PHYS3: Ideas and Implementation

1. Increased understandings of cathode rays led to the development of television

EXPLAIN WHY THE APPARENT INCONSISTENT BEHAVIOUR OF CATHODE RAYS CAUSED DEBATE AS TO WHETHER THEY WERE CHARGED PARTICLES OR ELECTROMAGNETIC WAVES

The following properties of cathode rays fitted the wave model Travelled in straight lines: a shadow of an opaque object appeared if the object was placed in the ray’s path Could pass through thin metal foils without disturbing them (Rutherford tested this with gold foil)

The following properties of cathode rays fitted the particle model Deflected by magnetic and electric fields Had momentum (small paddlewheels turned when placed in the ray’s path) Travelled considerably slower than light Left the cathode at right angles to the surface

o A luminous objects, e.g. hot metal plate, emits light in all directions, while a charged particle repelled by the cathode will travel perpendicular to it

______________ Heinrich Hertz performed an experiment that appeared to show that cathode rays were not deflected by

electric fields, and this was used as evidence that cathode rays were electromagnetic waveso Within the tube, cathode rays ionised the gas. These ions were attracted to the plate with the

opposite charge, effectively neutralising the charge on the plate and allowing cathode rays to pass by unaffected

J.J. Thomson observed that cathode rays were deflected in an electric field if the tube was evacuated (gas particles removed to create a partial vacuum)

o Cathode rays deflected towards the positive plate, proving that they were negatively charged particles Hertz’s argument that cathode rays were EM waves because they could penetrate thin metal foils would be

nullified later by a change in the model of the atom

EXPLAIN THAT CATHODE RAY TUBES ALLOWED THE MANIPULATION OF A STREAM OF CHARGED PARTICLES

A cathode ray tube is a glass tube containing two electrodes, from which most of the air is removed by a vacuum pump. A high voltage applied across the electrodes causes cathode rays, streams of negatively charged particles (electrons), to flow from the cathode (negative electrode) towards the anode (positive electrode), with little obstruction from the few remaining gas particles.

When electrons collide with gas particles, the gas is ionised, causing the emission of light (fluorescence) Discharge tubes produce different effects at different air pressures At lower pressures, electrons can accelerate to faster speeds before colliding with gas particles

PERFORM AN INVESTIGATION AND GATHER FIRST-HAND INFORMATION TO OBSERVE THE OCCURRENCE OF DIFFERENT STRIATION PATTERNS FOR DIFFERENT PRESSURES IN DISCHARGE TUBES

PERFORM AN INVESTIGATION TO DEMONSTRATE AND IDENTIFY PROPERTIES OF CATHODE RAYS USING DISCHARGE TUBES:

- CONTAINING A MALTESE CROSS- CONTAINING ELECTRIC PLATES- WITH A FLUORESCENT DISPLAY SCREEN- CONTAINING A GLASS WHEEL - ANALYSE THE INFORMATION GATHERED TO DETERMINE THE SIGN OF THE CHARGE ON CATHODE RAYS

IDENTIFY THAT CHARGED PLATES PRODUCE AN ELECTRIC FIELD

An electric field exists in any region in which an electrically charged object experiences a force. The observation that charged plates exert a force on other charged objects brought close to them indicates that an electric field is produced by charged plates.

DISCUSS QUALITATIVELY THE ELECTRIC FIELD STRENGTH DUE TO A POINT CHARGE, POSITIVE AND NEGATIVE CHARGES AND OPPOSITELY CHARGED PARALLEL PLATES

DESCRIBE QUANTITATIVELY THE ELECTRIC FIELD DUE TO OPPOSITELY CHARGED PARALLEL PLATES

Oppositely charged parallel plates separated by a small distance produce a uniform electric field E = V/d

IDENTIFY THAT MOVING CHARGED PARTICLES IN A MAGNETIC FIELD EXPERIENCE A FORCEa n d DESCRIBE QUANTITATIVELY THE FORCE ACTING ON A CHARGE MOVING THROUGH A MAGNETIC FIELD: F=qv B sin q

If a particle with charge q is moving with velocity v, perpendicularly to a magnetic field of strength B, the particle will experience a magnetic force F, given by F = qvB

If the particle’s direction of motion is at an angle to the magnetic field, then F = qvB sinθ

SOLVE PROBLEM AND ANALYSE INFORMATION USING:F=qvB sin qF=qE

Electric field around a positive charge Electric field around a negative charge

Electric field due to charged parallel plates

and

E=Vd

E = F/qo Magnitude of electric field (NC-1) is the force on a unit chargeo F= qE

E = V/do Proof: E = F/q = Fd/qd = Work/qd = V/d

OUTLINE THOMSON’S EXPERIMENT TO MEASURE THE CHARGE/MASS RATIO OF AN ELECTRON

First, Millikan’s oil drop experiment to determine charge on an electron:

An atomiser sprayed a fine mist of oil drops. Some of these drifted into the electric field, and became charged as a result of exposure to X-rays.

By adjusting the voltage, Millikan could balance the weight force of the oil drops with force upwards due to the electric field

Fg = mg; FE = Eq; mg = Eq; q = mg/Eo To find mass of oil drop [m], Millikan measured terminal velocity of oil drops to determine radius, and

then used the known density of the oil to calculate mass of oil drop He found that the charge on an oil drop was always a multiple of 1.6 x 10 -19C, the charge on an electron

Thomson’s experiment to measure charge/mass ratio of an electron: Cathode rays were passed through two narrow slits to

make a thin parallel beam aimed at the centre of a fluorescent screen. Electrodes were placed to create a uniform electric field that exerted a downward force on the beam. Electromagnets were placed to produce a uniform magnetic field that exerted an upward force on the beam.

Thomson adjusted the strengths of the two fields until the beam passed through both fields undeflected. This showed that the two forces were equal. By equating the expressions for the two forces, Thomson calculated velocity of the electrons

o FE=FBo Eq=qvBo v=E/B

Thomson removed the electric field and measured the radius of the circular path followed by the particles in the magnetic field alone. By equating force due to magnetic field with centripetal force, he was able to calculate that electrons had a charge/mass ratio of 1.76 x 1011 C kg-1

o FB=FC

o qvB=mv2

r

o qm

= vrB

OUTLINE THE ROLE OF: ELECTRODES IN THE ELECTRON GUN, THE DEFLECTION PLATES OR COILS, AND THE FLUORESCENT SCREEN, IN THE CATHODE RAY TUBE OF CONVENTIONAL TV DISPLAYS AND OSCILLOSCOPES

Electron guno Heating filament heats the cathode, releasing electrons by thermionic emissiono The positively charged anode exerts a force on negatively charged electrons, accelerating them along

the tube There may be a few anodes, for focusing and accelerating the beam

Deflecting plateso A pair of parallel deflecting plates produce an electric field that can deflect the beam up or downo Another pair of deflecting plates deflect the beam left or righto Magnetic fields produced the electromagnets (copper windings) may also be used to deflect the

electron beamo These electric and magnetic fields can be used to aim the electron beam anywhere onto the

fluorescent screen Fluorescent screen

o Glass screen is coated with a fluorescent material, which emits light when high energy electrons strike it

Television Displays: 3 electrons guns, one for each of the primary colours (red,

green, blue) Television tubes usually use electromagnetic deflection Fluorescent screen has three different types of coloured

phosphors; each electron gun stimulates its corresponding phosphor

o Phosphorescence continues to emit light for a longer time after being excited, minimising screen flicker

Each image is formed from two passes of the electron beam. The odd-numbered lines are drawn first, then the beam ‘flies’ back to the start and draws the even-numbered lines.

