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keep it simple science®

1

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

The History of Physics is marked by anumber of “landmark” discoveries that changedour understanding of the Universe...

Newton’s Laws of Motion, and Gravitation, andEinstein’s Theory of Relativity

have already been studied.

This topic covers a number of other greatdiscoveries, experiments and scientists, so it isdefinitely a study of the History of Physics, fromabout 1850 into the 20th century.

However, it is not just history. Along the way,you will be studying some concepts, theoriesand facts that are vital to your overallunderstanding of this subject.

In addition, as you learn both the history andsome of the foundation ideas of modernPhysics, you will see that much of our moderntechnology is a direct result these discoveries...

When “Cathode Rays”were being studied

between 1850-1900, peoplesaid “interesting, but

what’s the use of it??”

Little did they know...

...the study of CathodeRays led directly to the

invention of the TV set, sofamiliar today.

About the Same Time as Cathode Rayswere becoming understood, other scientistswere studying electromagnetic radiation andobscure phenomena such as the

“PhotoelectricEffect”.

and Meanwhile,the unravelling of atomic structure and study ofelectrical conductivity in “weird” substanceslike Germanium and Silicon, led to thediscovery of “semiconductors”.

The invention ofthe transistorfollowed... the

basis of allmodern

electronics and computer

systems.

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

The Study of Crystal Structureled to thediscovery ofSuperconductors, the applications ofwhich are onlyjust beginning tobe implemented.




2

Cathode Rays.Discovery &Properties

Behaviour of aCharged Particle

in a MagneticField

Conductivity inMetals.

Superconductivity

Current & PotentialApplications of

Superconductivity

Television

Einstein’sContribution:Particle-Wave

Dualityof Light

Discovery ofthe Electron...

Thomson’sExperiment

Differing viewson Science’s

place in society

Valves to Transistorsto Microprocessors...Impacts on Society

The Braggs & X-ray

Crystalography

AtomicStructure

&Structures ofSolid Lattices

PhotoelectricEffect

&Applications:• solar cells• photocells

From IDEASto

IMPLEMENTATION

From CATHODERAYS

to TELEVISIONFrom RADIO

to PHOTOCELLS:Quantum Theory

From ATOMSto COMPUTERS

From CRYSTALSto

SUPERCONDUCTORS

CONCEPT DIAGRAM (“Mind Map”) OF TOPICSome students find that memorising the OUTLINE of a topic helps them learn and remember the concepts and important facts. As you proceed through the topic,

come back to this page regularly to see how each bit fits the whole. At the end of the notes you will find a blank version of this “Mind Map” to practise on.

Hertz’s Discovery of Radio Waves Plank’s

QuantumTheory

“Band Theory” ofConductors,Insulators &

Semiconductors

Discovery of Cathode RaysBy the 1850’s, scientists had developed thetechnology to produce quite high voltages ofelectricity and to make sealed glass tubes from whichmost of the air had been removed using a vacuumpump.

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

You may have done some laboratory investigationswith “Discharge Tubes” as follows...

It was soon established that whatever wascausing these glows or “discharges” in thetubes was coming from the negative electrode,or “cathode”...so these emissions were called“Cathode Rays”.

Over the following 20 years these mysterious“rays” were studied by many scientists, mostnotably Sir William Crookes. He devised somany clever variations on these Cathode RayTubes (CRT’s) that they were known as“Crookes Tubes”.

You will have seen, in the school laboratory, anumber of different CRT’s and repeated many ofCrookes’s famous experiments...

Experiments with CRT’sMaltese Cross Tube

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

from the Cathode.

Furthermore, Crookes tried this experiment withmany different metals as his electrodes. Thetype of metal made no difference... CathodeRays are identical, regardless of the materialsused.

Tube With a Fluorescent Screen

Tube With a Rotating Paddle-Wheel

This evidence from these various experimentswas very inconsistent... some of the features ofcathode rays suggested they are particles, otherresults suggested they are waves.

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1. FROM CATHODE RAYS TO TELEVISIONkeep it simple science

®



Each 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.

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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 glowfrom gas in the tube, but the glass itself glows at

one end of the tube.

CATHODE( -vve)

ANODE (+ve)in the shapeof a Maltese

Cross

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

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Wheel spins when cathode rays strike the paddles

This shows that the rayshave momentum, and

therefore have mass

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

violet (UV) rays

4

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 platesabove, it follows that the particles must benegatively charged, because the beam isdeflected by repulsion from the negative plate,and attraction towards the positive.

Confusion About Cathode RaysUnfortunately, when the early experimenterstried something similar to the above, they didNOT detect a deflection of the beam. So, theyconcluded there was NO charge associated, andwere confused about the nature of the CathodeRays.

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

Evidence that CR’s were 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 discoveriesinvolved the Cathode Ray Tube. This is arelatively 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 “ForceField”.

WORKSHEET at the end of this section.




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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 perunit of charge experienced by a charge in thefield...

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 chargedparticle we deal with is the electron.

The value of its charge is Qe = ( -)1.602 x 10-19C.

Get used to this very small value.

Example Calculation:In the CRT shown at top left of this page, astream of electrons passes between 2electrically charge plates. The electric fieldstrength 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 thedirection of the force will be in the oppositedirection to the electric field.

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

field

Attraction

Repulsion

5

Electric Field Between Parallel Charged Plates

The field around and between point charges isirregular in direction, and varies in strength atevery point. The field between parallel chargeplates, however, is uniform in strength anddirection at every point (except at the edges).The direction of the field is the way a positivecharge would move.

WORKSHEET at the end of the section.

Force on a Moving Charge in a Magnetic Field

In the previous topic you learned that when anelectric current flows through a magnetic field, thewire experiences a force... the “Motor Effect”.

Now you need to realise that the reason is thatevery electric charge, if moving through amagnetic field, will experience a force.

You may have seen the following experimentwith a CRT in the laboratory:




The strength of the field depends on theVoltage applied to the plates, and the distancebetween 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 theplates?

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

CRT with fluorescentscreen.

Cathode Ray beamgoes straight across.

If a magnet isbrought near, the

beam deflects.

A force is actingon the moving

charged particles.

The size of the force can be calculated asfollows:

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

magnetic field vector lines.

Since sin90o = 1,and sin0o = 0,

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

BMMaagg..FFiieelldd

θ

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 = QvBsinθθ

= -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?

SS

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+

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Discovery of the Electron...Thomson’s Experiment

In 1897, the confusion and debate aboutCathode rays was settled by one of the mostfamous, and critically important, experiments inthe history of Science.

The British physicist J.J. Thomson set up anexperiment in which cathode rays could bepassed through both an electric field, andthrough a magnetic field, at the same time.

How a TV Screen WorksThomson used afluorescent screen at theend of his CRT to detectand measure thedeflection of the cathoderays (electrons).

Over the following 30years, CRT technologyevolved into the televisionscreen. By the middle ofthe 20th century, TV wasdeveloping to become themajor system for homeentertainment and by the1980’s the same screensbecame the vital displayunits for computers.

A TV “picture-tube” is really just a moresophisticated version of Thomson’s CRT. Theimage 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 areaccelerated towards a series of anodes by thehigh voltage electric field between them, justlike in the CRT’s of Crookes or Thompson.

The Deflection Platesare used to deflect the beam to create spots oflight at different points on the screen.

One set of charged plates are arranged so thefield can deflect the beam up or down. Anotherset are arranged at right angles to causedeflection left or right.

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

The Fluorescent Screenglows with light when the electron beam strikes thefluorescent chemical coated on the inside of theglass.

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.




