachother. This means electrons can freely enter theconduction band and t).........................................can flow through the substance.

In an u)....................................., these bands areseparated by a wide v)..............................................so that electrons can never reach thew)................................... band.

In a semiconductor, the valence and conductionbands are separately by a x).............................. gap.In the rest state, electrons cannot get across,and the substance does noty).......................................... However, it onlyrequires a slight increase in energy and suddenlymany electrons z)................. the gap and thesubstance begins aa).......................................

The semiconductor material can be made moresensitive and conductive if ab)..........................quantities of other elements are added to theatomic lattice. This is called ac..............................the semiconductor.

Semiconductors can carry electricity in 2 ways: bythe flow of ad).................................... which havereached the conduction band, or by the flow ofae)............................ left behind by departingelectrons.

If a af)....................................... is doped withatoms with 5 valence electrons, this results inag)........................................... in the lattice to carrythe current. This is an ah).......-Typesemiconductor.

If it is ai)................................... with atoms withonly aj)................. valence electrons, this createsextra ak)..................................... in the lattice tocarry current. This is a al)........-Typesemiconductor.

Before semiconductors, electronic switching wasdone by am).................................... valves. Thesewere an)................................................. tubes. Theao)............................................ was invented toreplace these valves. Compared to a valve, atransistor is ap) ........................ (size) and aq)................... (cost) consumes ar)...................... electricity andproduces almost no as)........................................ operates at a at)............................ rate does not need to au)............................................... is highly av)..........................................................

The early transistors were made fromaw)..............................., but this was later replacedby ax).................................. because it is moreay)................................ and a lot az)............................Miniaturization of electronics on siliconba)........................ has led to the development ofbb)................................... which are at the heartof all modern computers.

18




Investigating Crystal Structures...Bragg and Son

The regular shapes of crystals (such as salt) had long beenassumed to be due to a regular arrangement of the atomsor ions in a lattice-like structure. However, until the early20th century, there was no way to prove or confirm thisidea.

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.

The Braggs were able to analyse the interference pattern inorder to deduce the arrangement of the atoms within thecrystal. For this, they were jointly awarded the Nobel Prizefor Physics in 1915.

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

Crystal StructuresThanks to scientists like the Braggs, we now understand theatomic-level structure of most substances. You learnedpreviously how a substance like the semiconductor Siliconis a lattice of atoms chemically bonded together:

Crystal Structure of MetalsUnlike silicon, salt and other crystals, metal atoms are notchemically bonded to each other by the sharing orexchanging of electrons.

You will remember that the outer valence electrons inmetals are weakly held, and can access the conductionband at any time. The result is that the valence electronson each atom are NOT confined to that atom, but freelywander around from atom to atom.

Each metal atom is, therefore, ionized because its valenceelectron(s) are on the loose. The metal lattice is oftendescribed as an array of ions, embedded in a sea of electrons.

This sea of electrons shifts and flows freely. If an electricfield is present, the electrons will all flow in the samedirection as an electric current. Thats why metals are allgood conductors.

Resistance in MetalsSo why is there resistance in a metal wire?Although the electrons can flow quite easily, theirmovement is not totally free.

Any impurities in the metal distort the shape of the latticeand impede the electron flow. Also, as the ions vibrate dueto thermal energy, the vibration causes more collisionsamong electrons, so their flow is resisted. As temperatureincreases, the vibrations increase too, and thats whyresistance in metals increases with temperature.

Logically, if you re-read the previous paragraph and thinkbackwards, you might infer that if you had a really puremetal, and cooled it right down so that all lattice vibrationsstopped, then it would become a perfect conductor.

19



4. FROM CRYSTALS TO SUPERCONDUCTORS

Crystalx-raybeam

X-rays diffracted by the crystallattice, form Interference patternswhich are captured on the film.

Photographic filmsensitive to x-rays

SSii SSii SSii SSii

SSiiSSiiSSii

SSii SSii SSii SSii

SSii

Eachchemicalbond isformed byatoms

sharing 2electronswith eachneighbour

+

+

+

+

+

+

+

++

+

+

+

+ + + +

+

+

Superconductivity!