Cathode Ray Oscilloscope (CRO): Uses a cathode ray tube to display an input signal voltage as a waveform Oscilloscopes usually use electrostatic deflection Horizontal deflection of the beam is provided by a time base

o With no input voltage, the oscilloscope repeatedly draws a horizontal line across the screen from left to right

Vertical deflection is provided by the input voltage, allowing voltage to be plotted as a function of time

o If input voltage varies from zero, the cathode ray is deflected upwards for positive polarity or downwards for negative polarity

Controls on a CRO include:o Timebase control to set the speed at which the line is drawn, to select ‘time per division’o Vertical control sets the scale of vertical deflection, controlling amplitude of the trace

2. The reconceptualisation of the model of light led to an understanding of the photoelectric effect and black body radiation

OUTLINE QUALITATIVELY HERTZ’S EXPERIMENTS IN MEASURING THE SPEED OF RADIO WAVES AND HOW THEY RELATE TO LIGHT WAVES

Based on Oersted’s discovery that an electric current creates a magnetic field, and Faraday’s discovery that a changing magnetic field can induce an electric current, Maxwell postulated that oscillating magnetic and electric fields could propagate through space at the speed of light

Maxwell’s theories predicted the existence of electromagnetic radiation other than that of visible light Hertz’s experiments confirmed that electromagnetic radiation of other frequencies exist

Heinrich Hertz used an induction coil to cause rapid sparking between spherical electrodes, across a gap in a conducting circuit

o Maxwell had predicted that a rapidly varying electrical field may generate electromagnetic waves When a loop of wire with a small gap was held near the sparking induction coil but not electrically connected

to it, a spark would jump across the gap in the detecting loop even though it was not connected to a source of electrical current

o The spark frequency in the detecting loop was the same as that of the induction coilo This spark was evidence for electromagnetic waves travelling through space from the induction coil to

the detecting loop. It was thought to result from the electric field vibration of the waveso By examining AC voltage induced in a coil of

wire placed in the path of the waves, Hertz demonstrated that they also carried an oscillating magnetic field

o These waves exhibited reflection, refraction, interference, diffraction and polarisation, proving that they were transverse electromagnetic waves like light.

Hertz examined the interference pattern generated by one wave travelling directly from source to detector, and another wave reflected from a metal plate. By positioning the detector loop at various positions, he was able to obtain an interference pattern, and could calculate the wavelength using the interference pattern.

o By substituting this wavelength, and the known frequency of the radio waves (same as frequency of current in the wave generator) into v = λ x f, Hertz calculated the speed of these radio waves.

The calculated speed was approximately 3 x 108 ms-1, same as the value for speed of light measured by Fizeau.o This proved Maxwell’s theory that electromagnetic waves could exist with many different frequencies,

propagating through space at the speed of light

DESCRIBE HERTZ’S OBSERVATION OF THE EFFECT OF A RADIO WAVE ON A RECEIVER AND THE PHOTOELECTRIC EFFECT HE PRODUCED BUT FAILED TO INVESTIGATE

He observed that a spark between the gap in the transmitter loop caused a spark between the gaps in the detecting loop

o A radio wave caused an electrical disturbance in the receiver He found that the gap in the detector could be made larger and still generate sparks, when the radiation from

the transmitting spark shone directly into the gap in the detecting loop. o A glass panel placed between the source of EM waves and the receiver absorbed ultraviolet radiation

that assisted electrons in jumping across the gap, reducing the maximum spark lengtho A quartz barrier produced no observable decrease in spark length, as quartz does not absorb UV

radiation A charged object loses charge more readily when illuminated by ultraviolet light Irradiating the detector points using another source of UV increased sparking

Hertz thought that UV light was somehow increasing the conductance of air in the detector gap Hertz did not attempt to explain how this phenomenon was brought about, and did not recognise that the

ultraviolet (UV) component of the transmitter spark removed free electrons from the surface of the metal, allowing the discharge to occur across a wider gap

PERFORM AN INVESTIGATION TO DEMONSTRATE THE PRODUCTION AND RECEPTION OF RADIO WAVES

IDENTIFY PLANCK’S HYPOTHESIS THAT RADIATION EMITTED AND ABSORBED BY THE WALLS OF A BLACK BODY CAVITY IS QUANTISED

Black body radiationo Absorbs all incoming radiationo Walls absorb radiation and release

radiation of a different wavelength until an equilibrium situation is established depending on the temperature of the black body

o All radiation entering through the small hole is absorbed by the walls, so radiation leaving the hole is characteristic of the equilibrium temperature that exists in the furnace cavity

Ultraviolet catastropheo Classical wave-theory of light predicted that radiation intensity would increase without limit as

wavelength of radiation decreasedo This meant that intensity of the radiation emitted would approach infinity as wavelength decreased

into the ultraviolet portion of the spectrum; this increase in energy level would violate the Principle of Conservation of Energy

Experimental data from black body experiments showed that radiation intensity curve for a given temperature has a definite peak, passing through a maximum and then declining. The peak wavelength was lower when temperature was higher

Max Planck proposed that the radiant energy exchanged between particles of the black body and the radiant energy field may be treated statistically as if it was exchanged in multiples of a small ‘packet’, a quantum of energy E=hf

o The minimum packet of energy was called a quanta, and is now identified as a photon. The size of each quanta is directly proportional to the frequency of light emitted

o Planck believed that he had invented a mathematical trick to explain the results of black body radiation experiments, but failed to accept the quantisation of radiation until supporting evidence arose

EXPLAIN THE PARTICLE MODEL OF LIGHT IN TERMS OF PHOTONS WITH PARTICULAR ENERGY AND FREQUENCY

Some properties of light are best explained if light is considered to consist of a stream of particles, or discrete packets of energy, called photons

A photon is a ‘packet’ of energy relating to the quantum description of matter. In accordance with wave-particle duality of quantum theory, it is a particle of electromagnetic energy with zero rest mass, and travels at 3 x 108 ms-1 in vacuum.

