9.4 FROM IDEAS TO

IMPLEMENT-ATION

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

K1.1 Explain why the apparent inconsistent behaviour of cathode ray tubes caused debate as to whether they were charged particles or electromagnetic waves

• Tests were done in order to determine when nature of cathode ray tubes.• Some tests showed that cathode rays were charged particles:

• The Maltese Cross experiment showed a crisp, sharp lines outlining the shadow, which meant the cathode rays travel in straight lines.

• Deflection from magnetic fields (magnets): If opposite poles from magnets are placed on either side of the discharge tube, it will deflect along the perpendicular. Note: it will not move towards or away from the north pole, because charged particles (electrons and protons) are neither attracted or repelled by the magnet. It is the electric and magnetic fields which interact to cause the charged particles to experience a force.

• Paddle wheel experiment showed that cathode rays carry have mass because they are able to apply a force and cause the paddle wheel to rotate.

• Other tests conducted indicated that cathode rays were electromagnetic waves:• When Hertz had applied electric plates, it appeared to show that cathode rays were not deflected by electric

plates. His experimental results were incorrect, however, the results were used as evidence to support his theory that cathode rays were electromagnetic waves, just like light which is not deflected by electric fields. J.J.Thompson had conducted the same experiment to show that cathode rays were clearly deflected by electric plates, indicating that cathode rays were charged particles.

• Hertz also conducted other experiments which showed the 'light-like' properties of cathode rays that suggested they were electromagnetic waves. These experiments were the fact that they travelled in straight lines, caused fluorescence and caused chemical reactions. These indicated that they were electromagnetic waves, but this was become the definition of an atom was flawed.

• These apparently conflicting results arose from inadequacies in experimental design and the then current state of knowledge about the nature of atoms.

• The properties of cathode rays were clarified by later experiments, and were shown to be charged particles (electrons)

K1.2 Explain that cathode ray tubes allowed the manipulation of a stream of charged particles• A cathode ray tube is a highly evacuated glass tube containing two electrodes (cathode and anode). A high

potential difference applied across the electrodes causes cathode rays, streams of negatively charged particles (electrons), to be emitted from the cathode and accelerate towards the anode.

• By passing the electrons (cathode rays) between suitable electric and magnetic fields it is possible to manipulate their path so that they strike a screen at any point. Solid objects can also be placed inside the tube to block the path of the rays.

• Geissler invented a vacuum pump that was efficient enough to reduce the pressure inside a strong glass tube up to 0.01% of normal air pressures. Under normal temperature and air pressure conditions, air acts as an insulator. At reduced pressure, air (and other gases) conduct electricity.

• Plücker, a friend of Geissler, found that by placing metal electrodes in the ends of one of Geissler's tubes and joining the electrodes to a high voltage source ,he could get electricity to flow through the tube. It was shown that as pressure was reduced in the discharge tube, a series of changes progressively took place.

K1.3 Identify that a moving charged particle in a magnetic field will experience a force• A moving charged particle sets up an electric field, which when in a magnetic field causes an interaction between

the two fields and so the moving charged particle will experience a force. Alternatively: the charged particle will be cutting flux lines and so it will experience a force. This occurs when it moves through a magnetic field and it also occurs when a changing magnetic field moves past the charged particle. A stationary charged particle in a magnetic field does not experience a force.

• The direction of the force can be determined by using the right hand rule:• Fingers represent the direction of the magnetic field• Thumb as the direction of motion of positive charge • Direction which the palm points is the direction of the force on the charged particles

• The moving charged particle in a magnetic field will undergo circular motion.• Note: The following two equations: F = qvBsinA and F = mv2/r can be equated in order to determine any variable

because a charged particle moving in a magnetic field undergoes uniform circular motion. Hence, a centripetal force must be acting on it, and so the magnetic force equation can be equated to the centripetal force equation.

K1.4 Identify that charged plates produce an electric field• The electric field between two parallel plates is uniform in strength and directed from the high potential

(positively charged plate) to low potential (negatively charged plate).• An electric field exists in any region in which an electrically charged object experiences a force. Charged plates

exert a force on other charged objects brought close to them which indicates that an electric field is produced by charged plates.

K1.5 Describe quantitatively the force acting on a charge moving through a magnetic field F = qvBsinA• F = qvBsinA, where

• F is the force on the charged object, • q is the charge in coulombs, and F is proportional to q

• v is the velocity of the charged object in ms-1

, F is proportional to v • B is the magnetic field flux density (or magnetic induction) in Tesla (T), F is proportional to B• A is the angle between the velocity and the magnetic induction

K1.6 Discuss qualitatively the electric field strength due to a point charge, positive and negative, and oppositely charged parallel plates

• An electric field has both strength and directionThe strength of the electric field due to a positive point charge diminishes with distance from the object. The direction of the field is defined as pointing radially away from a positive point charge.

The strength of the electric field due to a negative point charge diminishes with distance from the object. The direction of the field is defined as pointing radially towards a negative point charge.

Opposite charges will attract

Like charges will repel

The electric field between two oppositely charged parallel plates is uniform in strength and direction. The field direction is defined as at right angles to the plates and away from the positive plate.

The number of the lines drawn to represent a field at any point indicates the electric field strength at that point. The stronger the field, the more lines are drawn in a given space.

K1.7 Describe quantitatively the electric field due to oppositely charged parallel plates• The formula is E = V/d, where E is the electric field strength, V is the potential difference and d is the distance

between the two parallel plates.• The electric field strength is proportional to the voltage of the plates, and is inversely proportional to their

separation. It is the same at all points in the region between the plates, and at right angles everywhere in the region between the plates.

K1.8 Outline Thomson's experiment to measure the charge/mass ratio of an electron• By assuming that cathode rays were negatively charged particles, Thomson conducted an experiment which not

only proved their nature but also the charge/mass ratio, i.e. q/m. • In his experiment, Thomson arranged electric and magnetic fields in his cathode ray tube so that the forces would

balance each other out.

