FROM IDEAS TO 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

Cathode rays were observed to have inconsistent behaviour with regards to its nature – particle or wave.

The English physicists believed that cathode rays were a stream of negatively charged particles. Cathode rays were observed to cause a paddlewheel to rotate, demonstrating that they had momentum and thus mass. They were also observed to deflect in a magnetic field and travelled more slowly that light did, all evidence for the particle nature of cathode rays.

However, German physicists including Heinrich Hertz believed that cathode rays were a form of wave. They found that an opaque Maltese cross placed in the path of the cathode rays caused a shadow to appear in green light, and they travelled in straight lines, both properties like light. Furthermore, they were found to be able to pass through thin films of metal without damaging them, something that no particle had ever been observed to do. Hertz believed they were waves because there was no observable deflection by an electric field; it was later found that this was caused by an insufficiently low pressure in his cathode ray tube.

The debate was eventually settled by Thomson’s experiment in 1897, and the particle nature of cathode rays proven without doubt.

- Explain that cathode ray tubes allowed the manipulation of a stream of charged particles

When a high voltage is applied across two parallel plates in a vacuum tube, cathode rays are emitted from the cathode and accelerate towards the anode. The glass behind the anode is hit by the cathode rays and causes a glow. Different pressures in the vacuum tubes were found to produce different fluorescent results. The cathode rays can also be manipulated by deflection in an electric or magnetic field.

- Identify that moving charged particles in a magnetic field experience a force

Charged particles passing through a magnetic field experience a force, the direction of which can be determined by the right hand palm rule.

- Identify that charged plates produce an electric field

Oppositely charged plates will produce and electric field, the field lines going from the positive plate to the negative plate.

- Describe quantitatively the force acting on a charge moving through a magnetic field

F=qvB sin θ

F = Force (N) q = Charge (C) v = Velocity (ms-1)B = Magnetic field strength (T)θ = Angle between direction of velocity and magnetic field

- Discuss qualitatively the electric field strength due to a point charge, positive and negative charges and oppositely charged parallel plates

A positive and negative point charge will create an electric field, as will oppositely-charged parallel plates. The strength of the electric field at any point is defined as the force per charge, and the direction is the direction of force on a positive charge when placed at that point. The force on a charge in an electric field is given by:

F=Eq

F = Force (N) E = Electric field strength (NC-1)q = Charge (C)

- Describe quantitatively the electric field due to oppositely charged parallel plates

E=Vd

E = Electric field strength (NC-1, Vm-1)V = Voltage between plates (V)d = Distance between plates (m)

- Outline Thomson’s experiment to measure the charge/mass ratio of an electron

J.J. Thomson conducted an experiment in 1897 to determine the charge to mass ratio of cathode rays. This experiment proved the particle nature of cathode rays.

Part 1

Firstly, the magnetic field supplied by the coil and the electric field supplied by the electric plates were turned on in such a way as to cancel each other out and allow the cathode ray to pass undeflected.

Thus;

FB=F E

qvB=Eq

v=EB

Part 2

Next, the electric field was turned off so that the cathode ray was deflected from the magnetic field. Since the cathode ray entered the magnetic field at 90°, the deflection was in a circular arc.

Thus;

FB=F c

qvB=mv2

r

qm

= vrB

Substituting from the first part;

qm

= Er B2

Since the electric and magnetic field strength were known and the radius of the arc of the cathode ray could be measured, the q/m ratio could be found. It was found to be 1.76x1011 C/kg regardless of the cathode materials used, proving without a doubt their particle nature. This high q/m ratio suggested that an electron was either very light or very highly charged. Thomson believed that it was very light and that it was a constituent of an atom, leading to his plum pudding model.

Later experiments (notably Millikan’s oil drop experiment) showed that the charge of an electron was 1.6x10 -19 C. Using this and the q/m value, the mass of the electron was found to be 9.11x10 -31 kg. This mass is less than a thousandth of the mass of an H atom and confirmed that electrons are a part of atoms.

- Outline the role of the following in the cathode ray tube of conventional TV displays and oscilloscopes:

Electrodes in the electron gunA cathode in the electron gun is heated up by a separate voltage supply or heating filament to emit electrons via thermionic emission. These electrons accelerate towards the anodes. A third electrode between the cathode and the anode, called the grid, can be used to control the velocity of the electrons by making it more positive or negative. This controls the brightness of the screen. In a black and white TV there is one electron gun while a colour TV has three.