+ve

-ve

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Magnetic Field Effect ((AAddjjuussttaabbllee EElleeccttrroommaaggnneettss))

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B iinnttoo ppaaggee

Thomson was able to adjust the strengths of the2 fields so that their opposite effects exactlycancelled out, and the beam went straightthrough 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 becalculated from the currents and voltages appliedto the plates and electromagnets, Thomson wasable to calculate the ratio between the charge andmass of the cathode rays.

Charge to mass ratio = Q m

This established beyond doubt that cathoderays were particles, 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 (thesmallest atom). It was apparent that the cathode rayparticle was much smaller than a hydrogen atom. Theconclusion was that all atoms must be made ofsmaller parts, one of which was the “cathode rayparticle”, soon re-named the “ELECTRON”.

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

VVaarriiaabbllee vvoollttaaggee

The discovery of a).......................... Rayswas made with simple“b)........................... tubes” from whichmost of the air was removed with ac)............................. pump. When highd)..................... was applied to electrodesat each end of the tube, it wouldproduce a variety of e)........................,............................ and ..............................The exact pattern changed as thef).......................... in the tube was altered.It was discovered that the effects weredue to mysterious emissions comingfrom the cathode (or g).............................electrode).

About the 1870’s, Sir Williamh)...................... and others, built specialCRT’s to study the cathode rays. Thefamous “i)............................. cross” tubeshowed that the rays travelled instraight lines. Tubes withj).............................. screens showed thatthe rays caused fluorescence, andtubes equipped with a “paddle-wheel”proved that the rays carried bothk).................... energy and l)......................

Unfortunately, attempts to detectdeflection by applying anm)...................... field were unsuccessful,so for many years there was confusionover whether CR’s were n)......................or ..........................

Evidence they were waves:• CR’s travel in o)................................ likelight.• 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 createdaround anything with electric charge.

The direction of the field is defined asv)........................ .......................................Any charge within a field will experiencea w)....................... The field between 2x)...................... ........................... platesis uniform in both y).............................and ..........................., and is determinedby the z)............................. applied to theplates and the aa).........................between them.

Electric charges also experience a forceif they are ab).................................through a ac)............................ field. Thisis easily observed by bringing aad).......................... near a CRT with afluorescent screen; the magnet causesthe beam to ae).........................................The direction of the force and thedeflection of the CR beam is easilydetermined by the“af).................................................. Rule.

In 1897, J.J. ag)........................................used the deflection of a CR beam byboth ah)........................ and ......................fields to measure the ratio ofai)......................................... of a cathoderay. This established, beyond doubt,that CR’s are aj).......................... and area small part contained within allak)................... Thomson had discoveredthe al).............................. The simple CRTwas later used as the basis to invent theam).......................................... screen.

The main parts of the “picture tube” are:• The an)............................. Gun, whichproduces a beam of ao)...........................from a ap)....................... and acceleratesthem towards a series ofaq)..................................• The ar).............................. plates, whichuse electric fields to as)............................the beam onto the screen.• The at)............................. screen, whichforms the image when fluorescentchemicals au)................... with spots oflight when struck byav).................................

7




Worksheet 1 From Cathode Rays to TelevisionFill in the blank spaces. Student Name...........................................


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 2.50mmapart.

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.

8.A particle of the solar wind with charge of(+)1.60x10-19C (it is in fact a proton) encountersthe Earth’s 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 theproton.

9.In an experiment similar to Thomson’s, 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 exactlycounteracted by an electric field produced bycharged plates which are 8.00mm apart.

b) What is the strength of the electric field?

c) What is the voltage being applied across theplates?

8



Worksheet 2 Practice ProblemsElectric Fields & Forces Student Name...........................................

Worksheet 3 Practice ProblemsForce on a Moving Charge in a Magnetic Field Student Name...............................

3. (cont)b) What charge does the speck of dust carry?c) The static charge was created when someelectrons were either removed from, or addedto, 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” ontothe paper by accelerating them in electric fieldsto achieve 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?

1.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 theforce experienced by the electron.

2. In a nuclear accelerator, a charged ion has beenaccelerated up to a velocity of 2.90x108ms-1. Asit enters a magnetic field of strength 8.05T (fieldis perpendicular to ion’s velocity vector) itexperiences a force of magnitude 3.75x10-9N.What is the magnitude of the charge on the ion?

Remember that for full marksin calculations, you need to show

FORMULA, NUMERICAL SUBSTITUTION,APPROPRIATE PRECISION and UNITS


Longer Response QuestionsMark values shown are suggestions only, and are togive you an idea of how detailed an answer isappropriate. Answer on reverse if insufficient space.

6. (5 marks)Explain why the apparent behaviour of cathoderays caused debate as to whether they werecharged particles or electromagnetic waves.

7. (6 marks)Two parallel charged platesare 6.00cm apart, in vacuum,and have a potential difference of 30.0Vbetween them. An electron (Qe = -1.60x10-19C)is located between the plates.

a) Find the magnitude of the electric fieldbetween the plates.

b) Calculate the force that will act on theelectron due to this field.

c) At what rate will the electron accelerate?(electron mass = 9.11x10-31kg)

8. (8 marks)An alpha particle (Qa = + 3.20x10-19C)is about to enter amagnetic field ofstrength 5.22T at avelocity of 2.95x103ms-1.a) Find the magnitude and (initial) direction ofthe force due to the magnetic field it willexperience.

b) A pair of charged plates (not shown in thediagram) are arranged so that the force due tothe magnetic field will be exactly cancelled outby the force due to the electric field.Sketch where the plates need to be to do this,and indicate the type of charge on each plate.

c) If these electric plates are 10.0cm apart, whatvoltage must be applied to exactly cancel themagnetic deflection?

9. (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.

9



Worksheet 4 Test Questions section 1 Student Name...........................................

Multiple Choice1. The “Maltese Cross” cathode ray tube provesthat 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 through an electric field directed as shown inthe diagram. E field

This is part of an experiment in which the beam will simultaneously cathodepass through a magnetic field. rays

The arrangement and strengths of the 2 fieldsis such that the effects will cancel out so thebeam travels through without deflection.

In which direction must the magnetic field bedirected in order to achieve this?A. into the pageB. up the pageC. to the leftD. out of the page

3. Which of the following diagrams correctlyshows the electric field between point chargesand/or charged plates?

4. Thomson’s famous cathode ray experimentwas able to get 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 the“electron gun” part of a TV picture tube, the firstthing to change would be: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.

++

- ++

A. B.

C. D.

-

- -

++ ++

++++

----

The Radio Experiments of HertzBy the 1880’s, the theory of electromagneticradiation (EMR) had been around for 20 years,but no-one had found proof that these wavesexisted. Until, that is, the famous experiment ofHeinrich Hertz in 1887.

Using the familiar “induction coil” to producesparks across a gap, Hertz showed that someinvisible waves were being produced... he haddiscovered radio waves.

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... thespeed of light!

This was powerful evidence supporting thetheory that light was just one of a wholespectrum of Electromagnetic waves that hadbeen predicted earlier.

In recognition of Hertz’s contribution to ourknowledge of waves, the unit of wave frequency(Hz) is named in his honour.

Within another 20 years, radio was being usedfor long-distance communications using morsecode. Within 100 years the world was blanketedwith radio transmissions for communicationand entertainment.

Investigating Radio WavesYou may have done some simple studies inthe laboratory, such as:

By adding a “tapping key” switch to thetransmitter circuit, it is easy to sendmessages to the receiver in the form of “dots-and-dashes” of static noise.

10

2. FROM RADIO to PHOTOCELLS: QUANTUM THEORYkeep it simple science

®



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rreecceeiivviinngg lloooopp

How did Hertz measure speed of the radio waves?

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

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

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

at the speed of light.