• Cathode rays pass through slots in the collimators (cylinders) making a near parallel beam that hits the end of the

glass tube in the centre. The presence of the electric field alone would cause the beam to be deflected down with a force of qE (E = F/q → F = qE). Similarly, a magnetic field would be arranged so that the beam would be deflected upwards in the arc of a circle, with a force of qvB (F = qvBsinA, where sinA = 1).

• Charges in a magnetic field travel with uniform circular motion, so the force is centripetal. This means that qvB = mv2/r → q = v

m Br• B could be calculated based upon the geometry of the circle and the current within it, and r could be determined

based upon the displacement of the beam.• By equating the strength of the two forces, the beam could be made to pass through undeflected. When this

occurs, the two forces are equal, hence qE = qvB, and so v = E/B. Since E and B can both be determined, v can similarly be calculated which means that the charge to mass ratio (q/m) can be calculated.

• Thomson successfully calculated this ratio. He discovered that regardless of the type of cathode material he used or the gas present in the tube, all cathode rays have the same q/m ratio, i.e. q/m = 1.76 x 1011 C.kg-1, which was

approximately 1800 greater than the ratio for hydrogen ions obtained by other methods such as electrolysis. This meant that either the charge of the cathode rays mas approx. 1800 times greater, or the mass was almost 1/1800 times the mass of the hydrogen ion. Thomson concluded that is was the latter, and so from these measurements he proposed that cathode rays are a constituent of all atoms.

K1.9 Outline the role of: electrodes in the electrode gun; the deflection plates or coils; and the fluorescent screen in the cathode ray tube of conventional TV displays and oscilloscopes

• Electrodes in the electron gun: The electron gun produces a narrow beam of electrons. It consists of a filament, a cathode, the grid (electrode whose charge will vary), and two open-cylinder anodes:• The anodes help to accelerate and focus the electrons. Each successive anode needs to be more positively

charged than than the previous one, in order to continue the force of attraction and therefore accelerate the beam towards the screen.

• The cathode focuses the cathode rays.• A ring-shaped electrode, the grid, between the cathode and the anodes, controls the brightness of the spot that

appears on the fluorescent screen. By changing the charge of the grid with relative to the cathode, the number of electrons emitted by the gun is controlled and hence the brightness can be controlled. E.g. by making the grid negative with respect to the cathode, the number of electrons, and hence the brightness, will be reduced.

• The deflection plates or coils consists of two pairs of parallel plates, one arranged horizontally and the other vertically. The parallel plates are connected to a potential difference which produces an electric field between the plates.• The Y-plates (refer to diagram) control vertical deflection of the beam and the position it will appear on the

fluorescent screen. There are three cases:• If there is no voltage between both sets of plates, then there is no effect on the electron beam. • If the plates are each given a steady voltage (and hence charge, since the higher potential plate is positive

and the lower potential plate is negative), then the beam would deflect towards the positive plate. • If the top plate was made to alternative between being positive and negative, the beam would move up

and down. This is what AC voltage would do, with the spot moving up and down at the frequency of the voltage. At a high voltage, the spot would move up and down so fast that it would appear to be a steady vertical line. For example, at 50Hz AC, the spot would move up and down 50 times per second.

• The X-plates control horizontal deflection. Similarly as the plates that control vertical deflection, by controlling the voltage of each plate, the spot where the beam will hit the screen can be controlled.

• Fluorescent screen: the inside glass of the end of the tube is coated with a fluorescent material, for example zinc sulphide. When an electron beam hits the screen, the coating causes fluoresces and a spot of light seen on the screen. The screen converts the kinetic energy into light energy.Cathode Ray Tube (CRT)

• A CRT television is similar to the CRO but more sophisticated. Instead of electric fields, it uses magnetic fields from electromagnets to deflect the electron beam because the magnetic coils allow for a wider angle beam/stronger deflection in order to cover the whole screen.

• It has two time-base circuits, one for horizontal deflection and the other for vertical. The result is that the electron 'zigzags' down the screen.

• Each picture is made from two passes of the electron beam, fir the odd-numbered lines are scanned followed by the even lines. Each line takes a fraction of a second to be produced, so no flicker is seen.

Cathode Ray Oscilloscope (CRO):• The cathode ray oscilloscope (CRO) is an electronic device to 'view' electrical signals, i.e. waveforms. • The oscilloscope 'graphs' the relationship between two or more variables. The horizontal axis usually represents

time and the vertical axis generally represents voltage that is produced by an input signal. The CRO is most of the most widely used test instruments since many physical phenomena can be converted into voltage. It is used acoustics, communication, electronics and heart monitoring.

• Vertical Deflection Grid: The vertical grid on the screen is generally divided into line 1cm apart and the deflecting control is expressed as volts/cm (V.cm-1). There is a range of values available depending on the size of the applied voltage.

• Horizontal Deflection Grid- Time base circuit: A time-base circuit in the CRO results in the formation of a wavy line- a wave form. A spot is swept horizontally across the screen and when it reaches the edge it goes back to the start. This is achieved by using a saw-tooth voltage (rapid changes makes it fly back to the start).

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

K2.1 Outline qualitatively Hertz's experiments in measuring the speed of radio waves and how they relate to light waves

• Using the apparatus set up above, Hertz was able to provide experimental evidence for the existence of electromagnetic waves first postulated by Maxwell. Hertz hypothesised that the sparks set up changing electric and magnetic fields, that propagated as electromagnetic waves. These waves, falling on the spark gap, set up electric and magnetic fields inducing a spark.

• Hertz measured the wavelength of the waves, by using a technique called Lloyd's mirror. Hertz used a reflection sheet to deflect some of the waves from a surface, making them travel a larger distance to the detector. Both beams interfere both constructively (maximum) and destructively (minimum). Hertz analysed the interference patterns these waves produced with the waves the travelled directly to the detector. Since the knew the frequency of the spark generator in loop A, and using the equation v = fλ he determined the velocity of the waves. The value Hertz obtained for the speed of these waves, matched the value found by Fizeau as the speed of light, which allowed him to identify them as electromagnetic radiation.