The deflection plates or coilsThe accelerated electrons travel towards the deflection system, which deflect the electrons in the desired direction towards the screen. In cathode ray oscilloscopes, the deflection is provided by electric fields from two pairs of parallel plates, the X and Y plates, while in TVs magnetic fields from coils are used for larger and more efficient deflections.

The fluorescent screenThe inside of the glass on the end of the vacuum tube is coated with fluorescent material which releases light when struck by electrons. Once the beam moves on from a pixel (or phosphor dot), the fluorescence fades after a short time. In a colour TV, each pixel has three sub-pixels for red, green and blue. In a CRO, a beam of cathode rays scans across the screen relatively slowly to trace out a single dot or line. In TVs, the electrons scan a series of horizontal lines across the entire screen 50 times a second. The odd line pixels are scanned first followed by the even lines. The scanning pattern is called a raster.

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

- Describe Hertz’s observation of the effect of a radio wave on a receiver and the photoelectric effect he produced but failed to investigate

In 1887, Heinrich Hertz verified the existence of electromagnetic waves, the first person to do so after Maxwell’s prediction.

In his experiment, he used an induction coil to produce sparks across the gap between the electrodes of a transmitter. The current oscillated back and forth, generating an e-m wave that was emitted. Hertz found that when sparks jumped across the gap in the transmitter, sparks would also jump across the gap in the receiver, even though it was not connected to a source of current. Hertz concluded that the sparks generated in the receiving loop were induced by an e-m wave that was produced by the transmitter.

Hertz then showed that these waves could be reflected, refracted, could be polarised and travelled at the speed of light, a strong confirmation of Maxwell’s theory. He also noticed that the intensity of the sparks in the detecting loop increased when illuminated by UV light, but did not investigate further.

- Outline qualitatively Hertz’s experiments in measuring the speed of radio waves and how they relate to light waves

Hertz was able to determine the velocity of the radio waves by measuring its frequency and wavelength (since v=fλ). The frequency was known since it was the same as the frequency of the oscillating current, while the wavelength

was found by analysing the interference pattern of two waves with slightly angled paths. He found that the speed of the radio waves was equal to the speed of light.

- Identify Planck’s hypothesis that radiation emitted and absorbed by the walls of a black body cavity is quantised

Classical physics predicted that as the wavelength of black body radiation decreased, the intensity of the radiation would increase. As the wavelength decreased to the UV region, the intensity would

increase without bound and approach infinity, contradicting experimental results. This was known as the ultraviolet catastrophe.

In 1900, Max Planck proposed a theory that was able to reproduce the experimental graphs. His theory was based on a radical assumption that the energy emitted from a black body was not continuous, but rather quantised and emitted as packets of energy called quanta. This suggested that the energy of any radiation could only be a multiple of a minimum value, equal to hf.

- Identify Einstein’s contribution to quantum theory and its relation to black body radiation

In 1905, Einstein extended Planck’s quantum assumption by proposing the particle nature of light – that light consists of quanta called photons, the energy of which was proportional to the frequency of light. Einstein used the photon theory of light to explain the photoelectric effect, for which he was awarded a Nobel Prize. Planck is believed to have been the initiator of quantum physics; however, when Planck first proposed the idea of the quantisation of energy, it was thought to be radical and even Planck himself could not be convinced that this was true. However, when Einstein used it to successfully explain the photoelectric effect, it provided convincing evidence to back up this radical hypothesis. Einstein’s contribution to quantum theory led to the beginning of a new era of modern physics.

The photoelectric effect is the phenomenon in which electrons are liberated from a metal surface struck by electromagnetic radiation with frequency above a certain value. Einstein explained that liberation of electrons is due to the collision of photons in the light with energy more than the work function of the metal.

ϕ=h f 0

ϕ = Work function (J) h = Planck’s constant (6.626x10-34 Js) f0 = Threshold frequency (Hz)

Electrons are only emitted if the energy of the incident radiation is higher than the work function of the metal. The intensity of the light, or the photons per unit area, determines the output current. The liberation of electrons follows the all or nothing principle – all of the energy from one photon must be absorbed and used to liberate one electron from atomic binding, or else no energy is absorbed. If the photon energy is absorbed, some of the energy liberates the electron from the atom, and the rest contributes to the electron's kinetic energy.