What Hertz Failed to InvestigateIn one of his many experiments with the newwaves he had discovered, Hertz found that his“receiving loop” became more sensitive andsparked more if it was exposed to otherradiations coming from his transmitter.

He didn’t realise the significance of thisobservation, and failed to follow up on it.

We now know (with perfect hind-sight) that hehad produced the “Photoelectric Effect”:

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

This Photoelectric Effect was exploited in the20th century to develop the technology ofphotocells and solar cells.

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SolarCells

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The induction coil’shigh-vvoltage

sparking producesall sorts of EMR,including radio,light, UV & even

X-rrays.

11

Black Body RadiationIn a previous Preliminary topic (“CosmicEngine”) you learned about the way that energyis radiated from hot objects. A “perfect” emitterof radiation had become known as a “black-body”...

It was well known that as a “black body” becamehotter, it not only emitted more energy as radiation,but that the wavelength of the peak of the radiationbecame shorter, and frequency became higher.

The problem was that the standard Physicstheories of the time could not explain the shape ofthese graphs, which were obtained fromexperiment.

Plank’s Quantum TheoryIn 1900, Max Plank proposed a radical new theoryto explain the black body radiation. He found thatthe only way to explain the exact details comingfrom the experiments, was that the energy wasquantised: emitted or absorbed in “little packets”called “quanta” (singular “quantum”).

The existing theories of “classical” Physicsassumed that the amount of energy carried (say)by a light wave could have any value, on acontinuous scale. Plank’s theory was that theenergy could only take certain values, based on“units” or quanta of energy.

It’s the same as with matter: The smallestamount of (say) carbon you can have is 1 atom.Then you can have 2 atoms, 3 atoms and so on,BUT you cannot have 1/2 atoms of carbon... thematter is quantised, with whole atoms as theminimum “quantum”. Well, says Plank, energyis the same!

Plank’s Quantum Theory proposed that theamount of energy carried by a “quantum” oflight is related to the frequency of the light:

Problems with Classical PhysicsAt the same time that Plank was proposing hisQuantum Theory to explain the Black Bodyradiation details, the “Photoelectric Effect” (thatHertz had observed but failed to study) wasbeing investigated by others.

Experiments on the photoelectric effect wereproducing results that could NOT be explainedby the existing theory of light. For a century ormore, light had been accepted as a wave. Thisexplained its reflection, refraction, interference,and many other phenomena.

However, the photoelectric effect experimentswere giving results that suggested light wasbest explained as a stream of particles... thiscould 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 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 thislight?

Solution:a) c =λλ.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 longerWavelength of Radiation

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hhoottoobbjjeecctt““ppeeaakk””

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BLACK BODYRADIATION

CURVES

12

Einstein and Quantum TheoryIt was Albert Einstein who came to the rescueand neatly combined Plank’s Quantum Theorywith the classical wave theory of light, in a waythat solved all the apparent conflicts, andexplained the Photoelectric Effect as well!

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

• Light is a wave, but • the energy of the wave is concentrated in little“packets” or “bundles” of wave energy, nowcalled “Photons”.• Each photon of light has an amount of energygiven by E = h.f, according to Plank’s QuantumTheory.• When a photon interacts with matter, it caneither transfer all its energy, or none of it... itcannot transfer part of its quantised energy.

Einstein’s model for light involves a “duality”...light must have a dual nature. Many of itsproperties are wave related; e.g. ability toreflect, refract and show interference patterns.In other cases, especially when energy transfersare occurring, the light photons are like littleparticles. This explained the Black BodyRadiation curves, and the weird features of thePhotoelectric 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 testEinstein’s predictions, and the results agreedwith his predictions precisely!

This was confirmation that the photon theory oflight, and the quantum theory of energy wereboth correct. Einstein was awarded the NobelPrize for Physics in 1921, for his contribution tounderstanding the Photoelectric Effect.

Applications of the Photoelectric Effect

Solar CellsSolar Cells (or “photovoltaic cells”) are deviceswhich produce electricity directly from lightenergy. They are very familiar in the populargarden lights which need no wiring or batteryreplacements.

During the day, the solar cell(s) charge up asmall re-chargable battery.

At night, the battery provides electricity to alow-power garden lamp.

More importantly, solar cells hold the promiseof cheap, efficient, environmentally-friendlyelectricity production. Already they are used inremote areas (see sketch on front page) and inspecial situations, such as power for orbitingsatellites.

Solar cells produce electricity from thePhotoelectric Effect:Light photons falling on the cell give up theirquantum of energy to electrons in a sandwich ofsemiconductor material, called a “p-n junction”.The energy gained by electrons causes them tobe emitted so that they travel through thesemiconductor structure and create a potentialdifference across it. This voltage causes acurrent to flow in the electrical circuit.

PhotocellsA photocell is a device which can detect andmeasure light. Photocells are used in light meters(photography), “electric-eyes” and a variety oflight-measuring scientific equipment, such asphotometers.

Once again, the photoelectric effect is involved.When a photon of light strikes the receivingsurface, its energy causes emission of anelectron, which is collected on a nearby anode.

A sensitive electric circuit is able to measure thelevel of electron emission, and this gives ameasure of the amount of light being received.




LLiigghhtt iiss NNOOTT

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




13

Assessment of Einstein’s Contribution to Quantum Theory“Assess” means to measure or judge the value of something. The syllabus requires you

to assess Einstein’s contribution to the Quantum Theory in relation to Black Body Radiation.

To begin with, you might note that Einstein did NOT think up the Quantum Theory... MaxPlank did that in 1900. However, it seems that Plank invented the quantum idea purely asa mathematical “trick” to explain the Black Body Radiation curves. Plank never proposedthat the quanta might give light a particle-like nature. Plank never suggested that the old

ideas of “classical” Physics might need changing.

It was Einstein who did that! His “particle-wave” (photon) idea combined Plank’s QuantumTheory with the classical idea that light is a wave. This totally new way to look at things

was one of the turning points of modern Physics, and set other scientists off into new andinnovative 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!

Is Science Research Removed from Social & Political Forces?Einstein was German-born, but became aSwiss citizen, and later American. In WW Ihe (and only 3 others) signed an anti-wardeclaration. He spent the war in neutralSwitzerland, lobbying for peace and an endto war. In the 1930’s he was forced to fleeNazi Germany because he was of Jewishdescent. In America, he fought against thedevelopment of the atomic bomb(developed directly from his own theories)and was appalled when it was used againstJapan in 1945.

Einstein believed that Science is a processthat should work for peace and the good ofall people, and not be involved in thepolitical & social forces that come and go.

Who was right? There is no correct, norsimple, answer to that. You must form yourown opinion... just be sure you have aninformed opinion.

In World Wars I & II, Science and scientists played amajor role in research and development of newweapons and war technologies. Some examplesinclude:

• 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 thatit was his duty to help his country fight a war. Hegladly contributed to weapons research in WW I, andleading up to WW II he was the director of the mainScientific Institute in Nazi Germany. Plank’s outlookseems to have been that Science is part of thepolitical & social structure, and must take an activerole in it.

Atom-bomb damageHiroshima, Japan

Einstein,1905


In 1887, Heinrich Hertz discovereda)....................... waves. His experiment involvedhigh voltage from an b)................................ coilwhich produced c)..................... across a gap.The sparking produced radio waves which hedetected with a d)............................................ inwhich a small gap also sparked. He was able toshow that the new radiations showed typicalwave properties such as e)............................ and............................ Hertz was also able to measurethe f)............................. of the waves, and show itwas equal to the speed of g)..............................He also produced evidence of theh)........................................ Effect, but failed toinvestigate it further.