• Hertz demonstrated that the waves exhibited the properties of reflection, refraction, diffraction, interference and polarisation, and that they travelled at c, properties also exhibited by light. This showed that light is a form of transverse electromagnetic waves.

K2.2 Describe Hertz's observation of the effect of a radio wave on a receiver and the photoelectric effect he produced but failed to investigate

• The photoelectric effect is the emission of electrons from substances, in particular metals, when they are bombarded with light (usually in the high frequency range, such as ultraviolet).

• The waves that Hertz had investigated fall under the category now called radio waves.• He had conducted his experiments in the dark because it enabled him to see the sparks more easily. When he

opened the curtains and shone ultraviolet light on the loops, the spark jumped the gap more easily. He realised that light and electricity must be connected in some way, and called this effect the photoelectric effect.

• When sunlight (containing UVL) fell on the emitting coil, the intensity of the spark that was produced increased. This is because the UVL in sunlight produced photoemission of electrons from the emitter, and so increasing the number of electrons led to a greater intensity of the spark.

• In his experiment, Hertz also noticed that the gap in the detector could be made greater and a spark would still occur if UV light was shone onto the gap. Hertz placed a sheet of glass (an absorber of UV light) between the transmitter and receiver and found that the glass shielded the spark so that no effect was produced at the receiver. Removing the glass resulted in the spark returning. Unfortunately, Hertz failed to investigate this phenomena further.

K2.3 Identify Planck’s hypothesis that radiation emitted and absorbed by the walls of a black body cavity is quantised (the quantum theory)

• A (perfect) black body will absorb all radiation falling on it, increasing its temperature. IT will then radiate this energy out again as temperature falls to room temperature. The peak radiation it emits reflects the temperature it reaches as it absorbs energy.

• Planck proposed that radiation (energy) is not emitted or absorbed by a black body continuously, but rather in little 'bursts' or 'packets of energy' called quanta (later changed to photons).

• The 'classical theory of light' predicted that, as the wavelength of the radiation emitted become shorter, the radiation intensity would increase. This means that as the energy (emitted from the black body then reabsorbed) decreased in wavelength from the visible into the UV portion of the spectrum, the intensity of the radiation emitted from the black body would approach infinity. This increase in energy would violate the principle of conservation of energy.

• Experimental data from black body experiments showed that the radiation intensity curve corresponding to a given temperature has a definite peak, passing through a maximum and then declining. This did not agree with the classical theory of light and could not be explained.

• Planck proposed a revolutionary explanation. He proposed that energy would be exchanged between particles of the black body and the equilibrium radiation field. He assumed that the radiant energy, although exchanged between particles of the black body and radiant energy field in continuous amounts, may be treated statistically as if it were exchanged in multiple small 'lumps'. Each lump is characteristic of each frequency of radiation emitted. He described this small, average packet of energy as a 'quantum' of energy, that could be described by hf (E=hf).

• Given that c = fλ → f = c/ λ, and E = hf, hence E = h.(c/λ). In this way, the energy of a light photon of any known wavelength can be determined.

K2.4 Identify Einstein's contribution to the quantum theory and its relation to black body radiation • Einstein realised that the only way to explain the photoelectric effect was to say that instead of being a wave, as

was generally accepted, light was actually made up of lots of small packets of energy called photons that behaved like particles. Einstein wasn't the first person to use the idea of photons, but he was the first to make it the starting point of an explanation.

• Einstein explained that the reason photocurrent (current produced by the movement of photoelectrons) increased as the intensity of light of an appropriate frequency shining on the emitter increased, was because photoemission increased as more photoelectrons could be emitted. This increased only to a maximum value since there is a limited number of valence electrons which can be emitted.

• Einstein adopted a particle model in conjunction with Planck's hypothesis, and proposed that:• The energy of light is not evenly spread out over the wavefront, but is concentrated in photons• Each photon has energy given by Planck's relationship: E = hf• 'The All or Nothing Theory': a photon could give up either all of its energy to one electron or none of it, it

could not give up only part of it• The maximum kinetic energy of the emitted electron, KEelectron- max, was equal to the initial photon energy

minus the work done in overcoming the attractive forces near the surface, i.e. KEelectron- max= hf – φ , where φ is the work function Explanation of the above:

• Einstein showed that by assuming that radiation was made up of bundles or photons with energy that fulfils this equation: E = hf, and by assuming one photon interacted with one atom, the photoelectric effect could be explained by conservation of energy.

• The work function, φ, is the threshold energy below which no photoelectrons (electrons emitted by the photoelectric effect) are released, i.e. φ = h.f0 where f0 is the threshold frequency. Photons of light below this frequency did not carry enough energy, so no photoelectrons could be emitted regardless of the intensity of light.

• It takes an amount of energy, φ(work function), to remove an electron from an atom on the metal surface. Hence, to remove an electron the incident photon would have at least this much energy or more. Hence, the threshold frequency (work function) for removal of an electron would be: E = hf0 = φ, where f0 is the threshold frequency.

• If the incident photon had more energy than this, then the left over energy after ejecting the electron would go into kinetic energy of the ejected electron E = hf = φ + Keelectron, or Keelectron= hf – φ. This kinetic energy would mean that the electrons could travel faster, and since current is a measure of the q/t, if the photoelectrons are travelling faster than more will pass through a given point per second and so the photocurrent will also increase.

• Milikan conducted an experiment which provide experimental evidence in support of Einstein's theory. The graphed results were as predicted by Einstein. The interception with the Kemax axis shows the work function, and the and the gradient of each slope is as follows:

gradient = rise = KE = h, and so the gradient of every curve obtained was constant, Planck's constant run f0

S2.1 Identify data sources, gather, process and analyse information and use available evidence to assess Einstein'scontribution to the quantum theory and its relation to black body radiation

• Einstein applied and expanded Planck's idea about quanta which had been developed based on Planck's observations of black body radiation when he used them to explain the photoelectric effect.