Ekmax=hf−ϕ

Ekmax = Maximum kinetic energy of an emitted electron (J)

h = Planck’s constant (6.626x10-34 Js) f = Frequency of incident light (Hz)ϕ = Work function (J)

- Explain the particle model of light in terms of photons with particular energy and frequency

Based on Planck’s quantum hypothesis and Einstein’s photon theory, the energy of light is quantised in packets called photons, the energy of which is proportional to its frequency. This phenomenon whereby light can act as a wave in some situations and a particle in others is known as wave-particle duality.

- Identify the relationships between photon energy, frequency, speed of light and wavelength

E=hf

E = Energy (J) [1 eV = 1.6x10-19 J] h = Planck’s constant (6.626x10-34 Js) f = Frequency (Hz)

c=fλ

c = Speed of light (3x108 ms-1) f = Frequency (Hz)λ = Wavelength (m)

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

A photocell is a device that uses the photoelectric effect to convert light energy into electrical energy. They consist of an anode and a cathode from which photoelectrons are liberated. Photocells are used in burglar alarms, automatic doors and other applications needing “electronic eyes”.

When incident light strikes the cathode, photoelectrons are emitted from the cathode due to the photoelectric effect and current flows. However, when the beam of light is interrupted, such as by a burglar passing by or a person walking towards an automatic door, the beam of light is interrupted and the intensity of the light falls. Consequently, the current drops, and an electronic sensor in the circuit sounds an alarm or opens the door.

- Process information to discuss Einstein’s and Planck’s differing views about whether science research is removed from social and political forces

Both Planck and Einstein lived in Germany during the early 20th century, and they were close friends. However, during World War One, Einstein’s pacifist views became clear as he openly criticised German militarism. Planck, however, supported the German cause, signing the Manifesto of the 93 intellectuals declaring their support for German military actions. After Einstein immigrated to America, he deeply regretted his advice to President Roosevelt to research nuclear bombs, due to his fear that the Germans were developing nuclear technology, which

led to death of many people in Japan. However, Planck continued his research in Germany during WW2, issuing a “persevere and continue working” slogan and concentrating on his research.

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 metals, metal cations are surrounded by a sea of delocalised electrons that can move freely and act as charge carriers.

- Describe the difference between conductors, insulators and semiconductors in terms of band structures and relative electrical resistance

When atoms are packed closely together in a crystalline lattice, the energy levels of the electrons vary slightly and wider energy bands are formed. The valence band is made up of the energy levels of the valence electrons, while the conduction band is the empty band above the conduction band, in which electrons are free to move.

In a conductor, the valence band is partially filled and the conduction band overlaps with the valence band. The forbidden energy gap is non-existent, allowing electricity to be conducted. As temperature increases, resistance increases since the lattice has more energy to vibrate and cause collisions with electrons. In a semiconductor, the energy gap between the valence and conduction band is small, and electrons can easily jump to the conduction band to conduct electricity. As temperature increases, resistance decreases as more and more electrons have sufficient energy to jump into the conduction band. In an insulator, the energy gap between the valence band and the conduction is too great for electrons to jump into the conduction band and conduct electricity.

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

In a semiconductor, when an electron gains sufficient energy to jump into the conduction band, it leaves behind a hole. In the valence band, the electrons then have room to move into the hole when a potential difference is applied, and the hole thus acts as a positive charge carrier moving in the direction of the electric field. In the conduction band, the freed electron can act as a charge carrier. Thus, both electrons and holes contribute to current flow.

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

Conductors have the highest relative number of free electrons, followed by semiconductors in which some electrons have enough energy to jump to the conduction band at room temperature. Insulators have the least number of free electrons.

- Identify that the use of germanium in early transistors is related to lack of ability to produce other materials of suitable purity

Earlier solid state devices used germanium as there was only technology to extract and purify Ge to a sufficient degree. Purification methods for silicon were only developed in the 1950s, and it became the preferred semiconducting material because it is more abundant than Ge, being one of the most abundant elements in the earth’s crust, and retains its semiconducting properties at higher temperatures than Ge.

- Describe how ‘doping’ a semiconductor can change its electrical properties

Pure semiconductors are called intrinsic semiconductors, while doped semiconductors are called extrinsic semiconductors. Doped semiconductors have impurities in their lattice structure; an n-type semiconductor is formed with a group V dopant while a p-type semiconductor is formed with a group III dopant.

An n-type semiconductor is formed with group V dopant atoms, with one extra electron not required for bonding. These electrons occupy the donor level, an energy level close to the conduction band, and thus can easily be promoted into the conduction band, thus increasing the number of free electrons to conduct electricity.