20

Superconductivity in Metals and CeramicsIn 1911, a Dutch physicist managed to cool mercury downto about 4oK (-269oC) and found that its electricalresistance dropped to zero.

Over the following years, various other metals were foundto become superconducting at very low temperatures. Thepotential to build electrical generators and equipment withzero resistance was a very attractive idea, but thetemperatures involved (no higher than about 20oK) were solow that there seemed no practical way to take advantage.

Then in 1986, Swiss scientists discovered some ceramicmaterials containing rare elements like Yttrium andLanthanum, which became superconductors at muchhigher temperatures. Still cold by human standards, but100o higher than the metal superconductors, these ceramicshad zero resistance at temperatures as high as 130oK(around -150oC). This is a temperature that is much morepractical to achieve.

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

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

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 whodeveloped the theory in the 1950s.

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 positive ions anddistorts the crystal structure slightly:

This distortion concentrates the positive charge in this partof the lattice, and attracts other electrons.

In a normal conductor, this distortion leads to collisionsand loss of energy by the flowing electrons which repeleach other... this is the normal electrical resistance withinthe conductor.

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

Due to quantum effects (which are beyond the scope ofthis Course... KISS Principle) each electron of the CooperPair helps the other to pass through the lattice without anyloss of energy. This means there is ZERO resistance.

However, at a temperature above the transition, thethermal vibrations in the lattice keep breaking up theCooper Pairs as fast as they can form. This destroys thesuperconductivity, and the normal electrical resistance ofthe substance returns.

The Meissner EffectYou may have seen a practical demonstration of asuperconductor in action, in class. The Meissner Effectis named after the scientist who discovered it.

If a disk of superconductor ceramic is chilled below itstransition temperature, a small magnet placed closeabove it will levitate; spinning freely if prodded, butheld up against gravity by unseen forces.

Explanation:As the magnet is brought near, its magnetic field inducescurrents in the ceramic. Since there is NO electricalresistance, the currents flow freely, non-stop and generatea magnetic field that repels the approaching magnet.

Superconductors will never allow an externalmagnetic field to penetrate them.

++ ++ ++++ ++

++ ++ ++++ ++

Approachingelectron

++ ++ ++++ ++

++ ++ ++++ ++

CCooooppeerr PPaaiirrof electrons forms

dish

LiquidNitrogen

Disk of SuperconductingCeramic

Small Levitating magnet



21

AdvantagesSuperconductor technology offers

high efficiency in any electricalsituation, because there is no energyloss due to resistance. the ability to generate extremelystrong magnetic fields fromsuperconducting electromagnets. faster operation of computers, sincesuperconducting switching devicescould be 10 times faster than asemiconductor transistor

Limitations Superconducting metals must bechilled with liquid helium. This isimpractical and expensive. New, superconducting ceramics canbe chilled with liquid nitrogen, which ischeaper and much more practical, BUTthese ceramics:

are fragile and brittle and difficult to make into wires.

can be chemically unstable and have a limited life span.

Possible Future Applications

Current computer technology is based onsemiconductor microchips. Although these becomefaster and more powerful every year, there is a limit tohow far they can go.

A superconductor computer could open a whole newlevel of enhanced performance due the possible highspeed switching of circuits.

Electricity generation & distribution could be mademuch more efficient with superconductor technology.

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

Generators lose energy by resistance heating in the coilsneeded to produce magnetic fields, and are limited inthe strength of the fields they can produce.Superconducting coils would allow generators to bemuch more powerful and efficient.

Greater efficiency generally in electrical technologywould reduce associated environmental problems, suchas Greenhouse gas emissions.

Using Superconductor Technology

The Maglev TrainThe idea of using superconducting electromagnets to levitate atrain above a magnetic guide-rail has been around for many yearsand experiments have been going on for decades.