• This added to the credibility of the idea of quanta (which wasn't widely supported because it went against the classical theory) and widened support for the theory.

• In addition, Einstein expanded quanta to apply to light and in this way opened a door for Bohr's mid o develop his solar system model of the atom. So, Einstein's application of quanta was important in two directions: it supported a newly developing idea and stimulated others.

K2.5 Explain the particle model of light in terms of photons with particular energy and frequency• Light is composed of particles called photons which have particular amounts of energy (quanta) dependent on

their frequency of oscillation. • Also, E = qV0 = hf – φ where q is the charge on the electron, and V0 is the stopping voltage. Stopping voltage is

the reverse potential applied across the electrodes in a CRT to stop the emission of photoelectrons from the emitter.

• Lenard studied the relationship between the energy of the emitted photoelectrons and he intensity and frequency of the incident light. The incident light caused photoelectrons to be given off from the emitter and move towards the collector (anode). The resulting current was registered on a ammeter. By making the collector negative, Lenard was able to reduce the photocurrent to zero. (The greater the energy of the photoelectrons the greater the energy required to repel them).

• The maximum kinetic energy of the photoelectrons, KEmax, could be found from the relationship between the work done in stopping the electrons and he energy, i.e. qV0 = ½ mv2 = KEmax (V0 is the stopping voltage).Characteristic Classical Predictions Experimental Results: by Lenard's experiment

Intensity As the light intensity increases, the photocurrent will increase

As the light intensity increased, so did the photocurrent

Emission time For low intensity light, the time for the electrons will be longer than high intensity light

If emission occurred, it was instantaneous (however, emission did not always occur)

Frequency Emission is independent of frequency Emission was frequency dependent. Below a certain frequency (f0), no electrons were emitted, regardless of the intensity of light

Energy As the light intensity increases, the kinetic energies of the photoelectrons will increase

As the intensity increased, the KEmax remained constant. KEmax was found to depend on the frequency of light used and the type of surface

• To apply Einstein's model to the factors tested in Lenard's experiment:• Intensity is measured by the number of photons per unit area, which means that as the intensity is increased

the number of photons will be increased hence the photocurrent is increased• A photon transfers all energy to the electron. Provided this energy is greater than the work function, the

electron will leave immediately (that is, in order of microseconds). If however, hf < φ, the electron will not be emitted.

• Since the energy of a photon is dependent on frequency, then it follows, if the frequency is too low the available energy may be too low to release the electron

• Intensity is independent of photon energy. Since it is the photon energy that determines the KEmax, then KEmax

must also be independent of intensity.

S2.2 Identify data sources, gather, process and present information to summarise the use of the photoelectric effect in photocells

• Photocells are cells in which electrons initiating an electric current are produced by the photoelectric effect.

• Essentially, the consist of an anode and cathode. The cathode is coated with a photosensitive material that emits electrons when light falls on it. The photoelectrons are accelerated to the anode (by a potential difference between them) resulting in a photocurrent that is proportional to the intensity of light falling on the cathode.

S2.3 Process information to discuss Einstein's and Planck's differing views about whether science research is removed from social and political forces

• Planck was a patriot for native Germany. Einstein sought to leave it from an early age and to take up Swiss citizenship.

• Planck was respected by the state and was a very stoic and proper man; a gentleman working his for native

Germany. Planck was for all intents and purposes a patriotic nationalist.• Einstein was more liberal in outlook, and was not apparently loyal to any government. At the beginning of WWI,

these differences become very apparent.• Planck has adopted the nationalist view that it was the role of science to support the was effort. He was one of the

first to sign a document entitled, Manifesto of the ninety-three German intellectuals to the civilised world which attempted to justify the role the State was playing in the war was justified. He supported the research that was supporting the war effort.

• The dictatorship of Hitler resulted in German science once again being marginalised. Many scientists, including Einstein fled from Hitler's regime in its early stages. Planck, however, stayed behind because at first he saw no moral imperative to oppose the Nazi regime. He was appointed as the president of the Kaiser Wilhelm Institute, Germany's most prestigious scientific research facility. He later resigned from the position in protest against the sacking of all Jewish teachers from German universities. He also refused to work on any war research projects. He paid the price though, when his son was killed for apparently apart of a process in the assassination of Hitler.

• Einstein was also invited to sign this document but blatantly disagreed. He refused to accept the role of science and the state in the war effort, Einstein adopted the views of those of a pacifist. He could not accept the role of science in killing his fellow human beings. Einstein even went as far as signing a rival agreement that supported the rights of all people to a peaceful world called Manifest to the European. Einstein clearly saw the role of science as something for the good of man that was not to be manipulated for the good of the state.

K2.6 Identify the relationships between photon energy, frequency, speed of light and wavelength: E=hf and c=f λ • E = hf• c = fλ• f = c

λ• E = h. c

λ

S2.4 Solve problems and analyse information using E=hf and c=f λ

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

K3.1 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. The metallic structure is proposed to be a lattice of positively charged nuclei 'floating in a sea of electrons'. The valence electrons are not attached to any particular nucleus and are free to move when even the smallest potential is applied.

K3.2 Compare qualitatively the relative number of free electrons that can drift from atom to atom in conductors, semiconductors and insulators

• Conductors contain high numbers of free electrons in the conduction band that can drift between atoms.• Under normal conditions, semiconductors and insulators have far fewer free electrons than conductors. • Raising the temperature, using certain lighting conditions or applying a potential difference can induce electrons

in semiconductors to move into the conduction band.