A photon carries an amount of energy that is directly proportional to the frequency of the radiation. Therefore all photons of light of a particular frequency have exactly the same amount of energy.

o E=hfo Aquantumof energy=Planck ’ sconstant (6.626×10−34 Js)×frequency

e.g. photons of ultraviolet light have higher energy than those of red light

IDENTIFY THE RELATIONSHIPS BETWEEN PHOTON ENERGY, FREQUENCY, SPEED OF LIGHT AND WAVELENGTH: E=hf AND c= fλa n d

SOLVE PROBLEMS AND ANALYSE INFORMATION USING: E=hf AND c=fλ

The energy of a photon is given by E=h× f , where: o E is the energy of the photon in joules (or electron volts) o h is Plank's constant: 6.626 X 10-34 J s o f is the frequency of the light in hertz (seconds-1).

The speed of light is given by the relationship c=f × λ, where o c is the speed of light: 3 x 108 m s-1 o f is the frequency of the wave o  is the wavelength of the wave.

By combining the two equations, E=hcλ

IDENTIFY EINSTEIN’S CONTRIBUTION TO QUANTUM THEORY AND ITS RELATION TO BLACK BODY RADIATIONa n d IDENTIFY DATA SOURCES, GATHER, PROCESS AND ANALYSE INFORMATION AND USE AVAILABLE EVIDENCE TO ASSESS EINSTEIN’S CONTRIBUTION TO QUANTUM THEORY AND ITS RELATION TO BLACK BODY RADIATION

The Photoelectric Effect Photoelectric effect: the release of electrons from a metal surface after absorption of energy from

electromagnetic radiation Wilhelm Hallwachs placed zinc on an insulating stand, and attached it by wire to a negatively charged gold leaf

electroscope. When the zinc was exposed to ultraviolet light, negative charge leaked away quickly from the electroscope

J.J. Thomson enclosed the metal surface to be exposed to UV light in a vacuum tube, and showed that UV light caused the same particles found in cathode rays (electrons) to be emitted

Philipp von Lenard studied how the energy and amount of emitted photoelectrons varied with frequency and intensity of light used.

o Rate at which electrons were emitted could be measured by the electric current produced o Energy of emitted electrons could be measured by charging the collector electrode negatively to repel

electrons. The voltage at which current drops to zero (stopping voltage) represents the maximum energy of emitted electrons.

o o Lenard found that rate at which electrons are emitted is directly proportional to light intensity, and maximum energy of electrons increases with frequency of light illuminating the metal

Classical physics could not explain the existence of a minimum threshold frequency which is necessary for electrons to be emitted from a material.

Einstein’s Explanation of the Photoelectric Effect Light exists as photons, each with energy given by E=hf

o Photons with higher energy correspond to light of higher frequency To produce the photoelectric effect, the energy contained in a photon must be greater than or equal to the

work functiono The work function is the electron binding energy, the energy required to release the electron from the

surface of a particular materialo hf ≥W

If the energy of the photon is greater than the work function, the additional energy of the photon over the work function will provide the kinetic energy of the photoelectron

o E=hf=W +EKo ∴EK=hf−W

Therefore, the kinetic energy which photoelectrons possess is determined by frequency of light source

The number of photons determines intensity of the lighto Therefore, a higher intensity light would have more photons, which causes electrons to be emitted at

a faster rate

Einstein’s Contribution to Quantum Theory, and its Relation to Black Body Radiation The photoelectric effect was impossible to understand in terms of classical wave theory. Einstein’s explanation

of the photoelectric effect contributed to concepts of wave-particle duality in quantum theory, and the photoelectric effect provided evidence for the particle nature of light.

Einstein explained Planck’s work on black body radiation as follows:o Radiation is absorbed/emitted in packets of energy called photons. A photon is the smallest amount of

radiation energy possible at a particular frequency, given by E=hf . A photon cannot transfer part of its energy (all-or-nothing).

o Amount of energy carried by a photon is proportional to its frequency. Intensity of light is proportional to the number of photons.

PROCESS INFORMATION TO DISCUSS EINSTEIN AND PLANCK’S DIFFERING VIEWS ABOUT WHETHER SCIENCE RESEARCH IS REMOVED FROM SOCIAL AND POLITICAL FORCES

Einstein: Einstein believed that science research was inextricably linked to social/political forces Due to his fame, he was able to speak and write frankly about politics. Some of his papers expressed leftist

political opinions about pacifism, socialism and Zionism (supported creation of a Jewish homeland in Palestine) During WWI, Einstein was a pacifist. He signed an anti-war counter manifesto Einstein formed a coalition that fought for a just peace, and for a supranational organisation to prevent future

wars He flouted the ascendant Nazi movement, and tried to be a voice of moderation in the tumultuous formation

of the State of Israel With the rise of Hitler, Einstein emigrated to the United States because he was Jewish Einstein wrote affidavits recommending U.S. visas to European Jews who were trying to flee persecution. He wrote a letter to U.S. President Franklin Roosevelt, urging U.S. development of an atomic bomb before the

Germans did. This effort became the Manhattan Project, which developed the first atomic bombs.o Therefore, he believed that scientific research was linked to social/political forces, and encouraged the

U.S. government to use scientific research to develop the atomic bomb before the Germans. However, he later lobbied to stop nuclear testing and future bombs.

Planck: Planck believed that science research was removed from social and political forces He signed the “Manifesto of 93 Intellectuals” defending Germany’s war conduct. In the turbulent years after WWI, Planck issued the slogan “persevere and continue working” to his research

colleagues. This demonstrates his belief that scientific research must continue independent of social/political happenings.

o He tried the same “persevere and continue working” slogan when the Nazis seized power in 1933, asking scientists who were considering emigration to remain in Germany.

Planck continued his academic career in Berlin, trying to compromise and work within the system while avoiding open conflict with the Nazi regime

o He refused to issue a public proclamation against the treatment of Jewish professors

IDENTIFY DATA SOURCES, GATHER, PROCESS AND PRESENT INFORMATION TO SUMMARISE THE USE OF THE PHOTOELECTRIC EFFECT IN PHOTOCELLS

Photocell: a device that uses the photoelectric effect, including photovoltaic cells, photoconductive cells, and phototubes

Photovoltaic cell A photovoltaic cell (solar cell) converts energy from sunlight into electrical energy In a semiconductor diode where p-type silicon is in contact with n-type silicon, the n-type layer is exposed to

light and the p-type layer is not. A fine metal grid acts as electrical contacts on the light exposed side, collecting photoelectrons emitted from

the n-type silicon surface, and returning these electrons to the p-type silicon through an external circuit to form a DC current

Photoconductive cells A photoconductive cell is made of semiconductor material. Light energy causes electrons to be released from

their valence bonds, increasing the number of mobile electrons in the material and raising conductivity. o The increase in conductivity can be detected by measuring current produced by a constant applied

voltage Used as switches to turn street lights on and off depending on amount of light

energy naturally available Used as light gates to detect when a beam of light from a light source is

interruptedo Counting devices in production lines; alarm systems where an intruder

cuts a beam of light Breathalysers: person breathes a known quantity of breath into a chamber, and

a beam of light passes through it towards a photovoltaic cell. Alcohol particles from breath sample condense and reflect light, reducing intensity of light hitting photovoltaic cell and reducing current produced.