Meanwhile, other researchers had studied theway energy is emitted from hot objects. The“i)................................ Radiation” curves showeda shape that could not be explained by theaccepted theories. In 1900, j)........................proposed the “k)............................. Theory” toaccount for the problem. The basic idea of histheory is that the energy of light (or other EMR)is “l)............................” the same way thatmatter is. The minimum quantity of matter is onem)........................, and fractions cannot occur.Plank proposed that the energy of EMR is thesame, and that the amount of energy carried byone “n)......................” is related to theo)................................ of the wave.

4. To cause emission of an electron from thesurface of a certain metal requires the electronto gain a minimum of 2.38x10-20J of energy.a) Find the frequency and wavelength of thephoton of EMR which carries this “threshold”amount of energy.b) What happens if the electron is struck by aphoton with a longer wavelength than this?c) What will happen if the electron was struck bya photon of higher frequency than calculated in(a)?

5.An electron was emitted from a metal surfaceafter being struck by a photon of EMR.The electron left the surface with energy of6.22x10-17J. It firstly had to “use” 9.28x10-19J ofenergy to escape the metal surface. All of thisenergy was gained by interaction with a singlephoton.Find the frequency and wavelength of thephoton.

14



Worksheet 5 Quantum TheoryFill in the blank spaces. Student Name...........................................

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

In 1905, Einstein used Plank’s s)...........................Theory to explain all the difficulties. His ideawas:• Light is a wave, but the energy is concentratedin “bundles” called “t)................................”• Each bundle carries a u)............................ ofenergy, as described by Plank’s theory.• When a photon interacts with matter, it caneither transfer v)............... of its energy, orw).................... of it, but cannot transferx).....................................................

This idea allows light to have its “waveproperties” such as y)......................................,................................. and .......................................,but to also sometimes show z)............................-like properties when it transfers energy.Based on his theory, Einstein made certainmathematical aa)............................... regardingthe ab).................................. Effect. These wereconfirmed by ac).................................. in 1916.This confirmed Plank’s ad).............................Theory, and explained all the “problems” withae).............................. ...................... radiation &the af).................................. Effect.

Worksheet 6 Practice ProblemsQuantum Theory Student Name..........................................(Plank’s 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 carriedby a photon of

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

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

Multiple Choice1. Which of the following best describes theoutcome of Hertz’s famous experiments of1887?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 speedof light.

2. According to “Quantum Theory”, if youcompared the energy of 2 photons of light andfound that one had more energy than the other,then the one with more energy must have:A. more mass.B. longer wavelength.C. higher frequency.D. a higher velocity.

3. The “Photoelectric Effect” involves:A. emission of electrons that have absorbed a

quantum of energy from a photon.B. emission of a photon of light that has

absorbed the excess energy from a falling electron.

C. using photographic film to get an image of x-ray interference patterns.

D. using an electrical induction coil to causesparks in a separate receiving coil or antenna.

4. According to Einstein, light often behaves likea wave, but sometimes acts like a particle. Aphenomenon in which the particle nature of aphoton 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.


5. (4 marks)As part of your studies you have carried out aninvestigation to demonstrate the productionand reception of radio waves.Describe briefly how you did this.

6. (6 marks)Two different photons of light have wavelengthsof 5.00x10-7m (photon P) and 2.40x10-8m(photon Q).Qualitatively (no calculation required) compareP & Q’s:a) speed

b) frequency

c) energy

Explain your answers in each case.

7. (4 marks)For an electron to escape from the surface of aparticular metal, it needs to absorb a minimumof 6.75x10-19J of energy. Calculate the a) frequency

b) wavelengthof a photon with just enough energy to causethis.

8. (3 marks)Identify the contribution made by Einstein toQuantum Theory.

9. (4 marks)a) What is the “photoelectric effect”?

b) Summarize how this effect is used in a “solarcell”.

15





Revision of Atomic StructureAfter Thomson identified the electron as aparticle present in all atoms, it didn’t take longfor scientists to figure out the details of atomicstructure. You are reminded of the basic modelof a typical atom:

Electrical 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.

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

If an electric field is present (due to a voltage beingapplied) billions of electrons begin moving in thesame direction... an electric current is flowing, andwe say the metal is a good Conductor.

In other solids such as plastic or glass, the outervalence electrons are more strongly attracted totheir own atom, and cannot easily escape from it,to move from atom to atom. We say these thingsare poor conductors, or good Insulators.

Band Structure TheoryThe explanation just given for conductors andinsulators is OK, until you find out about“Semiconductors”. Elements such as Siliconand Germanium have a number of “strange”properties including being rather poorconductors of electricity until given a little jolt ofenergy. Then, suddenly they become quite goodconductors.

To understand semiconductivity, you need tolearn about “Band Structures”.We have known since the early 20th century thatthe electrons around an atom can occupydifferent “orbits” or energy levels surroundingthe nucleus. These energy levels are“quantised” (Quantum Theory applies) so theremay be “forbidden energy zones” betweenthem. An electron cannot exist in this “fobiddenzone” because the energy level there does NOTcorrespond to a whole quantum.

16

3. FROM ATOMS to COMPUTERS: SEMICONDUCTORSkeep it simple science

®



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This ability, called “Semiconductivity”,allows these materials to act as

electrical switches, turning electricalcurrents 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. Tojump up over a “forbidden zone” they must haveenough energy to achieve the quantum energylevel required to occupy the next band.

In any atom in its “rest state”, the highest bandoccupied by electrons is the “Valence Band”. Ifan electron has enough energy to get to theunoccupied levels above there, the electron iseffectively free to “wander off”. If an electricfield is applied, the electron becomes part of aflowing current, and the substance isconducting electricity.

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

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Structureof an ATOM -

Electrons in orbit at different“Energy Levels”

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17

Conductors, Insulators &Semiconductors

In terms of “Band Theory”, the difference inconductivity between different substances issimply the relationship between the ValenceBand 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 conductionband at any time, so the solid array of atoms is agood conductor at all times.

In an insulator, such as plastic, the electrons cannever achieve the conduction band unless they aregiven a huge boost of energy. At normaltemperatures and voltage levels, the substance willnot carry a current.

A semiconductor, like Silicon, will not normallycarry current, because electrons lack the energy tojump the “forbidden energy gap”. However, if thetemperature is increased, and a voltage applied,there comes a point when electrons jump the gapin great numbers, and the substance suddenlyconducts very well indeed.

This effect does not occur at room temperatureunless the semiconductor substance is “Doped”.

Doping a Semiconductor“Doping” means to add a very small quantity ofa different type of atom to an otherwise puresolid lattice of semiconductor atoms.

Conduction of Electrons & HolesNormally we imagine that an electric current iscomposed of a flow of negative electrons. However, in asemiconductor, when an electron jumps out of thevalence band and flows off somewhere, it leaves behinda “hole” in the valence band. This hole, is a space thatan electron from elsewhere can jump into.

Imagine a line of atoms in a semiconductor lattice:

Now imagine a sequence of movements in which thenext electron in the valence band has enough energy tojump into the 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 tothe right and the hole flows to the left.

In fact, in terms of electrical energy, it makes nodifference whether the current really is negativeelectrons going one way, or “holes” going theother way... either way, it constitutes an electriccurrent. The holes are considered as positivelycharged spaces (relative to the electrons) andso the flow of positive holes may be thought ofas genuine “Conventional Current”.

So, there is another way to “Dope” asemiconductor. The diagram on the left showsthe use of atoms with an “extra” valenceelectron. The other way to do it is to use atomswith only 3 valence electrons, creating extra“holes” in the lattice.

Conduction Band

Valence Band

Conduction Band

ForbiddenEnergy gap

ValenceBand

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DOPING increases the conductivity of the lattice

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

18

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

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

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

History: Electronics & ComputersThe concept of a machine to carry out highspeed calculations and “logical” operations hasbeen around for centuries. Prior to the 20thcentury, any such device had to be mechanical,using “clockwork” gears and so on. There weresome notable successes with control devicesfor weaving looms, and mechanical “addingmachines”, but applications were very limited.