K3.3 Describe the difference between conductors, insulators and semi-conductors in terms of band structures and relative electrical resistance

Definitions:• Valence band: contains the valence electrons and is partly or completely filled

• Conduction band: upper energy band and corresponds to the higher energy levels in the isolated atom. It is empty• Forbidden energy lap: lies between the valence and conduction band. It corresponds to the gap between the

energy levels in the isolated atom Conductors:

• In a conductor there is no forbidden energy gap, because the valence band and conduction band overlap. • This means that when a potential difference is supplied and an electric field is set up, the electrons within this

band will have a force applied to them causing them to move and conduct current relatively easily. • Also, every atom in a conductor contributes valence electrons for conduction, there is a huge number of electrons

to provide the electric current.• Hence, conductors have low electrical resistivity or high conductivity.

Semi-conductors: • In a semi-conductor, the forbidden energy gap is large (but smaller than that of an insulator) which means that

electrons in the valence band need to be supplied with thermal energy which will raise/increase their potential.• This means that they will 'jump the gap' to the conduction band making them available to conduction. This

means, that whilst they are conductors, they do not conduct as well as conductors. • They have an electrical resistance between that of a conductor and an insulator.• NOTE: the moving electrons leaves the positive 'holes' in the atom

Insulators: • The forbidden energy gap is very large in insulators and so it is very difficult for electrons to 'jump the gap'. No

electrons are normally available for conduction. • Insulators have high levels of electrical resistance and no/very low levels of conduction.

The effects of heating on Conductors and Semi-conductor• In conductors, the increased motion of atoms and other electrons hinders the flow of valence electrons because of

an increase in collisions between the positive lattice and the mobile electrons, and so the electrical resistance increases.

• In semi-conductor, nuclei and other electrons are held more tightly in lattice positions so heating allows any conducting electrons to move more rapidly through the semi-conductor, and so the resistance decreases. The additional energy also allows more electrons to break free of their lattice positions.

• When the semi-conducting material becomes overheated, it loses its conducting properties all together and adopts the properties of a insulator.

K3.4 Identify absences of electrons in a nearly full band as holes, and recognise that both electrons and holes help to carry current

• In semi-conductors, when valence electrons 'jump the gap' to the conduction band, it leaves behind positive holes• This hole is equivalent to a positive charge (an absent electron) and moves in the opposite direction, but this does

not happen at room temperature unless the sample is doped. The hole does not actually move, but the electrons jump from one hole to another in order to fill the previous gap. But by jumping, they leave behind a hole which in turn is filled by other electrons who also leave holes.

• Conduction of electricity in semi-conductors is enhanced by the presence of positive holes which provide ready positions for the electron to jump into. In addition, as each electron moves into a positive hole, the position it used to occupy becomes a positive hole and allows the next electron in line easier access forwards.

• Movement of both electrons and holes constitutes an electric current. We consider the conduction of semi-conductor to be a flow of electrons towards positive terminal and positive holes towards the negative terminal.

K3.5 Identify the use of Germanium in early transistors is related to lack of ability to produce other materials of suitable purity

Germanium:• Germanium is a Group IV element which is never found free in nature and is a very rare metal. It gained

economic significance after its semiconducting properties were recognised as being used in electronics.• Dissolving the appropriate mineral ores in hydrochloric acid produces Germanium. The resulting chloride is

converted into germanium oxide, which is reduced with hydrogen. The powdery metal produced is then melted and cast in ingots. For use in electronic devices, the germanium ingots are further purified and then doped using minute amounts of arsenic, gallium or other elements. Using a 'seed crystal', single crystals are obtained.Silicon:

• Silicon is also a Group IV element and is similarly too reactive to be found free. It is the second most abundant element (after oxygen) by weight in Earth's crust.

• It is produced by reduction of silica (silicon dioxide, common sand) with coke in an electric furnace. As with Germanium, for use in electronics single crystals are grown by slowly withdrawing a seed crystal from molten silicon.Silicon vs. Germanium

• For germanium, it is important that the semi-conductor be as pure as possible; contamination as low as one ppb affects its conducting properties.

• Suitable purification methods for germanium had been developed and remained until 60s when germanium was the favoured conductor. In 50s, purification methods were devised for silicon, and so germanium was then surpassed by silicon as the preferred semiconductor material for solid-state devices.

• The reasons why silicon is preferred over germanium is:• Silicon is much more abundant than germanium (and hence cheaper)• Silicon retains its semi-conducting properties at higher temperatures than germanium (we say Si has a much

lower 'leakage current' than Ge). Germanium conducts a greater current when heated which can lead to melted circuits. Silicon maintains its conductivity and semiconducting properties at high temperatures because it is able to forms an electrically insulating oxide layer when heated to high temperatures in an aerobic environment (in presence of oxygen). This film is critical in the manufacturing of integrated circuits.

• The molecular structure of a crystal of silicon is uniform, ensuring the consistency of properties.

K3.6 Identify differences in p- and n-type semiconductors in terms of the relative number of negative charge carriers and positive holes

• Semiconductors typically come from Group IV elements; which means they have 4 valence electrons.• N-type semiconductors involve replacing an atom with a Group V element (5 valence electrons). This means that

there is an extra negative charge carrier (electron) compared to the pure element, and two extra negative charge carriers than the p-type semiconductor.

• P-type semiconductors involve replacing an atom with a Group III element (3 valence electrons). This means there is one less electron, or one extra positive hole. This means that p-type semiconductors have less negative charge carriers but more positive holes than both the pure element and n-type semiconductors.

• N-type semiconductors are slightly better conductors than p-type semiconductors, since the extra electrons in the n-type semiconductor are more mobile than the extra positive holes in the p-type semiconductor.

K3.7 Describe how 'doping' a semi-conductor can change its electrical properties• Pure semiconductors have too little electrical conductivity to be of any use in an electrical circuit, their

conductivity can be increased by adding impurities- doping.• Doping a semiconductor involves reducing the forbidden energy gap between valence and conduction bands by

pre-introducing a hole or an extra electron. This is done by placing an atom with a Group 3 metal (an extra hole: p-type) or a Group 5 metal (an extra valence electron: n-type).

• Doping affects the electrical conductivity because it increases the number of positive holes (p-type) or electrons available for conduction (n-type). It improves the electrical conductivity by a factor of about 100 times.