The guiderail(s) under the train contain conventionalelectromagnets. On board, helium-chilled super-conducting electromagnets produce powerful magneticfields.

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 andrepelling it. The actual motive power is supplied fromthe rail, not from onboard the train.

The big advantage is the high speed possible withoutany rail friction, and the low maintenance and low noisethat goes with this. A disadvantage is the very high costof building the guide rail track.

Experiments have been going on for years in Germanyand in Japan. The first truly operational Maglev nowconnects the city of Shanghai in China, with its airport30km away. German built, it cost US$1.2 billion, andreaches speeds of around 430km/hr.

MAGLEV = MAGnetic LEVitationShanghai Maglev TrainPhoto 2004 Matthew Hillierused with permission

Worksheet 4

Fill in the blanks.Check your answers at the back.

Sir William Bragg, and his son Lawrence beameda).................................. through crystals. The waveswere b)......................................... by the atom/ionarray, and formed c).........................................patterns, which were recorded ond).................................... film. By measurements ofthese images, they could deduce the exactstructure and geometry within thee)..................................................

Unlike other crystals, metals have a structuredescribed as an array of f)..........................,enbedded in a sea of g)......................................The electrons have free access to theh)..................................... band, so the metal is agood i).................................................... ofelectricity. There is some j)........................................because of collisions caused by thermalk).................................. of the lattice.

l).......................................... was first discovered inmercury metal which had beenm)............................. to a temperature of aboutn).......................... In the 1980s, a new class ofsuperconducting o)....................................... werediscovered, with transition temperatures uparound p)..........................

If a magnet is placed above a superconductor, itwill q)......................................, being held up byr)........................... forces. The field is created bys)......................................... in the superconductor,induced by the external t)........................................Superconductors will never allow an externalfield to u)...................................................... them.

The explanation of superconductivity isv)...................... Theory, which states: an approaching electron causes a slightw)............................ of the ion lattice. this concentrates the density ofx)............................ charge, which attracts moreelectrons. 2 electrons can form ay)..................................................... which resultsin both of them z)............................................... thelattice without energy loss, due toaa)....................................... energy effects.

The advantages and possible applications offeredby superconductor technology include highab)............................................. of electricalgeneration and ac).............................................,because it could eliminate energy losses due toad)..........................................Another possiblity is in computers, which couldoperate ae)............................................ because asuperconducting af)...................................... canwork ag)............. times faster than aah)................................................

A limitations of superconductor technology isthe need to ai)............................... a metal usingaj)......................................, which is veryak).................................. and ...............................The higher temperature al)................................superconductors solve part of this problem, butthey are am).................................. and................................... and difficult to make intoan)........................................ They may also beao)............................................................................and have a short life-span.

One superconductor technology that has beenimplemented is the ap)................................ train,which uses superconductor magnets toaq)......................................... the train above itsguide rail.

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FFrroomm IIDDEEAASSttoo

IIMMPPLLEEMMEENNTTAATTIIOONN

From CATHODE RAYSto TELEVISION From RADIO

to PHOTOCELLS:Quantum Theory

From ATOMSto COMPUTERS

From CRYSTALSto

SUPERCONDUCTORS

CONCEPT DIAGRAM (Mind Map) OF TOPICSome students find that memorizing the OUTLINE of a topic

helps them learn and remember the concepts and important facts.Practise on this blank version.

Practice QuestionsThese are not intended to be "HSC style" questions, but tochallenge your basic knowledge and understanding of thetopic, and remind you of what you NEED to know at theK.I.S.S. principle level.

When you have confidently mastered this level, it isstrongly recommended you work on questions from pastexam papers.

Part A Multiple Choice1.The Maltese Cross cathode raytube proves that cathode rays:A. travel from anode to cathode.B. travel in straight lines.C. are particles with mass.D. are electrically charged.

2.A cathode ray beam is passing throughan electric field directed as shown inthe diagram. E field

This is part of an experiment in which the beam will simultaneously cathode rayspass through a magnetic field.The arrangement and strengths of the 2 fields is such thatthe effects will cancel out so the beam travels throughwithout deflection.