During World War II the first electroniccomputers were built (in tight secrecy) to helpdecode enemy radio messages. Instead of gearsand dials, the “Collosus” computer usedthermionic valves to electronically switchcircuits on and off, to store and manipulate data.

Invention of the TransistorThermionic valves had been widely used inradios for some years and were vitalcomponents of the new industry of television.

Valves were also important in the switching ofconnections in telephone exchanges, where thegrowing communication demands requiredautomatic dialing and connection technology.(The original system involved human“operators” manually plugging wires intosockets to connect phone calls.)

However, the valve-based technology wasproving too slow, too unreliable and tooexpensive for the booming telephone industry.The major U.S. phone company “BellTelephone” set its scientists the task ofresearching new materials and processes toreplace the valves.

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

Because of the properties of the semiconductor(conductivity that can be switched on and off)the transistor can do the same job as thethermionic 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: Cathode Ray Tubes“Thermionic” refers to the way these CRT’s would

emit many electrons from the cathode (andthereby carry a current) when the cathode became

hot. Once “warmed up” the valve can act as anelectronic “switch” in a circuit, when the voltage

to the anode is varied.

Characteristics:• relatively large &

expensive

• consume relatively largeamounts of electricity

• produce large amountsof “waste” heat

• although faster thanmechanical switches,

valves are slow-acting bymodern standards

• require time to “warm up”

• have a limited lifetime,and can “burn out” like alight bulb. Therefore their

reliability is low, and maintenance needs are high.

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

10 c

m

Despite these limitations,“Collosus” was very

important in helping to win the war.

19

Silicon v GermaniumTo make semiconductor material with thedesired conductivity properties, it is necessaryto firstly prepare extremely pure samples, thenadd minute amounts of the “doping” chemical,and finally grow crystals of the semiconductorfrom the molten material in a furnace.

The original transistors were made fromGermanium because the technology to producecrystals of the pure element was already known.However, Germanium is a rare element, whereasits close “sister element” Silicon, is one of themost abundant elements on Earth.

By the 1960’s, the technology to obtain purecrystals of Silicon had been developed, andbecause Silicon is so abundant and thereforecheaper, it quickly replaced Germanium.Silicon’s electrical properties turned out to bebetter too. For example, it held itssemiconductive properties constant over awider range of temperatures.

Also in the 1960’s, the technology of thecomputer began to emerge for financial andcommunication uses. The “solid-state”transistor technology allowed a computer to bebuilt to fit a table-top, rather than fill a room.Every teenager had a brick-size “transistorradio”, in the same way that in this decadeeveryone has an MP3 and a mobile phone thesize of a matchbox.

The miniature“integratedcircuit board”led to thetechnology ofthe “siliconchip” wherethousands, andnow millions of transistor-equivalents can beprinted microscopically in the space of apostage stamp... a “microchip”.

In the 1980’sthe first cheapPC’s (personal

computers)could processa magnificent2x103 “bytes”of information.

Twenty years later, these notes are being typedon an even cheaper PC which can process2x109 bytes, (2GB). The computers havebecome a million times more powerful!

Assessment of Impacts of the Transistor on Society

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

Fire, 500,000 years ago. Fire transformed human society because of itspower to warm people, cook food and protectfrom predators.

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

Metallurgy & the Industrial Revolution,which led to new tools, machinery, massproduction, urbanisation, and mass transportsystems.

The transistor helped create the “Information & Communication Revolution”, which is still developing today. Electroniccircuits, using microchips, are the basis of allthe computers which allow:

• instant access to (virtually) all the information on the planet via the internet.

• instant access to money from your bankaccount from (virtually) any town or cityanywhere in the world.

• instant communication via your mobile phoneto and from (virtually) anywhere.

Computers are the key to the global economyand mass consumerism which keeps thingcheap through mass production & distribution.Computers keep track of the billions of businesstransactions that feed us, clothe us, entertainus, transport us and service all our needs.

Like it or hate it, (some people think we shouldhave stayed in the trees) the modern worldcould not exist without the invention of thetransistor!




Computer “motherboard”

a)...................... orbit around the nucleusof atoms at various b)................... levels.Basically, a substance will be anelectrical conductor if c)..........................can move fromd)................................................... freely.If electrons cannot do this at all, thesubstance is an e).....................................

A “semiconductor” is a substancewhich has very low f)................................until its electrons are given just a littleenergy. Then, at a certain point, itsuddenly becomes g)..............................This allows electrical circuits to beh)................................. on and off, and isthe basis of modern i)...........................and j).................................

The best explanation ofsemiconductivity involves “k)........................................ Theory”, summarized asfollows:• the highest energy level in an atomthat has electrons in it, is called thel).................................. band.• any further (unoccupied) levels abovethis are called m)......................................bands.• If an electron has enough energy to getto a m).............................. band, then it isfree to flow, and form an electricn).......................................

However, between the bands there maybe “forbidden” o)......................................The energy levels are quantised, so a“forbidden” level is where the energy isnot equal to a whole p).............................

In a conductor, the q)...................... bandand r)...................... bandss)................................... each other. Thismeans electrons can freely enter theconduction band and t).............................can flow through the substance.

In an u)....................................., thesebands are separated by a widev).............................................. so thatelectrons can never reach thew)................................... band.

In a semiconductor, the valence andconduction bands are separately by ax)........................ gap. In the “rest” state,electrons cannot get across, and thesubstance does not y)..............................However, it only requires a slightincrease in energy and suddenly manyelectrons z)................. the gap and thesubstance begins aa)...............................

The semiconductor material can bemade more sensitive and conductive ifab).......................... quantities of otherelements are added to the atomic lattice.This is called “ac..............................” thesemiconductor.

Semiconductors can carry electricity in2 ways: by the flow ofad)........................... which have reachedthe conduction band, or by the flow of“ae)....................” left behind bydeparting electrons.

If a af)................................. is doped withatoms with 5 valence electrons, thisresults in ag)......................... in the latticeto carry the current. This is an “ah).......-Type” semiconductor.

If it is ai)......................... with atoms with onlyaj)............ valence electrons, this creates extraak).................. in the lattice to carry current. Thisis a “al)........-Type semiconductor.

Before semiconductors, electronic switchingwas done by am)...................... valves. Thesewere an).............. ................. tubes. Theao).......................... was invented to replacethese valves. Compared to a valve, a transistoris• 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 laterreplaced by ax).......................... because it ismore ay)......................... and a lotaz)....................... Miniaturization of electronicson “silicon ba)....................” has led to thedevelopment of “bb)...................................”which are at the heart of all modern computers.

20




COMPLETED WORKSHEETSBECOME SECTION SUMMARIES

Worksheet 8 From Atoms to ComputersFill in the blank spaces. Student Name...........................................

Multiple Choice1.According to “Band Structure Theory” ofelectrical conductivity, the “valence band” andthe “conduction band” 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.

2.Which line of information below, best describesa “p-type” semiconductor?

Valence of atoms Current mainly used to dope lattice carried by

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

3.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.

4.The original transistors were made fromGermanium, but the technology later switchedto 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 couldn’t be miniaturised

as well.


5. (5 marks)In relation to the “Band Structure Theory” ofconductivity, a) what is meant by the “valence band” of anatom?

b) what is meant by the “conduction band” of anatom?

5. (cont.)c) explain the difference between

conductorsinsulatorssemiconductors

6. (5 marks)Compare and contrast a “p-type”semiconductor and an “n-type” semiconductor.