A p-type semiconductor is formed with group III dopant atoms, with one less electron than the lattice around it. As a result, the doped semiconductor has holes in the lattice that creates an energy level near the valence band, called the acceptor level. Electrons in the valence band can easily jump to the holes in the acceptor level, leaving holes in the valence band that can conduct electricity. As a result, the conductivity of the semiconductor is increased due to the larger number of holes.

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

A p-type semiconductor has an abundance of holes in the valence band, while an n-type semiconductor has extra electrons that can carry charge in the conduction band. Both holes and electrons in p and n-type semiconductors improve the conductivity of the semiconductor.

- Describe differences between solid state and thermionic devices and discuss why solid state devices replaced thermionic devices

Thermionic devices used thermionic emission in vacuum tubes to liberate electrons from a hot material. The filament in the vacuum tube is heated and emits electrons towards the anode. Thermionic valves can be either diodes or triodes, which can switch current on and off, rectify AC into DC or amplify current/voltage. Solid state devices serve the same function as thermionic valves, utilising the properties of semiconductors. A p-type and n-type semiconductor joined forms a semiconductor diode, acting as a device that only allows electric current in one direction. A thin layer of a doped semiconductor sandwiched between two semiconductors of the opposite type forms a transistor, which can amplify voltage/current and act as switches.

Solid state devices replaced thermionic devices because:- they were less expensive, fragile and bulky compared to the large vacuum tubes - thermionic devices required a large amount of energy and start-up time for thermionic emission- thermionic devices invariably need replacement as the filament burnt out, the glass could break or the vacuum lost

- 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 development of the transistor led to the development of integrated circuits, also called microchips. Each integrated circuit is a complex circuit with many circuit elements such as resistors and transistors on a single silicon wafer. A microprocessor is a complex microchip that can perform arithmetic, logic and control operations. The integrated circuit allowed extremely complicated circuits to be miniaturised, and allowed great speed of operation due to the extremely small distances that the signals travel.

The invention of integrated circuits formed the basis of modern computers and information systems, having a huge impact on society. They have significantly advanced our methods of communication, research and information storage.

- Identify data sources, gather, process and present information to summarise the effect of light on semiconductors in solar cells

Photovoltaic (solar) cells convert light energy into electrical energy using semiconductors and the photoelectric effect. A solar cell consists of a n-type and p-type semiconductor joined at a p-n junction, where the excess free electrons in the n-type semiconductor diffuse to fill the holes in the p-type semiconductor. As a result, the p-type semiconductor accumulates a negative charge and the n-type semiconductor is left deficient in electrons, giving it a positive charge and setting up an electric field in the depletion zone between the two.

When sunlight strikes the n-type semiconductor, some electrons are promoted to the conduction band via the photoelectric effect. These freed electrons can only move from the n-type to the p-type via an external circuit because of the electric field set up in the depletion zone. As a result, an electric current is created in the external circuit.

4. Investigations into the electrical properties of particular metals at different temperatures led to the identification of superconducti66vity and the exploration of possible applications

- Outline the methods used by the Braggs to determine crystal structure

Sir William Henry Bragg and his son Lawrence Bragg conducted an experiment using X-ray diffraction, on the basis that X-rays shone towards a lattice would scatter and create an interference pattern due to the different planes of the lattice. In Braggs’ experiment, an X-ray tube operating at about 40000V emits X-rays that pass through a collimator, causing the X-ray beam to become parallel. The parallel X-ray bean strikes the crystal sample and the scattered X-rays interfere on a detector. The interference pattern is then analysed and can reveal the lattice structure of the crystal.

The diffraction pattern is analysed with Bragg’s Law:

nλ=2d sinθ

Using this equation, the Braggs were able to discover the crystal lattice structure of metals.

- Identify that metals possess a crystal lattice structure

Metals consist of a lattice of metal cations surrounded by a sea of delocalised electrons.

- Describe conduction in metals as a free movement of electrons unimpeded by the lattice

In a metal, the delocalised electrons can act as free moving charge carriers. The electrons move randomly through the lattice and there is no net electric charge. When an electric field is applied, the electrons drift in the direction opposite to the electric field.

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

The resistance of the metal comes from collisions between the drifting electrons and the metal cations in the lattice. As the temperature increases, the metal cations in the lattice gain more energy and vibrate with a larger amplitude, increasing the chance of collision with the drifting electrons and thus increasing the resistance of the metal. Impurities also increase the resistance of the metal, by adding more obstacles, increasing the probability of a collision.