K3.8 Describe differences between solid state and thermionic devices and discuss why solid state devices replaced thermionic devices

• A thermionic device is basically a cathode ray tube, whilst a solid state device is made from semi-conducting materials.

• Thermionic emission is the spontaneous emission of electrons from solids (and liquids) when heated to high temperatures. It is utilised in thermionic devices such as valves, which consist of an evacuated/vacuum tube with an electron-emitting cathode and at least one other electrode. A diode has a total of two electrodes, a triode has three electrodes etc. Diodes are used as rectifiers, whilst triodes and pentodes as amplifiers.

• Thermionic devices were one used extensively in electronics, but have been superseded by solid state devices, because they:• are much bigger than solid state devices, which limited the minimum size of any electronic device

(miniaturisation was essential to the progress of electronics)• consume a lot more electrical energy than solid state devices and produce much more heat, they are less

efficient and waste a lot of energy• more expensive to produce and hence to sell, not very affordable and were considered a luxury by many• are more fragile than solid state devices, since they are mainly made of glass which also made them less

reliable, they also had fragile internal components which tended to 'crystallise' with continual heating and cooling, they also break very easily if bumped, especially when hot

• components were mounted on metal bases, insulated by bakelite rings and connected to each other by insulated wires. Insulation often cracked or degraded due to movement and heat

• cannot operate as fast as solid state devices• require a 'start-up' time to become operational while semiconductor devices do not• portable devices require a large energy source to run (up to 12D batteries)• relative high grid voltages were required to run amplifiers whereas equivalent transistors can work with 0.6V

for same output

S3.1 Gather, process and present secondary information to discuss how shortcomings in available communication technology led to an increased knowledge of the properties of materials, with particular reference to the invention of the transistor

• The advancement of technology which has led to the development of iPods, mp3 plates, small mobile phones and GPS systems had occurred because of the research and development of solid state devices.

• There wasn't much room in the advancement of thermionic devices; the miniaturisation of appliances had to occur for any appreciable level of development to occur. This led scientists to discover new pathways, which effectively led to the research and development of solid state devices.

• Three scientists were looking for replacements for vacuum tubes whose technology was limiting long distance phone calls and prohibiting further technological development in this field. It was these scientists who had invented the transistor (this point is debatable). It was through the shortcomings in available technology that the transistor was invented. By the 60s transistors had begun to rapidly replace vacuum tubes in electronics.

• Solid state devices rely on p-n junctions. Operation of p-n junction:• Semiconductors, pure of doped, allow current to flow equally well in both directions. A price of p-type next

to a region of n-type however, has unidirectional properties (current easily flows in one direction only, there is almost no current flowing in the opposite direction

• The p-n junction acts as a rectifier or diode. In n-type region there are many electrons (but element is neutral) and in p-type there is relatively few electrons. Consequently, when two pieces are joined electrons from the n-type diffuse into the p-type and recombine with holes near the boundary leaving positive ions behind.

• Similarly, holes from the p-type silicon diffuses into the n-type (where they recombine with electrons near the boundary) leaving negative ions behind.

• This diffusion of carriers results in a depletion zone near the boundary (depleted of free majority carriers). The semiconducting element is now electrically charged, the p-type is negative and the n-type is positive. As a result, an electric field is set up that creates a potential barrier opposing further diffusion. Under the effect of this field, minority carriers (electrons in p-type and holes in n-type) drift across the boundary. At

equilibrium, the drift of minority carriers in the opposite direction balances the diffusion of majority carriers across the boundary. This barrier will also act to move any electrons created in the silicon into the n-type and any holes into the p-type.

• Forward bias:• When connected in a circuit (as above), the depletion region is decreased as positive holes are replaces

by the positive battery terminal and are injected into the n-type material where they become minority carriers and recombine with electrons in n-type, thus ceasing to be mobile charge carriers. The electrons move in the opposite direction, towards p-type.

• For each electron-hole combination near the junction, a covalent bond near the positive terminal breaks down and an electron is liberated which enters the positive terminal. The newly created hole then move to the left towards the p-n junction.

• In the n-type region near the negative battery terminal, electrons enter the n-type material to replace those electrons lost be recombination of holes near the junction. These electrons move toward the p-n junction where they recombine with new holes continually arriving there. When connected this way so that a relatively large current can flow across the junction, it is referred to as forward bias.

• Reverse bias:

• If battery voltage is reversed in polarity as shown, holes in the p-type region are attracted to the negative battery terminal while electrons in the n-type are attracted to the positive terminal.

• As a result the depletion region is increased and current flow ceases almost completely. In this state the diode is said to be reverse biased.

• The p-n junction therefore presents an easy pathway for current in one direction, but acts ideally as an open circuit in the other direction. Such junctions are also used in photovoltaic cells.

• Transistors:

• Triode valves that are used for amplification were replaced by transistors. The transistor consists of two p-n junctions of close proximity. There are two types of transistors, npn and pnp:

• Transistors consist of three regions: the emitter, base and the collector. The base is much thinner than the other two components. Electrons pass from an outer layer (emitter), to a central layer (base) and then to the other outer layer (collector).

• For amplification uses, the emitter-base junction is forward biased and base-collector is reverse biased (npn).

• The bias voltage VEB in above diagram reduces the potential barrier at the emitter junction and facilitates the injection of electrons (in npn transistors) or holes (in pnp transistors) into the base.

• The base is so thin that almost all the electrons (97-99.9%) injected into the base (npn) drift towards the collector at a steady velocity where they recombine with a hole from the external battery (external battery removes electrons by providing holes for recombination).

• In the base region some recombination occurs (0.1-0.3%) and a small number of holes pass into the base to make up for the ones that have recombined, they constitute a base current.

• The reverse bias collector-base junction ensures that practically no electrons are injected into the base from the collector. The net result is the transfer from the emitter circuit to the collector circuit of a current which is nearly independent of the collector-base voltage, VCB but highly dependent on VEB. By varying VEB, a small signal voltage varies the emitter current and hence the collector current> This leads to a relatively large voltage change across a load in the collector circuit; the transistor is acting as an amplifier.