7. (4 marks)Describe the differences between a solid stateand thermionic device in terms of structure anddiscuss why solid state devices replacedthermionic devices.

8. (4 marks)Assess the impact of the invention of thetransistor on society, with particular referenceto their use in microchips.

21





Investigating Crystal Structures... Bragg and Son

The regular shapes of crystals (such as salt) hadlong been assumed to be due to a regulararrangement of the atoms or ions in a lattice-likestructure. However, until the early 20th century,there was no way to prove or confirm this idea.

The discovery of high frequency EMR in the form ofX-rays opened up a new line of investigation. SirWilliam Bragg and his son Lawrence, beamed X-rays through crystals and studied the diffractionpatterns which were formed as the crystal latticescattered the X-rays.

The Braggs were able to analyse theinterference pattern in order to deduce thearrangement of the atoms within the crystal. Forthis, they were jointly awarded the Nobel Prizefor Physics in 1915.

This opened up a whole new investigativetechnique, allowing scientists to probe thestructure of matter as never before. It was X-raydiffraction crystallography, for example, thatallowed the structure of DNA to be determined inthe 1950’s.

Crystal StructuresThanks to scientists like the Braggs, we nowunderstand the atomic-level structure of mostsubstances. You learned previously how asubstance like the semiconductor Silicon is alattice of atoms chemically bonded together:

Crystal Structure of MetalsUnlike silicon, salt and other crystals, metalatoms are not chemically bonded to each otherby the sharing or exchanging of electrons.

You will remember that the outer “valence”electrons in metals are weakly held, and canaccess the “conduction band” at any time. Theresult is that the valence electrons on eachatom are NOT confined to that atom, but freelywander around from atom to atom.

Each metal atom is, therefore, ionised becauseits valence electron(s) are on the loose. Themetal lattice is often described as

“an array of ions, embedded in a sea of electrons”.

This “sea of electrons” shifts and flows freely. Ifan electric field is present, the electrons will allflow in the same direction as an electric current.That’s why metals are all good conductors.

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

Any impurities in the metal distort the shape ofthe lattice and impede the electron flow. Also, asthe ions vibrate due to thermal energy, thevibration causes more collisions amongelectrons, so their flow is resisted. Astemperature increases, the vibrations increasetoo, and that’s why resistance in metalsincreases with temperature.

Logically, if you re-read the previous paragraphand think backwards, you might infer that if youhad a really pure metal, and cooled it right downso that all lattice vibrations stopped, then itwould become a perfect conductor.

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4. FROM CRYSTALS TO SUPERCONDUCTORSkeep it simple science

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Superconductivity!

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Superconductivity in Metals and Ceramics

In 1911, a Dutch physicist managed to cool mercurydown to about 4oK (-269oC) and found that itselectrical resistance dropped to zero.

Over the following years, various other metals werefound to become superconducting at very lowtemperatures. The potential to build electricalgenerators and equipment with zero resistance was avery attractive idea, but the temperatures involved(no higher than about 20oK) were so low that thereseemed no practical way to take advantage.

Then in 1986, Swiss scientists discovered someceramic materials containing rare elements likeYttrium and Lanthanum, which becamesuperconductors at much higher temperatures. Stillcold by human standards, but 100o higher than themetal superconductors, these ceramics had zeroresistance 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 avery short 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 ofsuperconductivity?

The accepted explanation is known as “BCSTheory”, where “BCS” are the initials of the 3scientists who developed the theory in the1950’s.

Imagine part of the solid lattice of positive ionsin a conducting metal or ceramic. As an electron(part of an electric current) approaches, itattracts the positive ions and distorts the crystalstructure slightly:

This distortion concentrates the positive chargein this part of the lattice, and attracts otherelectrons.

In a normal conductor, this distortion leads tocollisions and loss of energy by the flowingelectrons which repel each other... this is thenormal electrical resistance within theconductor.

But in a superconductor below its “transitiontemperature”, something very strange occurs;due to Quantum Energy Effects, 2 nearbyelectrons “pair up” to form what is called a“Cooper Pair”: (Cooper is the “C” in “BCSTheory”)

Due to quantum effects (which are beyond thescope of this Course... KISS Principle) eachelectron of the Cooper Pair helps the other topass through the lattice without any loss ofenergy. This means there is ZERO resistance.

However, at a temperature above the“transition”, the thermal vibrations in the latticekeep breaking up the Cooper Pairs as fast asthey can form. This destroys thesuperconductivity, and the normal electricalresistance of the substance returns.




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

If a disk of superconductor ceramic is chilled belowits “transition temperature”, a small magnet placedclose above it will “levitate”; spinning freely ifprodded, but held up against gravity by unseenforces.

Explanation:As the magnet is brought near, its magneticfield induces currents in the ceramic. Since

there is NO electrical resistance, the currentsflow freely, non-stop and generate a magnetic

field that repels the approaching magnet.

Superconductors will never allow an externalmagnetic field to penetrate them.

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AdvantagesSuperconductor technology

offers

• High efficiency in anyelectrical situation, because

there is no energy loss due to resistance.

• The ability to generateextremely strong magnetic

fields from superconductingelectromagnets.

• Faster operation ofcomputers, since

superconducting switchingdevices could be 10 times

faster than a semiconductortransistor

Limitations• Superconducting metals must

be chilled with liquid helium.This is impractical and

expensive.• New, superconducting

ceramics can be chilled withliquid nitrogen, which ischeaper and much more

practical, BUT these ceramics:• are fragile and 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 thesebecome faster and more powerful every year,

there is a limit to how far they can go.

A superconductor computer could open a wholenew level of enhanced performance due thepossible high speed switching of circuits.

Electricity generation & distribution could bemade much more efficient with

superconductor technology.

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

power lines were superconductors.

Generators lose energy by resistance heating inthe coils needed to produce magnetic fields, andare limited in the strength of the fields they can

produce. Superconducting coils would allowgenerators to be much more

powerful and efficient.

Greater efficiency generally in electricaltechnology would reduce associated

environmental problems, such as Greenhouse gas emissions.



The Maglev TrainThe idea of using superconducting electromagnetsto “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 containconventional electromagnets. On board, helium-chilled super-conducting electromagnets producepowerful magnetic fields.

The fields in the rail and the train repel each otherso that the entire train is levitated 1-2cm above thetrack.

Propulsion and braking is also done magnetically, bythe fields 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 possiblewithout any rail friction, and the low maintenanceand low noise that goes with this. A disadvantageis the very high cost of building the guide rail track.

Experiments have been going on for years inGermany and in Japan. The first truly operational

Maglev now connects the city of Shanghai in China,with its airport 30km away. German built, it cost

US$1.2 billion, and reaches speeds of around 430km/hr.

MAGLEV = MAGnetic LEVitation

ShanghaiMaglevTrain

Using Superconductor Technology

Sir William Bragg, and his son Lawrencebeamed a)..................... through crystals.The waves were b)....................................by the atom/ion array, and formedc)..................................... patterns, whichwere recorded on d).................................film. By measurements of these images,they could deduce the exact structureand geometry within the e).......................

Unlike other crystals, metals have astructure described as “an array off).........................., embedded in a sea ofg).............................” The electrons havefree access to the h).................................band, so the metal is a goodi)...................................... of electricity.There is some j)........................................because of collisions caused by thermalk).................................. of the lattice.

l).......................................... was firstdiscovered in mercury metal which hadbeen m)............................. to atemperature of about n)..........................In the 1980’s, a new class ofsuperconducting o)..................................were discovered, with “transition”temperatures up aroundp).........................

If a magnet is placed above asuperconductor, it will q).........................,being held up by r)...........................forces. The field is created bys)...................................... in thesuperconductor, induced by theexternal t)........................................Superconductors will never allow anexternal field to u).....................................them.