In which direction must the magnetic field be directed inorder to achieve this?A. into the pageB. up the pageC. to the leftD. out of the page

3.Which of the following diagrams correctly shows the electricfield between point charges and/or charged plates?

4.Thomsons famous cathode ray experiment was able toget a value for:A. the charge to mass ratio, of cathode rays.B. the mass of the electron.C. the strength of crossed electric and magnetic fields.D. the electric charge of an electron.

5.If you were to alter the voltage to the anode in theelectron gun part of a TV picture tube, this would alter:A. the position of the image on the screen.B. the speed of the cathode ray beam.C. the brightness & colours of the fluorescent image.D. the size of the image.

6.Which of the following best describes the outcome ofHertzs famous experiments of 1887?A. His discoveries led to the Quantum Theory of light.B. He showed that light gives interference patterns.C. He confirmed that light is an electromagnetic wave.D. He determined a more accurate value for the speed of

light.

7.According to Quantum Theory, if you compared theenergy of 2 photons of light and found that one had moreenergy than the other, then the one with more energy musthave:A. more mass.B. longer wavelength.C. higher frequency.D. a higher velocity.

8.The Photoelectric Effect involves:A. emission of electrons that have absorbed a quantum ofenergy from a photon.B. emission of a photon of light that has absorbed theexcess energy from a falling electron.C. using photographic film to get an image of x-rayinterference patterns.D. using an electrical induction coil to cause sparks in aseparate receiving coil or antenna.

9.According to Einstein, light often behaves like a wave, butsometimes acts like a particle. A phenomenon in which theparticle nature of a photon is noticeable, is:A. interference of photons scattered by crystals.B. refraction of light by a glass lens.C. photoelectric effect occurring in a solar cell.D. polarization of light by sunglasses.

10.According to Band Structure Theory of electricalconductivity, the valence band and the conductionband in a semiconductor:A. overlap each other.B. are sparated by a very wide forbidden energy gap.C. are inverted in reverse order to normal.D. are separated by a narrow energy gap.

24



++

- ++

A. B.

C. D.

-

- -

++ ++

11.Which line of information below, best describes a p-typesemiconductor?

Valence of atoms Current mainly used to dope lattice carried by

A. 5 electronsB. 3 holesC. 5 holesD. 3 electrons

12.Which of the following is NOT an advantage of atransistor, compared to a thermionic valve?A. consumes less power.B. needs time to warm up.C. operates faster.D. smaller and more reliable.

13.The original transistors were made from Germanium, butthe technology later switched to use Silicon, because:A. Silicon crystals are easier to grow.B. Germanium supplies were running out.C. Silicon is more abundant and cheaper.D. Germanium crystals couldnt be miniaturized as well.

The following diagram describes a famous experimentcarried out by Sir William & Lawrence Bragg.The diagram refers to Q 14 & Q15.

14.The radiation used by the Braggs was:A. x-raysB. radio wavesC. ultra-violetD. visible light

15.The pattern captured on the photographic film was due tothe phenomenon of:A. refraction.B. photoelectric effect.C. polarization.D. interference.

16.Superconductor technology is currently limited by:A. lack of suitable applications to apply it to.B. superconducting chemicals being fragile and brittle.C. the operating temperatures being too low to achieve.D. semiconductors do the same job more efficiently.

17.In a superconductor above its transition temperature:A. lattice vibrations break up the Cooper Pairs as fast as

they can form.B. lattice distortions attract electrons to form Cooper Pairs.C. the Meissner Effect can levitate a magnet.D. the holes in a doped lattice allow electrons to tunnel.

Longer Response QuestionsMark values shown are suggestions only, and are to give youan idea of how detailed an answer is appropriate.

18. (5 marks)Explain why the apparent behaviour of cathode rays causeddebate as to whether they were charged particles orelectromagnetic waves.