o Therefore, reduction in current produced measures amount of alcohol on the person’s breath

Phototubes Incoming photons strike a cathode in a vacuum tube, releasing electrons

which are attracted to an anode. Thus current flow is dependent on frequency/intensity of incoming light

Used as ‘electric eyes’ to open automatic doors in shopping centres, and turn taps on and off when people wash their hands in public toilets

Photomultiplier tubes also take advantage of secondary emission (electrons striking an electrode cause the release of more electrons) to amplify incoming signals

o Used in scintillation counters to detect levels of nuclear radiationo Used in blood analysis devices to determine concentrations of various components in blood samples,

by measuring transmission of light through the sample

3. Limitations of past technologies and increased research into the structure of the atom resulted in the invention of transistors

IDENTIFY THAT SOME ELECTRONS IN SOLIDS ARE SHARED BETWEEN ATOMS AND MOVE FREELY

In some solids, the outer electrons are very loosely bound to particular atoms. These electrons can therefore move across the entirety of the solid.De Broglie’s wave model of electrons

De Broglie proposed that electrons should demonstrate a wave-like nature Electron orbiting the atom must have a standing-wave pattern of vibration so that its orbit does not

destructively interfere with itself Therefore only energy levels where the electron orbits the nucleus with a circumference equal to a multiple of

the wavelength would be stable The electron can absorb/release energy and change to a different energy level, but this energy transfer must

be of a specific amount, a quantum.

o E=mc2 ; E=hf ;m c2=hf ;mv×v=hf ; pv=hf ; vf= hp; λ= h

mv

o Wavelength of an electron is given by: λ= hmv

COMPARE QUALITATIVELY THE RELATIVE NUMBER OF FREE ELECTRONS THAT CAN DRIFT FROM ATOM TO ATOM IN CONDUCTORS, SEMICONDUCTORS AND INSULATORS

Conduction strength of a material depends on the ease with which electrons are able to move through the crystal lattice

Insulators: atoms in the lattice are held by strong covalent bonds, so there are few/no free electrons that can drift between atoms to conduct electricity

Metal lattices: Valence electrons are delocalised, and are free to move throughout the lattice of positive ions. The large number of free mobile electrons makes most metals good conductors

o Under the influence of an electric field, electrons begin to have a net motion in a direction opposite to the electric field, producing an electric current

Semiconductors have far fewer free electrons than conductorso Certain conditions such as raising the temperature, applying a potential difference, or lighting, can

induce electrons in semiconductors to move into the conduction band

DESCRIBE THE DIFFERENCE BETWEEN CONDUCTORS, INSULATORS AND SEMICONDUCTORS IN TERMS OF BAND STRUCTURES AND RELATIVE ELECTRICAL RESISTANCE

When atoms of any substance are very close together, their highest electron energy levels overlap in a continuous fashion, in regions called energy bands

o The conduction band: electrons are free to move through the material and conduct electricityo The valence band: The energy band in which the outermost electrons are found in atoms. Under the

right conditions, these electrons can be induced to move into the conduction bando There is often a ‘forbidden energy band’ between the valence and conduction bands

In a conductor, the valence band is partly filled, and the conduction and valence bands overlap. This allows valence electrons to easily move along the conduction band, giving the material low electrical resistance.

In insulators, the valence band is completely filled. There is a large forbidden energy band, which makes it difficult for valence electrons to move into the conduction band, giving the material a high electrical resistance.

o Under the influence of a large enough electric field (breakdown voltage), electrons can be given sufficient energy to cross the energy gap

o In an ionic crystal such as sodium chloride, the ions have full valence bands, and hence the material behaves as an insulator.

In semiconductors, the valence band is partly filled, and the forbidden energy band is smaller that that of an insulator.

o Under certain conditions (e.g. higher temperature), electrons in the valence band can gain sufficient energy to cross the gap, reducing the electrical resistance of the material.

o In many materials, such as metals, resistivity increase with temperature because more conduction band electrons collide with cations of the lattice

o In semiconductors, resistivity decreases with increase in temperature, because the increased thermal energy causes more electrons to move from the valence band to the conduction band. Electricity can then be conducted by the movement of electrons, and the movement of ‘holes’ left behind in the valence band.

IDENTIFY ABSENCES OF ELECTRONS IN A NEARLY FULL BAND AS HOLES, AND RECOGNISE THAT BOTH ELECTRONS AND HOLES HELP TO CARRY CURRENT

When an electron in a semiconductor leaves the valence band it leaves a positive “hole”, that is, an atom with one less valence electron than normal. An electron from a nearby atom’s valence band can move and fill the hole, creating a new hole in that valence band.

The creation of holes and movement of electrons to fill them can be considered as a flow of positive charges

Holes act as a positive flow of current in the valence band moving towards the negative potential, and electrons act as a negative flow of current in the conduction band moving towards the positive potential.

The speed of the electron current flowing through overlapping conduction bands is much greater than the hole current moving from atom to atom.

PERFORM AN INVESTIGATION TO MODEL THE BEHAVIOUR OF SEMICONDUCTORS, INCLUDING THE CREATION OF A HOLE OR POSITIVE CHARGE ON THE ATOM THAT HAS LOST THE ELECTRON AND THE MOVEMENT OF ELECTRONS AND HOLES IN OPPOSITE DIRECTIONS WHEN AN ELECTRIC FIELD IS APPLIED ACROSS THE SEMICONDUCTOR

IDENTIFY THAT THE USE OF GERMANIUM IN EARLY TRANSISTORS IS RELATED TO LACK OF ABILITY TO PRODUCE OTHER MATERIALS OF SUITABLE PURITY

Germanium was widely used as a semi-conductor in early transistors because it was easier to produce germanium of suitable purity than silicon

o Relatively rareo Becomes a good conductor when it gets too hot. Hot germanium components allow too much current

to pass through, which can damage electronic equipment

Silicon is more difficult to purify, but it is now widely used as a semi-conductor in transistors because of various advantages

o Very common in Earth’s crust, making it relatively cheap if it can be purified Silicon is a far more plentiful raw material than rare germanium

o Retains its semi-conducting properties at higher temperatures than germanium Can handle higher electric currents before it overheats and its semi-conducting properties are

destroyedo Forms a silicon dioxide layer that can be doped and made into thin layers. The oxide lawyer protects

against oxidationo Processing techniques were developed to produce very pure, single crystal forms

In single-crystal form, molecular structure of silicon is uniform, ensuring consistent properties

DESCRIBE HOW ‘DOPING’ A SEMICONDUCTOR CAN CHANGE ITS ELECTRICAL PROPERTIES

A dopant is a tiny amount (about one part per million) of an impurity added to an otherwise pure crystal lattice to alter its electrical properties

o If the dopant atom has a different number of valence electrons from the semiconductor atoms, extra energy levels can be formed between the valence and conduction bands, reducing the energy gap between valence and conduction bands, and making it easier for these materials to conduct

Intrinsic semiconductors: Semiconducting properties of the material occur naturally without doping of the crystal lattice E.g. Silicon: Each silicon atom has four valence electrons. It fills its valence band by sharing an electron with

each of four adjacent atoms in a covalent bond. Each silicon atom is bonded to four other atoms in a tetrahedral lattice.

o Heating silicon enables some electrons to jump to the conduction band, leaving a hole in the valence band.