S3.2 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

• Since the invention of the transistor, electronic systems have become smaller, more sophisticated and cheaper. • One of the great breakthroughs was the development of the integrated circuit- a single silicon chip with currently

millions of transistors. The development of integrated circuits led way to the computer revolution, as it found itself at the heart of microprocessors.

• Microprocessors contain three essential elements of a computer's central processing unit (CPU): an arithmetic circuit, a logic circuit and a control circuit. The invention of microprocessors has paved the way for future developments, are have become essential parts of radios, TVs, mobile phones, computers, scanners, video recorders, digital cameras, calculators, digital clocks, cars, planes, in guidance systems, satellites etc. They have become an essential part of many of the products that many people have become dependent upon.

• It has led to changes in lifestyles for people, making them relatively much easier and simpler, allowed for the ease of travel, growth in entertainment sophistication, medical technology and benefits for many people.

• It has also meant that there has become a reduction in the demand for unskilled work which left many people unemployed and facing long-term unemployment issues and the associated welfare implications.

• There has been a growth in computing industries and the demand for work has sky-rocketed, which also has meant a need for tertiary training. This has encouraged a lot of people to pursue further education and raised the standard level of education required to find suitable employment.

S3.3 Identify data sources, gather, process and present information to summarise the effects of light on semiconductors in solar cells

• Solar cells, or photovoltaic devices, employ the semiconductor silicon and utilise the photoelectric effect. Solar cells and the photoelectric effect:

• When a p-n junction is exposed to light, photo-ionisation creates electron-hole pairs. The potential barrier causes separation of these pairs (electrons are attracted into the positive n-type material), setting up an emf. This emf is dependent of the rate of creation of these electron-hole pairs and hence the intensity of the incoming radiation.

• Metal contacts on the surface of the n-type silicon conduct the electrons away, and if there is a complete circuit they pass through the load and move back to the p-type silicon doing useful work in the process. The emf acts as a source of electric energy and so light energy has been directly converted into light energy.

Construction of the Photovoltaic Cell/Solar Cell:• The glass covering protects the semi-conductor material; the anti-reflection layer maximises the amount of light

the penetrates the cell; the contact grid collects the photo-electrons; the n and p type material form the p-n junction and the back contact provides a pathway back for the electrons.

The general structure of a photovoltaic cell:

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

K4.1 Outline the methods used by the Braggs to determine crystal structure • The British physicist Sir William Bragg and his Australian born son Lawrence applied X-ray diffraction to the

study of crystals. Diffraction is a characteristic of light and so if a phenomenon shows diffraction, then it is wave-like. Diffraction occurs when light reflects off the surface causing interference, either constructive or destructive depending on the angle of incidence.

• In constructive interference two or more waves interact to produce maximum amplitude, whilst in constructive interference at least two waves interact to product minimum amplitude.

• It was hypothesised by Max von Laue after the discovery of X-rays, that if atoms in a crystal were arranged in a regular manner they might act as a three-dimensional diffraction grating for X-rays. Von Laue and two technicians conducted an experiment and found diffraction patterns from the crystal, and so in doing so succeeded in proving the wave nature of X-rays but also the regular arrangement of crystals.

Bragg Diffraction• Braggs proposed that the short wavelength X-rays could penetrate and reflect from the atomic planes. In some

directions, constructive interference would occur; in others, destructive interference occurs. • They devised a formula for diffraction nλ = 2dsinθ where n is the 'order' of the diffraction pattern, d is the inter-

atomic spacing and θ is the angle of reflection. Depending upon what is known, this formula allows wavelength or d to be found (the angle can usually be measured easily)

• The Braggs experiment examined the interference patterns they observed when they developed and used a X-ray spectrometer. In doing this they bombarded metals with X-rays and analysed the interference pattern formed by the X-rays reflected from the surface of the metal interacting with the X-rays reflected from lower layers of atom.

K4.2 Identify that metals possess a crystal lattice structure • Crystals are composed of an orderly three-dimensional arrangement of atoms. In each type of crystal structure a

certain fundamental grouping of atoms is repeated indefinitely in three dimensions, called a unit cell.• Metals also possess this crystal lattice structure

K4.3 Describe conduction in metals as a free movement of electrons unimpeded by the lattice • Metals usually have either one, two or three valence electrons which are loosely bound to the positive nucleus. • In the crystal lattice structure, the positive ions form the fundamental structure within which is a 'sea' of electrons

that are free to move under the influence of an electric field.• In a conductor (not connected to a source of potential difference), the electrons travel at fast speeds (~106 ms-1)

colliding with each other and other 'impurity' atoms or imperfections within the lattice. During the collision, energy is lost as heat energy. After the collision, electrons accelerate again then decelerate at the next collision.

• Due to the random motion, there is no net movement of charge. When the conductor is connected to a source of potential difference such as a battery, an electric field is set up in the wire. This field exerts a force on the electrons that results in a net drift or electrons towards the positive terminal. This 'drift' velocity is proportional to the applied electric field and is usually very small (~10-4 ms-1)

• The conduction in metals involves the free movement of electrons unimpeded by the lattice, because the electrons are not attached to any particular nucleus and so are free to move when even the smallest voltage is applied.

K4.4 Identify that resistance in metals is increased by the presence of impurities and scattering of electrons in lattice vibrations

• It is a result of the collisions of the electrons with impurity ions or imperfections that the metal offers resistance to the current. The impurities distort the lattice structure and hinder the flow of electrons through them.

• As the temperature increases, the ions of the lattice vibrate more increasing the probability of collisions between conducting electrons and the lattice structure, thereby increasing the resistance.

• Since resistance in metals increases with increasing temperature, it was reasoned that decreasing the current should lead to a decrease in resistance. It was a result of this research that superconductivity was discovered.