The explanation of superconductivity is“v)..................... Theory”, which states:• an approaching electron causes aslight w)........................ of the ion lattice.• this concentrates the “density” ofx)............................ charge, whichattracts more electrons.• 2 electrons can form a“y).................................................” whichresults in both of themz)............................................... the latticewithout energy loss, due toaa).................................... energy effects.

The advantages and possibleapplications offered by superconductortechnology include high ab).....................of electrical generation andac).................................., because itcould eliminate energy losses due toad)..........................................Another possiblity is in computers,which could operate ae)............................because a superconductingaf)..................... can work ag).............times faster than a ah).............................

A limitations of superconductortechnology is the need to ai)....................a metal using aj)......................................,which is very ak).............................. and...............................The “higher temperature” al)....................superconductors solve part of thisproblem, but they are am).........................and ............................. and difficult tomake into an)........................................They may also be ao)............................................................ and have a shortlife-span.

One superconductor technology thathas been implemented is theap)................................ train, which usessuperconductor magnets toaq)............................ the train above itsguide rail.

25




COMPLETED WORKSHEETSBECOME SECTION SUMMARIES

Worksheet 10 Crystals to SuperconductorsFill in the blank spaces. Student Name...........................................

Multiple ChoiceThe following diagram describes a famousexperiment carried out by Sir William &Lawrence Bragg.

The diagram refers to Q1 & Q2.1.The radiation used by the Braggs was:A. x-raysB. radio wavesC. ultra-violetD. visible light

2.The pattern captured on the photographic filmwas due to the phenomenon of:A. refraction.B. photoelectric effect.C. polarization.D. interference.

3.Superconductor technology is currently limitedby: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.

4. In a superconductor above its transitiontemperature: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”.


31. (3 marks)Outline the methods used by the Braggs todetermine crystal structure.

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

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

26





Crystal

Photographic film




27

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

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

From IDEASto

IMPLEMENTATION

28

Answer SectionWorksheet 1a) 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

Worksheet 2Electric 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 = -7.82x10-15/-1.60x10-19

= 48.9x104 NC-1.b) E = V/d, so V = E.d = 48.9x104 x 0.00250 = 122V.

3.a) E = V/d = 200/0.0500 = 4.00x103NC-1.b) F = Q.E, so Q = F/E = 2.29x10-12/4.00x103

= 5.73x10-16C.c) No. electrons = charge on speck/ Qe

= 5.73x10-16/1.60x10-19 = 3.58x103 electrons.d) Attracted to -ve plate, therefore speck must have+ve charge, therefore must have lost electrons.

4.First find field, from force on electron:

E = F/Q= -5.88x10-17/-1.60x10-19

= 368NC-1.Now use E = V/d, d = V/E = 40.0/368 = 0.109m.(10.9cm)

5.Find E: E = V/d = 100/0.00500 = 20,000NC-1

Next use: F = Q.E = 9.75x10-10x20,000 = 1.95x10-5N.

Worksheet 3Moving Charges in Magnetic Fields1.F = QvBsinθθ

= -1.60x10-19x3.00x107x4.96x10-2xSin90o

= -2.38x10-13N. (Negative sign indicates direction is opposite towhatever the RH Palm rule tells us)RH Palm rule: if v vector is north, and B vectorvertically up, then F vector is east for a +ve charge.Therefore, for -ve electron, is west.Force = 2.38x10-13 N, west.

2. F = QvBsinθθ, so Q = F/vBsinθθ

= 3.75x10-9/(2.90x108x8.05xsin90o)= 1.61x10-18C.

3.F = QvBsinθθ, so v = F/QBsinθθ

= 7.40x10-15/(1.60x10-19x5.48x10-4xSin25o)= 2.00x108ms-1. (2/3 light speed!)

4.a) F = QvBsinθθ, so B = F/QvSinθθ

= 4.06x10-15/(1.60x10-19x7.80x106xsin90o)= 3.25x10-3T.

b) The force due to the electric field must be equal, so F = 4.06x10-15N.

E = F/Q = 4.06x10-15/1.60x10-19

= 2.54x104NC-1.c) E = V/d, so V = E.d = 2.54x104x0.00800

= 203 V.

Worksheet 41. B 2. D 3. D 4. A 5. B6.Cathode rays were found to have some wavesproperties (e.g. travel in straight lines, fluorescence, exposephoto film) and also to have some particle properties(e.g. carry kinetic energy and momentum). Thiscaused confusion and debate about their nature,finally resolved when Thomson measured thecharge/mass ratio, proving them to be particles.

7. a) E = V/d = 30.0/.0600 = 500NC-1.b) F = Q.E = -1.60x10-19x500

= -8.00x10-17N. (left in diag.)c) F = ma, so a = F/m = -8.00x10-17/9.11x10-31

= 9.78x1013ms-2.8.a) F = QvBsinθθ

= 3.20x10-19x2.95x103x5.22xSin90o

= 4.93x10-15N. Initial direction up the page. b) Plates need tobe as shown indiagram.

c) Force on particle must be equal to (a)F = 4.93x10-15N.

E = F/Q = 4.93x10-15/3.20x10-19 = 1.54x104NC-1

and E = V/d, so V = E.d = 1.54x104x0.100 = 1.54x103V.




+ve

-vve

29

Worksheet 4 (cont)9.a) Electron gun has a cathode to act as a source ofcathode rays (electrons), and a series of anodes toaccelerate the electrons up to the desired velocity.b) The deflection plates are parallel charged plates(or magnetic coils) which deflect the electron beamwith the electric (or magnetic) field, to steer thebeam to any point on the screen. One set ofplates/coils deflect left/right, another set deflectup/down.c) Fluorescent screen glows when struck byelectrons. The image is formed by many glowingspots of fluorescence.

Worksheet 5a) radio b) inductionc) sparks d) wire loop antennae) reflection & diffracted f) velocityg) light h) Photoelectrici) Black Body j) Max Plankk) Quantum l) quantisedm) atom n) quantumo) frequency p) light energyq) electron r) emitteds) Quantum t) photonsu) quantum v) allw) none x) part of its energy.y) reflection, refraction and diffraction (plus others)z) particle aa) predictionsab) Photoelectric ac) experimentad) Quantum ae) Black Bodyaf) Photoelectric

Worksheet 6Quantum Theory1.a) c = λλ.f, so f = c/λλ

= 3.00x108/4.25x10-7

= 7.06x1014Hz.b) E = h.f = 6.63x10-34x7.06x1014 = 4.68x10-19 J.

2. i) infra-red ii) UVc = λλ.f, so f = c/λλ c = λλ.f, so f = c/λλ

= 3.00x108/5.45x10-6 = 3.00x108/5.45x10-9

= 5.50x1013 Hz = 5.50x1016 HzE = h.f E = h.f

= 6.63x10-34x5.5x1013 = 6.63x10-34x5.5x1016

= 3.65x10-20 J. = 3.65x10-17 J.Comparison: UV photon carries 1,000 times moreenergy

3.a) E = h.f, so f = E/h = 8.75x10-14/6.63x10-34

= 1.32x1020 Hz.b) c = λλ.f, so λλ = c/f = 3.00x108/1.23x1020

= 2.44x10-12m.(this is extremely high energy, high frequency, shortwavelength EMR in the range of “hard” x-ray orgamma radiation)

4. a) E = h.f, so f = E/h = 2.38x10-20/6.63x10-34

= 3.59x1013 Hz.c = λλ.f, so λλ = c/f = 3.00x108/3.59x1013

= 8.36x10-6 m.