Extrinsic semiconductors: Semiconducting properties of the material are modified by addition of dopant atoms The atoms of the doping element need to fit reasonably well into the lattice structure so as not to distort it

and impede electron flow The doping element needs to have either one more or one less valence electron than the semi-conductor itself Doping increases the potential conductivity of the semiconductor (extra electrons or holes to act as charge

carriers)

IDENTIFY DIFFERENCES IN P AND N-TYPE SEMICONDUCTORS IN TERMS OF THE RELATIVE NUMBER OF NEGATIVE CHARGE CARRIERS AND POSITIVE HOLES

N-type semiconductors Formed when a Group 5 impurity atom (e.g. phosphorus or arsenic) is substituted into the silicon crystal

lattice, replacing an atom of silicon. 4 of the 5 valence electrons from the donor atom fill the valence band just like electrons from a silicon atom

would. The extra electron is promoted to the conduction band Extra electrons can carry a negative charge, so a n-type semiconductor has an excess of negative charge

carriers over positive holesP-type semiconductors

Formed when a Group 3 impurity atom (e.g. boron, aluminium, gallium) is substituted into the silicon crystal lattice

A Group 3 dopant atom has only 3 valence electrons, so there is a hole in the tetrahedral structure where an electron is missing.

This hole can act as a mobile positive charge carrier, so a p-type semiconductor has an excess of positive holes over negative charge carriers

Under the influence of an electric field, electrons move towards the positive terminal of the source, into the hole, creating new holes in adjacent atoms

DESCRIBE DIFFERENCES BETWEEN SOLID STATE AND THERMIONIC DEVICES AND DISCUSS WHY SOLID STATE DEVICES REPLACED THERMIONIC DEVICES

Thermionic devices: Prior to the invention of solid state devices, thermionic devices controlled

direction of current flow, converted AC into DC, amplified currents, etc. Valve: a thermionic device in which two or more electrodes are enclosed in a

glass vacuum tube Diode:

o Cathode is heated by current or with a heating filament, causing it to liberate electrons

Electrons liberated are accelerated through the vacuum towards an anode, by the high potential difference, creating an electric current

o If the negative terminal is attached to the cathode, current will flowo If the positive terminal is attached the cathode, and negative

terminal to the anode, current will not flow because the anode is not heated, and so electrons will not leave the anode.

Therefore, such a diode only allows unidirectional conduction, and can convert AC into DC Triode:

o Thomas Edison added a third electrode, the ‘grid’, to make a current amplifier known as a triode. o Positively charged grid draws more electrons from the cathode. These electrons pass through holes in

the grid and accelerate towards the even more positively charged anode. o A voltage placed on the grid has a much larger effect on the electric field in the valve, and can be used

to control anode current (amplification).

Solid state devices are based on the P-N junction:

In a P-N junction, electrons tend to move from the N region to the P

region by diffusion, leaving positively charged ions in the N region. Holes begin to diffuse into the N-type region leaving ions with an extra electron and a negative charge.

Hence, a depletion zone is formed at the junction where electrons and holes recombine. This region is charged negatively on the P side, and positively on the N side. The electric field created by the space charge region tends to repel electrons from the P side, and repel holes from the N side, opposing the diffusion process.

An equilibrium is created between the diffusion process that tends to generate more space charge, and the electric field generated by the space charge that tends to counteract the diffusion. The depletion zone acts as an insulator.

The upward direction in the adjacent diagram represents increasing electron energy. The electric field generated by the space charge means that you would have to supply energy to move an electron up in the diagram, into the p region

Reverse Bias: Because the P side has been made even more negative, the energy gap for electrons to move from the N side to the P side has been made greater, and so no current will flow. Effectively, the external voltage moves negative charge into the P side and positive charge into the N side. This increases the width of the depletion zone, preventing conduction.

Forward Bias: The P side is made more positive, so the energy gap for electrons to move from the N side to the P side has been reduced. An electron can move across the junction and fill the hole near the junction. It can then move from hole to hole towards the positive terminal, which can be described as a hole moving right. Effectively, the external voltage decreases the width of the depletion zone, allowing conduction.

A P-N junction diode allows electric charges to flow in one direction, but not in the opposite direction. When the P-N junction is forward-biased, electric charge flows freely due to reduced resistance of the P-N junction. When the P-N junction is reverse-biased, the junction barrier (and therefore resistance) becomes greater and charge flow is minimal.

Why solid state devices replaced thermionic devices:

A thermionic device contains a cathode that emits electrons only when heated to a high temperature. It requires a separate heating circuit to heat the cathode, which takes time to heat up. A solid state device uses semiconductors to generate a flow of electrons and does not require a heating circuit.

Thermionic devices are bulky, whereas solid state devices can be far smaller Thermionic devices use a lot of power, for heating the cathode and maintaining a high voltage to correctly bias

triodes, requiring large/many batteries. A silicon transistor requires around 0.6V Valves develop a large amount of heat, which may damage the surrounding electronics

o The heat also boiled off the cathode’s metal coating, and the coating would react with gases present in the tube

The cathode is slowly ‘poisoned’ by other elements in the tube, damaging its ability to emit electrons

Thermionic devices require a near vacuum to allow electrons to flow between the electrodes; they are commonly packaged in an evacuated glass tube

o Glass tubes were fragile, and the vacuum seals frequently broke, allowing air into the valveo Solid state devices operate at normal air pressure

Thermionic devices take time to start working as the cathode must first be heated up, whereas solid state devices work immediately.

Therefore, solid state devices are smaller, cheaper, more efficient, and more reliable than thermionic devices. This is why solid state devices replaced thermionic devices in many electronics applications

However, thermionic vacuum tubes are still used in specialised applications Audio amplification: guitarists often prefer tube amplifiers over

transistorised amplifiers for their aesthetic appeal, and warmth of the tone Vacuum tubes are less susceptible than solid-state components to the

electromagnetic pulse effect of nuclear explosions, so they are kept in use in some military applications

Used in high-power radio-frequency transmitters

GATHER, PROCESS AND PRESENT SECONDARY INFORMATION TO DISCUSS HOW SHORTCOMINGS IN AVAILABLE COMMUNICATION TECHNOLOGY LEAD TO AN INCREASED KNOWLEDGE OF THE PROPERTIES OF MATERIALS WITH PARTICULAR REFERENCE TO THE INVENTION OF THE TRANSISTOR

Radio and electronics required the ability to increase the voltages of signals. This could be achieved by vacuum tubes. The tubes however were fragile, lost vacuum slowly, used too much power, produced too much heat, and the electrodes corroded.