19. (6 marks)Two parallel charged platesare 6.00cm apart, in vacuum,and have a potentialdifference of 30.0V betweenthem.An electron (Qe = -1.60x10-19C) is located between theplates.a) Find the magnitude of the electric field between theplates.b) Calculate the force that will act on the electron due tothis field.c) At what rate will the electron accelerate?(electron mass = 9.11x10-31kg)

20. (8 marks)An alpha particle (Q = + 3.20x10

-19C) isabout to enter a magneticfield of strength 5.22T ata velocity of2.95x103ms-1.a) Find the magnitude and (initial) direction of the forcedue to the magnetic field it will experience.b) A pair of charged plates (not shown in the diagram)are arranged so that the force due to the magnetic fieldwill be exactly cancelled out by the force due to theelectric field.Sketch where the plates need to be to do this, and indicatethe type of charge on each plate.c) If these electric plates are 10.0cm apart, what voltagemust be applied to exactly cancel the magnetic deflection?

21. (6 marks)A TV picture tube is made up of several maincomponents. Outline the role of the a) electrodes of the electron gun.b) deflection plates or coils.c) fluorescent screen.

22. (4 marks)As part of your studies you have carried out aninvestigation to demonstrate the production andreception of radio waves.Describe briefly how you did this.

25



Crystal

Photographic film

++++

----

23. (6 marks)Two different photons of light have wavelengths of5.00x10-7m (photon P) and 2.40x10-8m (photon Q).Qualitatively (no calculation required) compare P & Qs:a) speedb) frequencyc) energyExplain your answers in each case.

24. (4 marks)For an electron to escape from the surface of a particularmetal, it needs to absorb a minimum of 6.75x10-19J ofenergy. Calculate the a) frequencyb) wavelength

of a photon with just enough energy to cause this.

25. (3 marks)Identify the contribution made by Einstein to QuantumTheory.

26. (4 marks)a) What is the photoelectric effect?b) Summarize how this effect is used in a solar cell.

27. (5 marks)In relation to the Band Structure Theory of conductivity,a) what is meant by the valence band of an atom?b) what is meant by the conduction band of an atom?c) explain the difference between

conductorsinsulatorssemiconductors

28. (5 marks)Compare and contrast a p-type semiconductor and ann-type semiconductor.

29. (4 marks)Describe the differences between a solid state andthermionic device in terms of structure and discuss whysolid state devices replaced thermionic devices.

30. (4 marks)Assess the impact of the invention of the transistor onsociety, with particular reference to their use in microchips.

31. (3 marks)Outline the methods used by the Braggs to determinecrystal structure.

32. (3 marks)Discuss the BCS Theory of superconductivity.

33. (3 marks)Outline the possible benefits from applyingsuperconductor technology to computers, generators andelectrical transmission systems.

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Answer Section

Worksheet 1Part Aa) cathode b) dischargec) vacuum d) voltagee) glows, streamers and striationsf) gas pressure g) negativeh) Crookes i) Maltesej) fluorescent k) kineticl) momentum m) electricn) waves or particles o) straight linesp) fluorescence q) photographic filmr) kinetic energy & momentums) mass t) negativeu) fieldv) the direction a positive test charge would movew) force x) parallel, chargedy) strength & direction z) voltageaa) distance ab) movingac) magnetic ad) magnetae) deflect af) Right-Hand Palmag) Thomson ah) electric & magneticai) charge to mass aj) particlesak) atoms al) electronam) TV an) electronao) electrons ap) cathodeaq) anodes ar) deflectionas) steer/direct at) fluorescentau) glow av) electrons

Part B Electric Fields & Forces1.a) E = V/d = 50.0/0.0400 = 1250 = 1.25x103NC-1.b) F = Q.E = 3.20x10-19x1.25x103 = 4.00x10-16N.c) i) Field is directed from +ve plate to -ve plate.

ii) Force is also directed towards -ve plate.2.a) F = Q.E, so E = F/Q = -7.82x1

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