4. (cont)b) A longer wavelength photon would have lowerfrequency, and therefore less energy. Since thiswould be below the “threshold” energy for emissionof an electron, no emission would occur.c) A higher frequency photon will transfer all itsenergy to an electron. The “threshold” energy is usedto “escape” the metal surface, while any excessbecomes the kinetic energy of the electron.

5.Total energy of the photon = 6.22x10-17 + 9.28x10-19

= 6.31x10-17J.Frequency, f = E/h = 6.31x10-17/6.63x10-34

= 9.52x1016Hz.Wavelength, λλ = c/f = 3.00x108/9.52x1016

= 3.15x10-9m.

Worksheet 71. C 2. C 3. A 4. C5.(many possible answers) A “spider web” of wire wasconnected to an induction coil. When switched on,the fluctuating, high voltage from the coil caused thewire to act as a tranmitting antenna, giving off radiofrequency waves.

This was proven by placing a modern radio receiveron the other side of the room. It picked up loud burstsof “static” noise whenever the coil was on.6.a) both travel at the same velocity (= 3x108ms-1 invacuum) because ALL EMR waves travel at this“speed of light”.b) Photon Q has a shorter wavelength, and thereforemust have higher frequency. c) Photon Q carries more energy, because quantumenergy is proportional to frequency.7.a) E = h.f, so f = E/h = 6.75x10-19/6.63x10-34

= 1.02x1015 Hz.b) c = λλ.f, so λλ = c/f = 3.00x108/1.02x1015

= 2.94x10-7m.8.Quantum Theory was proposed by Max Plank as a“mathematical convenience” to explain the shape ofthe “Black Body Radiation” curves. However, it wasEinstein who used quantum theory to cause a majorchange in the direction of Physics. His “photon” ideachanged “classical” Physics, and caused physiciststo look at the things quite differently.

Einstein’s contribution was to combine Plank’s theorywith classical wave ideas so that phenomena (like thephotoelectric effect) could be explained andunderstood.

9.a) The photoelectric effect occurs when light wavesare absorbed by a metal surface so that the energy ofthe light causes electrons to be emitted from thesurface.




30

Worksheet 7 (cont)9.b) In a solar cell (or “photovoltaic cell”) thephotoelectric effect occurs in a sandwich ofsemiconductor materials called a “p-n junction”. Thelight energy promotes valence electrons to theconduction band in such a way that a potentialdifference is set up across the junction. This cancause a flow of current in an external circuit, so thedevice is a way to generate electricity directly fromlight energy.

Worksheet 8a) Electrons b) energyc) electrons d) atom to atome) insulator f) conductivityg) conductive h) switchedi) electronics j) computersk) Band Structure l) valencem) conduction n) currento) energy gaps p) quantumq & r) conduction & valences) overlapt) currents/electricity u) insulatorv) forbidden energy gap w) conductionx) narrow y) conductz) cross aa) to conductab) minute / very small ac) dopingad) electrons ae) holesaf) semiconductor ag) extra electronsah) n-type ai) dopedaj) 3 ak) holesal) p-type am) thermionican) cathode ray ao) transistorap) much smaller aq) cheaperar) less as) waste heatat) faster au) warm upav) reliable aw) Germaniumax) Silicon ay) abundant/ commonaz) cheaper ba) chipsbb) microchips / microprocessors

Worksheet 91. D 2. B 3. B 4.C5.a) The valence band is the highest “orbit” or energylevel of an atom that has electrons in it (when theatom is in its “ground state”)

b) The conduction band is any energy level above thevalence band. In an atom’s ground state, theconduction bands would normally be empty. If anelectron can reach a conduction band it is effectivelyfree to flow through the lattice of the substance.

c) In a conductor, the valence & conduction bandsoverlap with each other, so that valence electrons canaccess the conduction band at any time, and therebyflow as a current.

cont...

5. c) (cont)In an insulator the valence and conduction bands areseparated by a very wide “forbidden energy gap” inwhich an electron cannot exist because the energyrequired does not correspond to a whole quantum ofenergy. To get to the conduction band, an electronneeds a huge amount of energy, and at normaltemperatures and voltages, this does not happen.

In a semiconductor, the gap between valence andconduction bands is narrow. A small energy input can“kick” and electron up so the substance goes fromnon-conducting to conductor.

6.“Compare and contrast” means to point outsimilarities and differences... be sure to shown both.SimilaritiesBoth types of semiconductor are solid crystals ofsilicon with a lattice structure made up of atomschemically bonded to 4 neighbours. The atoms havea narrow “forbidden gap” between valence andconduction bands, and can switch from being a non-conductor, to a good conductor with a very smallchange in energy. The sensitivity to this “switchingon” can be increased by “doping” the lattice withother atoms.

DifferencesIn “n-type” semiconductors, the lattice has beendoped with atoms with 5 valence electrons. Thisplaces “extra” valence electrons in the lattice andincreases the sensitivity of the substance to carryingcurrents by the flow of negative electrons.

In “p-type” semiconductors, the lattice has beendoped with atoms with only 3 valence electrons. Thisleaves extra “holes” in the lattice and increases thesensitivity of the substance to carrying currents bythe flow of positive holes.7.A Thermionic valve is a cathode ray tube: a glass tubecontaining metal electrodes in a vacuum. Typicallythe valve is 10-20cm in size. A solid state transistor isa “sandwich” of n-type and p-type semiconductormaterial (i.e. doped silicon crystals). A transistor canrange in size from 1-2 cm, down to microscopic layersetched into the crystal in a “microchip”.

Transistors can do exactly the same job as valves,but • are much smaller and cheaper.• use much less electricity, and produce hardly anywaste heat.• are faster, more reliable, and do not need time to“warm up”.For all these reasons, the transistor replaced thevalve in electronics.8.The invention of the transistor has had an enormousimpact on society by making possible thedevelopment of electronics, especially computers,operated by cheap, efficient and miniature“microchips”. This has allowed the development of:• business and financial systems for cheap, efficientoperation of a global economy.• instant access to information, communication andentertainment via the internet, TV, DVD technology,mobile phones, etc.





31

Worksheet 10a) x-rays b) diffractedc) interference d) photographice) crystal lattice f) ionsg) electrons h) conductioni) conductor j) resistancek) vibrations l) Superconductivitym) cooled n) 4oK (-269oC)o) ceramics p) 125oK (approx -150C)q) levitate r) magnetics) currents t) fieldu) penetrate v) BCSw) distortion x) positivey) Cooper Pair z) pass throughaa) quantum ab) efficiencyac) distribution/ transmissionad) resistance heatingae) much faster af) switchag) 10ah)semiconductor/ transistorai) cool/chill aj) liquid heliumak) expensive & impractical al) ceramicam) brittle & fragile an) wiresao) chemically unstable ap) Maglevaq) levitate

Worksheet 111. A 2. D 3. B 4. A5.Sir William & Lawrence Bragg beamed x-rays throughcrystals. The atomic or ionic lattice in the crystaldiffracted the x-rays, which then formed interferencepatterns. These were captured as geometric patternson photographic film. Analysis of the geometry of the“x-ray diffraction pattern” allowed them to calculatethe spacing and geometry of the lattice.

6. BCS theory states that:• an electron (in the conduction band of a conductor)causes a slight distortion of the ionic lattice.• This increases the density of +ve charge in this area,which attracts more electrons.• Normally this causes electron collisions and heatingin a conductor, resulting in “electrical resistance”.• In a superconductor below its “transitiontemperature”, the electrons can form “cooper pairs”which use quantum effects to “tunnel” through thelattice with zero resistance.

7. Possible benefits:• faster computers, because superconducting“switches” are 10 times faster than transistors.• more efficient generation of electricity fromsuperconducting coils producing more powerfulmagnetic fields in generators.• Elimination of restistance heating losses intransmission lines could save energy, and reducecosts and environmental impacts.



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