Semiconductor crystals had been used as current-rectifiers in radios in the 1930s, but how they worked was not understood. The ease with which these rectifiers could burn out led to increased research into the properties of semiconductors. Russell Ohl discovered the unique properties of the silicon p-n junction in 1940. Lark-Horovitz and Benzer discovered the excellent rectifying properties of germanium crystals, and that adding impurities could enhance the crystal’s current-carrying capacity.

As telephone networks began to expand rapidly in the late 1940s, the unreliability of the vacuum tubes as electronic relays began to be intolerable. Scientists at the Bell Laboratory were set the task of using semiconductors to replace the problematic vacuum tube.

Using improved knowledge of semiconductors from previous research, Brattain and Bardeen built the first transistor amplifier, the point-contact transistor, in 1948. Shortcomings in the point-contact transistor led to William Shockley’s proposal of a junction transistor with npn sandwich structure

Haynes showed that current could flow through a crystal of germanium (required for Shockley’s idea to work). Gordon Teal developed a technique for producing very pure single crystals of germanium (single crystals are better current-carriers than slivers cut from a larger ingot). Gordon Teal and Morgan Sparks invented the first npn junction transistor in the 1950s

These transistors could only amplify very small signals because of the thick middle p-type layer. Greater amplification was needed to effectively transmit communications signals (e.g. telephone) over vast distances. This led Morgan Sparks to improve the crystal manufacture technique. In 1951, he produced single crystals with middle p-type layers thinner than a sheet of paper. This improved the transistor’s performance.

As germanium transistors heated up, too many electrons were produced, reducing functioning. In 1954, Gordon Teal produced a working silicon transistor to remedy this problem.

The Transistor: A transistor is a tiny switch that changes the size or direction of electric current as a result of very small

changes in the voltage across it npn transistor

o When no wire is connected to the base, any electron flow from emitter to collector is stopped because of the p-type base region. Free electrons recombine with holes in the base, changing the base p-region from neutral to negative. This prevents any further flow of current.

o If a wire is connected to the base, and current is applied such that the trapped electrons in the base are removed, then current will flow from emitter to collector. The greater the number of electrons removed, the less negative repulsion there is, and the greater the emitter-collector current.

o Hence, a small emitter-base voltage produces a much larger (amplified) copy of itself in the collector-emitter circuit.

IDENTIFY DATA SOURCES, GATHER, PROCESS, ANALYSE INFORMATION AND USE AVAILABLE EVIDENCE TO ASSESS THE IMPACT OF THE INVENTION OF TRANSISTORS ON SOCIETY WITH PARTICULAR REFERENCE TO THEIR USE IN MICROCHIPS AND MICROPROCESSORS

The discovery of the transistor led to the development of integrated circuits (microchips) Each IC is formed as a single unit on a single crystal of silicon.

1) Wafer of silicon is produced with a silicon dioxide coating, and photoresist2) Light is projected onto parts of the wafer. Light makes the photoresist insoluble in developer

solution. The developer solution is then used to remove any photoresist coating which has not been exposed to light

3) Exposed areas are etched away4) Doping by showering with ions of impurity to produce transistor5) New photoresist is spun on the wafer, and previous steps are repeated to form a three-

dimensional circuit in layers6) Transistors are connected by metal connections

Large-scale integrated circuits (LSI) have > 5 million circuit elements on one square of silicon.

o Connections between components are built into the chip, reducing problems of variable resistance and improving signal transmission

Microprocessors are among the most advanced in integrated circuits. They control computers, mobile phones, digital microwave ovens, and other digital appliances

o Miniaturisation has allowed for faster transfer, storage and processing of information

o This has led to developments in areas such as medical diagnosis and treatment, entertainment, commerce, industrial design, and communications

Impact on transistors on societyo Key active component in practically all modern electronicso Can be mass produced using a highly automated and cheap process (fabrication)

o Transistorised mechatronic circuits have replaced electromechanical devices in controlling machinery

IDENTIFY DATA SOURCES, GATHER, PROCESS AND PRESENT INFORMATION TO SUMMARISE THE EFFECT OF LIGHT ON SEMICONDUCTORS IN SOLAR CELLS

A solar cell consists of a p-n junction with the n region placed on top, closer to the sun. A metallic grid contacts the n region. A back electrode similarly contacts the p region.

Anti-reflective coating is applied to the top of photovoltaic cells to reduce reflection losses The p-n junction creates an electric field, as electrons from the n side move into the p side by diffusion, until

an equilibrium is establishedo At equilibrium, the n side is positively charged and the p side is negatively chargedo This field acts as a energy hill, allowing electrons to flow from the p side to the n side, but not the

reverse When a photon with more than the minimum energy level

hits a cell, its energy is absorbed, freeing electron-hole pairs. The electric field propels that electron to the n side and the hole to the p side. Since an external circuit is provided, electrons will flow through it to the p side

o The potential difference between the n side and the p side allows the electrons to do work

o An external circuit is needed so that negative and positive charge don’t accumulate

4. Investigations into the electrical properties of particular metals at different temperatures led to the identification of superconductivity and the exploration of possible applications

OUTLINE THE METHODS USED BY THE BRAGGS TO DETERMINE CRYSTAL STRUCTURE

The Method: Sir William and Lawrence Bragg proposed that X-rays, because of their short wavelength, could penetrate the

surface of matter and scatter from the atomic lattice planes within the crystal. Therefore, crystal structure could be studied using X-rays

X-rays were produced by allowing high energy cathode rays to strike a metal anode. These X-rays were collimated using parallel plates of metal covered in molybdenum, causing the beam to become parallel.

These rays were directed at a crystal of a metal salt (e.g. sodium chloride NaCl, zinc sulfide ZnS)

A photographic plate was placed in the path of X-rays exiting the crystal. A pattern of bright spots was detected due to the interference of scattered X-rays.

Calculation of the angles between bright spots on the photographic plate allowed the Braggs to determine spacing and arrangement of the crystal

o Calculations can be modelled using reflection instead of scatteringo Maxima (constructive interference) occur when: nλ=2dsinθ

Impacts on Science Provided a method to determine simple crystal structures Direct evidence for the periodic atomic structure of crystals Crucial in determining the structure of important biological

substances such as DNA, and in the development of the transistor/microchip

IDENTIFY THAT METALS POSSESS A CRYSTAL LATTICE STRUCTURE

A crystal lattice is defined by a repeated three-dimensional unit. The basic building block of these crystalline structures is known as the “unit cell” and this “unit cell” repeats

itself over and over to form a lattice. When a pure metal starts to form from a cooling molten state, the atoms arrange themselves in an ordered

geometrical pattern that is repeated over and over again producing a crystalline structure. Metals consist of delocalised electrons in a lattice of positive ions

DESCRIBE CONDUCTION IN METALS AS A FREE MOVEMENT OF ELECTRONS UNIMPEDED BY THE LATTICE

When an electric field is applied, it produces a net movement of electrons in the direction opposite to the field Drift velocity: the average velocity of electrons in a conductor under the influence of an electric field Let the drift velocity be v, density of electrons be p, charge on each electron be q, volume of wire be V , cross-

sectional area be A, length be L, total charge beQ, and current be Io Q=p×q×Vo V=A× Lo ∴Q=p×q× A× L

o v=Lt

o ∴Q=p×q× A×v×t

o I=Qt

o ∴ I=pqAv Current is proportional to drift velocity. If electron

movement is impeded by scattering from the lattice, drift velocity is reduced, and therefore conductivity/current are reduced.