K4.5 Describe the occurrence in superconductors below their critical temperature of a population of electron pairs unaffected by electrical resistance

• It was discovered that if some metals were cooled below specific, individual temperatures that resistance disappeared. This was discovered when a Dutch physicist discovered that resistance disappeared in Mercury when he cooled it to a temperature below 4.2 K.

• Superconductivity is the phenomenon exhibited by certain conductors where they have no resistance to current below their critical temperature (temperature from resistance to no resistance). There is a maximum level of current that a superconductor can carry, and if this is exceeded they revert to being normal conductors.

K4.6 Discuss the BSC theory• The resistance of a metal is the result of the collisions of the free electrons with impurity atoms or imperfections

in the crystal lattice. In a superconductor, however, these electrons are able to pass unimpeded through the lattice. Since no interaction occurs with the lattice, the electrons don't lose energy and so the resistance is effectively zero.

• Whilst decreasing the temperature was thought to reduce vibrations and so reduce resistance, and it does happen to a degree, it does not explain superconductivity. The lattice vibrations are thought to be what causes superconductivity.

• Phonons (packets of sound energy present in the vibrating lattice) mediate the process as follows:• A cooper pair is a pair of electrons which travel together through the lattice of a superconductor• As it moves through the lattice, the negative charge of the electrons attracts the positive nuclei causing

them to move inwards towards the electron, distorting the lattice. As a result, phonons are emitted, forming a 'trough' of positive charge density around the electron.

• Before the electron can pass these ions and before the lattice rebounds to its normal position, a second electron becomes attracted to this positive space and pairs up with the first electron, forming a cooper pair. This pair of electrons easily pass through the lattice.

• When one of the electrons of the cooper pair passes near a positive ion, attraction between the opposite charges causes a vibration to pass from lattice ion to lattice ion until it is absorbed by second electron in the pair. Effectively, one electron emits a phonon and the other absorbs it; it is this exchange of phonons that keeps the cooper pair of electrons together. Cooper pairs are continuously being formed, broken and reformed.

• The pairing of electrons in this way is favoured because it results in a lower energy state. As the temperature rises, the lattice vibrations increase and break apart the Cooper pairs causing superconductivity to diminish. It is for this reason that metals need to be cooled to temperatures just above absolute zero for superconductivity to occur.

S4.1 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 grids

Computers:• The capacity of microprocessors and the efficiency of their operation is, in part, limited by the heat produced in

the devices they operate in. Having zero resistance circuits would allow their capacity and speed of operation to be increased a predicted 250 times.

• The use of superconductors in transistors (Josephson Junctions) and superconductive connection films also allow the processing of signals through those transistors to be speeded up significantly. This could result in the transmission of information in computers to be speeded up several orders of magnitude.

• Superconducting switching devices are being tested for use in high-speed computers since they can operate at speeds up to ten times the speed of an equivalent semiconductor switch

• Superconductors can also carry large electric currents without heat losses, they can be used to generate very large magnetic fields. Such fields are used in a variety of devices including magnetic resonance imaging (MRI) machines, which are used to provide doctors with 3D imaging of the internal structures of the human body

Motors and generators: • Motors and generators made using superconducting coils would be much lighter and smaller (no need for soft

iron cores, and the coil could be much smaller). • Less energy input would be needed to sustain their operation, thus saving environmental effects from fossil fuels. Transmission of electricity through power grids: • Zero resistance in coils would mean zero energy losses through heat and much more efficient transmission to

distribution points. It would allow the transmission of higher currents and so reduce the cost of energy production and eliminate the need for as many power stations.

• It would also allow power stations to be built near consumers rather than near fuel suppliers. Alternatively, it would also allow power plants with the environmental problems such as greenhouse gas emissions, to be built away from centres of population.

K4.7 Discuss the advantages of using superconductors and identify limitations to their use • The main advantage of superconductors relates to their ability to minimise energy loss due to resistive heating

and to allow the generation of intense magnetic fields.• Limitations:

• Original superconductors only operated at temperatures above absolute zero, which is still the case for metals. Over the years, high temperature superconductors have been produced which has enabled the use of liquid nitrogen to cool then down to the required temperatures rather than using liquid helium (is still required for metals) which is more expensive and less efficient.

• These superconductors (high temperature ones) are ceramic and are brittle, difficult to manufacture, are not very ductile or malleable and so are difficult to form into cables (for electricity transmission); and are chemically unstable in some environments.

• The ability to maintain superconductors below their critical temperature except in laboratory transmission is severely limited. In addition,the cost of developing technology and infrastructure for larger scale applications (generators and electrical transmission) would be fairly large.

S4.2 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

• A superconductor is repelled by a magnetic field, and so when a magnet is placed above the superconductor, it will ‘hover’ above the superconducting material (when it has reached its critical temperature). The ability of magnets to hover over superconductors is called magnetic levitation.

• This effect is called the Meissner Effect. This basically is that a superconductor does not allow a magnetic field to penetrate its interior. The external field induces a current to flow through and produce a magnetic field inside the superconductor, that just balances out the field that would have otherwise penetrated the substance

S4.3 Gather and process information to describe how superconductors and the effects of magnetic fields have been applied to the Maglev train

• One of the major applications of superconductivity is in the Maglev Train, (Magnetically Levitated Vehicle). This is an extremely high speed transport system that utilises magnets and linear electric motor technology.

• The advantages of the Maglev train are that they are reliable, safe, require minimum maintenance, have a low environmental impact and are very high speed.

• Magnetic levitation can be achieved in two ways. Both systems use a ‘magnetic wave’ travelling along the track to propel the suspended vehicle:

• In Germany, levitation results from attraction. Devices on the sides on trains wrap around the guideway. Electromagnets energised in ‘skirts’ are attracted up to iron rails on the guideway’s edge and lift the train.

• In Japan, levitation results from repulsion. Helium-cooled superconducting magnets on the vehicle interact with coils in the guideway in such a way that repulsion between ‘like poles’ causes the train to lift off the guideway.

S4.4 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