Average velocity of electrons is proportional to strength of electric field. If there is no complete circuit, then electrons will ‘pile up’ due to the electric field. This will create a reverse potential, resulting in no current.

IDENTIFY THAT RESISTANCE IN METALS IS INCREASED BY THE PRESENCE OF IMPURITIES AND SCATTERING OF ELECTRONS BY LATTICE VIBRATIONS

Resistance in metals increase as a result of electrons colliding with impurities which deform the crystal lattice Resistance in metals is also caused by rapid vibrations of the lattice.

o The vibrating lattice collides with free moving electrons, scattering them from their linear progress through the crystal

o Atoms that form the lattice vibrate more as their temperature increases. This is why resistance usually increases with temperature

PROCESS INFORMATION TO IDENTIFY SOME OF THE METALS, METAL ALLOYS AND COMPOUNDS THAT HAVE BEEN IDENTIFIED AS EXHIBITING THE PROPERTY OF SUPERCONDUCTIVITY AND THEIR CRITICAL TEMPERATURES

Material Type Critical Temperature TC (Kelvin)Uranium Metal 0.8Aluminium Metal 1.20Tin Metal 3.69Mercury Metal 4.15Lead Metal 9.20Tin-noibium alloy Metal alloy 18Noibium-aluminium-germanium alloy Metal alloy 21YBa2Cu3O7 (YBCO) Metal oxide ceramic compound 90TIBaCaCuO (TBCCO) Metal oxide ceramic compound 125HgBa2Ca2Cu3O8 (Hg-1223) Metal oxide ceramic compound 133

DESCRIBE THE OCCURRENCE IN SUPERCONDUCTORS BELOW THEIR CRITICAL TEMPERATURE OF A POPULATION OF ELECTRON PAIRS UNAFFECTED BY ELECTRICAL RESISTANCE

Matter can be cooled to near absolute zero using a succession of liquefied gases down to about 4.2K Lower temperatures may be achieved by successive magnetisation and demagnetisation Critical temperature (TC): the temperature at which a metal becomes superconducting Superconductivity describes the state reached in a conductor when resistance to electron movement drops to

zero. o Type I: For some pure metals, superconductivity occurs at temperatures up to 23Ko Type II: Occurs at higher temperatures, in the range of 120K

At temperatures below the critical temperature, lattice effects impeding electron movement change to assisting electron flow

o Comes about by an effect that pairs electrons and allows them to move freely through the material unaffected by electrical resistance (BCS theory)

DISCUSS THE BCS THEORY

The BCS theory (named after its proponents John Bardeen, Leon Cooper and John Schrieffer) explains superconductivity in terms of Cooper pairs and packets of sound waves related to lattice vibrations (called phonons).

At temperatures below the critical temperature, the following occurs:o When a negatively charged electron travels past cations in the lattice, the lattice distorts inwards

towards the electrono This distortion forms a ‘trough’ of positive charges around the electron. We consider that the electron

has emitted a phonon. A phonon is a quantised mode of vibration occurring in a rigid crystal lattice. Phonons

correspond to sound waves in a solid lattice structure.

o A second electron with opposite spin is drawn into the region of higher positive charge density before the crystal lattice can spring back to its normal position

The paired electrons comprise a Cooper pair. The net effect is that first electron emits a phonon and second electron absorbs the phonon

The attractive force exerted by phonons overcomes the electrons’ natural repulsion. If this binding energy is higher than the energy of ‘kicks’ from oscillating atoms in the conductor (which is true at low temperatures), then the Cooper pair will stay together and be unaffected by resistance.

o The electrons in a Cooper pair are separated from the state of normal conduction by a band gap. Their energies are quantized such that they do not have any available energy levels within reach of the energies of interaction with the lattice.

o Cooper pairs are constantly breaking and forming as the material carries current The pair of electrons act more like bosons (force carrier particles) which can condense into the

same energy level If the temperature is higher than the critical temperature, Cooper pairs cannot remain together due to

increased molecular motion. As the movement of electron pairs occurs without a collision, resistivity is zero. Therefore, very narrow wires

can carry large currents. o Current in the conductor produces a magnetic field. The strength of the magnetic field will reach a

point where it will cause the loss of the superconducting state. This limits the current limit of the wire o Critical current density increases if the wire if cooled further below TC

ANALYSE INFORMATION TO EXPLAIN WHY A MAGNET IS ABLE TO HOVER ABOVE A SUPERCONDUCTING MATERIAL THAT HAS REACHED THE TEMPERATURE AT WHICH IT IS SUPERCONDUCTING

When a superconducting material in its normal state is placed in a magnetic field, the magnetic field strength inside the material is almost the same as that outside it.

Meissner effect: When cooled below its critical temperature, a superconductor expels all magnetic flux, i.e. magnetic field inside the material is zero

If a material is placed in a magnetic field when it is in the superconducting state, surface ‘persistent currents’ flow with a depth equal to the penetration depth, to produce a magnetic field that cancels the applied magnetic field inside the superconductor. These currents do not decay with time due to zero electrical resistance

There is an exponential decay of the external magnetic field into the sample over a distance, the penetration depth. The magnetic field is completely expelled from the interior of the superconductor

o Unlike a regular magnet with a fixed North and South pole, a superconductor can create many poles to ensure that all poles are repelled

Therefore, if a small magnet is placed above a superconductor, the persistent currents in the superconductor create magnetic poles which cause repulsion between the magnet and superconductor, causing the magnet to be suspended above the superconductor

PERFORM AN INVESTIGATION TO DEMONSTRATE MAGNETIC LEVITATION

GATHER AND PROCESS INFORMATION TO DESCRIBE HOW SUPERCONDUCTORS AND THE EFFECTS OF MAGNETIC FIELDS HAVE BEEN APPLIED TO DEVELOP A MAGLEV TRAIN

Magnetic levitation provides a frictionless contact with the ground, allowing trains to reach higher speeds with greater efficiency.

o The train is levitated using permanent magnets, electromagnets, or superconducting electromagnets

o Alternating current flowing through propulsion coils on the guideway generates a continually varying magnetic field that moves forward along the track, with the frequency synchronised to match the speed of the train. Attraction/repulsion between the train’s magnets and these magnetic fields accelerates the train forward

A small percentage of the power input is required to maintain levitation, and the main obstacle to higher speeds is air resistance encountered by the train.

o Enormous amount of electrical power needed is an obstacle to its wider use Advantages of Maglev

o Faster than conventional trains ( > 500 km/h)o Minimum of moving parts reduces maintenanceo Because trains don’t touch the guideway, track deterioration

and misalignment over time is not a problemo Faster acceleration and braking due to no friction, greater

climbing capacity, unaffected operation in rain and iceo Energy efficient: for long distance travel, uses about half the

energy per passenger compared to a conventional traino Less air and noise pollution

Disadvantages of Maglevo Strong magnetic fields onboard would make train

inaccessible to passengers with pacemakers or magnetic data storage, necessitating the use of magnetic shielding

o Vehicle must be wheeled for travel at low speeds

Electromagnetic suspension system (EMS):

Electromagnets attached to the train are oriented toward the steel rail from below to provide an attractive magnetic force which levitates the train

The system is inherently unstable because of varying distances between magnets and guideway; as the gap between electromagnet and guideway decreases, the attractive force increases.

o Instability is monitored closely, and computer-controlled feedback systems maintain train at a constant distance from the track

Electrodynamic suspension system (EDS): Train is levitated by repulsive force between electromagnets in the guideway, and superconducting

electromagnets on the train Advantages

o Does not require same degree of computer monitoring/adjustment as EMSo Requires less energy to maintain onboard superconducting electromagnets, as there is no energy loss

due to resistive heatingo Higher train speeds and load capacities than EMS

Disadvantageso Very low temperatures needed

PROCESS INFORMATION TO DISCUSS POSSIBLE APPLICATIONS OF SUPERCONDUCTIVITY AND THE EFFECTS OF THOSE APPLICATIONS ON COMPUTERS, GENERATORS AND MOTORS AND TRANSMISSION OF ELECTRICITY THROUGH POWER GRIDSa n dDISCUSS THE ADVANTAGES OF USING SUPERCONDUCTORS AND IDENTIFY LIMITATIONS TO THEIR USE

Computers: Speed and further miniaturisation of computer chips are limited by resistive heating.

o New superconductive films used as conductors may result in more densely packed semiconductor chips, transmitting information orders of magnitude faster while consuming less power and producing less heat.

SQUIDs (Superconducting Quantum Interference Devices) are sensitive enough to detect the very weak magnetic fields caused by electrical currents in the human brain, and have allowed doctors to develop better images of brain disorders. If implemented into computing, SQUIDS may allow manufacturers to begin the mainstream release of quantum computers

Josephson effect: If two superconducting metals are separated by an insulating barrier, it is possible for electron pairs to pass through the barrier without resistance.

o A tunnelling current will flow across a Josephson junction even in the absence of potential differenceo When a constant potential difference is applied, current will oscillate with a constant frequency

A Josephson junction acts as a superfast switch, which can be advantageous in computers where processing time depends on speed at which signals are transmitted

It has been proposed that devices with Josephson junctions will be necessary to achieve the next level of processing speeds.

Generators and Motors: Using superconducting wires for rotor coils in electric

generators will reduce energy losses due to resistive heating in copper wires, making electricity cheaper Using superconducting wires for the field coils in electromagnets will further reduce energy losses

o Electric generators made with superconducting wire are over 99% efficient, and their size is about half that of conventional generators, making them very profitable for power generation

o Less fossil fuel would be required to produce electricity, and emissions of greenhouse gases and other pollutants from power plants would also be reduced.

Similarly using HTS wire for the field coils in motors will allow the wire to carry significantly larger current with less energy input than copper wires, to generate much more powerful magnetic fields with smaller size

o Using HTS wire for rotor coils will reduce energy losses due to resistive heating, allowing rotor coils to be much smaller while carrying the same current

o Motors without iron cores could be produced, making them lighter and more portableo Increased efficiency of superconducting motors would decrease the demand for electricity

Transmission of Electricity: Electrical transmission lines lose significant amounts of energy due to resistance in the wires Superconducting wires carrying DC current would be far more efficient in terms of energy losses due to zero

resistance, and could be far thinner to carry the same current. Since superconducting cables can have a smaller cross-section, existing conduits will be big enough to cope

with increasing demand, saving the cost of additional conduits. Greater efficiency in generating/distributing power means less fossil fuels must be burnt, leading to lower

emissions and a cleaner environment. Also, the ability to transport electricity efficiently over long distances means that renewable energy sources which are distant from population centres can be harnessed.

Consists of a HTS material wound around a hollow core which carries liquid nitrogen coolant

Superconducting transmission is currently impractical due to the high cost of cooling long distances of superconducting wire, and brittle nature of high-temperature superconductors

o AC current cannot be used because the constant switching of direction causes energy losses and heating

o The cost required to replace existing wires with superconducting wires, and the necessary infrastructure, prohibits immediate conversion to HTS transmission

Fault-Current Limiters: A fault-current limiter uses superconducting materials to limit the fault current when a fault occurs in a power

transmission network Current passes through the superconductor. When a high fault current begins, the superconductor quenches

and becomes a normal conductor. Resistance rises sharply and quickly, reducing the prospective fault current. HTS fault limiters can respond in just thousandths of a second to limit tens of thousands of amperes of current

Superconducting Magnetic Energy Storage (SMES): Uses a large toroidal or solenoid structure constructed of a HTS material, cooled to superconducting

temperatures Electrical energy is introduced into the device as DC current, and it can flow around the device’s circular path

indefinitely without energy loss. When required, it can be retrieved and either converted into AC, or

transported by a superconducting transmission system as DC Allows generators to operate at peak efficiency levels no matter

whether demand is maximum or minimum, and store excess energy as needed.

Magnetic Resonance Imaging (MRI): [Present Use]

How MRI workso A strong background magnetic field lines up atoms in the braino A second magnetic field is turned on and off at a frequency, causing some atoms to line up with this

second field. When the second field is turned off, atoms that were lined up release energy at a frequency that can be detected as they swing back to align with the background field.

o The detected waves are displayed graphically by a computer, allowing imaging of soft tissue To produce the 4 tesla magnetic field, massive solenoid windings with a constant power input and large

energy losses would be necessary. Superconducting materials cooled with liquid helium allow such large magnetic fields to be easily established,

and the currents and magnetic fields are maintained with no further energy input

Other Advantages: Particle accelerators that use superconducting electromagnets are cheaper to run because they use less

electricity to produce the needed magnetic fields.

Limitations: Technical difficulties and costs of sustaining the extremely low temperatures required to achieve

superconductivityo Researchers are working on high-temperature superconductors which retain superconducting

properties at higher temperatures Materials from which superconducting wires are usually made, are often hard to manufacture, brittle, and

hence difficult to make into wire. Current above a maximum threshold (critical current density) will revert the superconductor into its normal

state. The value of critical current density increases as temperature is decreased.