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

Superconductivity is the phenomenon exhibited by certain materials in which they exhibit zero resistance when they are cooled to a temperature below their critical temperature.

- Discuss the BCS theory

The Bardeen Cooper Schrieffer (BCS) theory can explain the superconductivity of Type I superconductors (metals) but fails to explain the superconducting properties of Type II superconductors. It is an explanation heavily embedded in quantum theory.

Beneath the critical temperature, the vibration of the metallic lattice is minimal. As an electron moves through the lattice, it attracts the positive ions as it passes, causing the lattice to vibrate. This causes a temporary positive region behind the first electron (the lattice moves more slowly than the electron because the cations are heavier) and a second electron is attracted towards the first. The two electrons form a Cooper pair, despite their columbic repulsion; they must be a considerable distance apart, otherwise this columbic repulsion overcomes their attraction. This attractive interaction between the electrons in the pair is brought about indirectly by the interaction between the electrons and the phonons of the vibrating crystal lattice. Many Cooper pairs in the superconductor overlap strongly, forming a large group of Cooper pairs that pass through the lattice unimpeded like a line of linked ice-skaters.

- Discuss the advantages of using superconductors and identify limitations to their use

Superconductors have zero resistance, which is hugely advantageous in power transmission, generation, computers, trains etc. However, they are limited by their low critical temperatures, and thus cost money to keep cool (liquid nitrogen/helium). With further research, superconductors with critical temperatures above room temperature may be found, removing this limitation. Type II superconductors are also brittle and not ductile, making them difficult to make into wire.

- 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

Type I

Material Tc (K)Aluminium 1.20Lead 7.22Mercury 4.15Niobium-aluminium-germanium alloy 21.0

Type II

Material Tc (K)YBa2Cu3O7 (YBCO) 90HgBa2Ca2Cu3O8+x 133

Type I superconductors are more workable; being metals they are malleable and ductile. They are also more easily produced, as they are simple metals or alloys. However, they have lower Tcs than Type II semiconductors, and are thus more expensive to cool using liquid helium as compared to liquid nitrogen. However, Type II semiconductors are brittle and fragile, making them harder to use for wiring. Furthermore, they are more difficult to produce than Type I superconductors.

- 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

The Meissner effect is the phenomenon in which a superconductor expels magnetic fields such that its internal magnetic field is zero.

When an external magnetic field attempts to penetrate a superconductor, it is expelled from superconductor and induces surface eddy currents on the surface of the superconductor, which flow in such a direction as to oppose and cancel the external magnetic field. These surface eddy currents experience no resistance in the superconductor, and cause repulsion between the superconductor and a small permanent magnet, allowing it to levitate over a superconductor.

- Gather and process information to describe how superconductors and the effects of magnetic fields have been applied to develop a maglev train

Maglev trains are levitated off the guardrail using superconducting electromagnets on the base of the train, which are repelled from normal electromagnets in the track below. The superconducting electromagnets provide a strong enough repulsion to be lifted off the tracks and hover.

Propulsion is provided by superconducting electromagnets on the side of the train which are both attracted and repelled to electromagnets on the guideway. The electromagnets on the guideway are synchronised to alternate polarity to attract the train forward and repel it from behind.

Maglev trains do not experience any friction with the track, meaning there is no energy lost as friction meaning more of the forward force is used to propel the train. Furthermore, their levitation means that there is less wear and tear resulting in less maintenance, and makes the train a smoother ride. However, they are expensive to build and run as coolants are required to maintain superconductivity and electromagnets are required along the whole length of the guiderail.

- 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

Using superconductors, computers will no longer be limited by the heat produced by processors, and allows faster electric signals with better energy efficiency. These computers would be able to perform complex operations at a very fast speed.

Motors and generators

The use of superconducting magnets in both generation and in electric motors minimises the power loss as heat. Superconducting motors would be more powerful due to the large amounts of current that could flow through them and the strong magnetic fields from the superconducting magnets. Superconducting generators would reduce power losses as heat, increasing their efficiency and converting more of the kinetic energy inputted into electrical energy. This would also mean that the consumption of fossil fuels required to power generators would be decreased.

Power transmission

The ability to transmit electricity without power loss is possible with superconductors, making thinner wires viable to reduce manufacturing costs. This also allows power stations to be built far away from urban areas, reducing pollution near cities. However, cooling is a difficult and expensive problem, and power would have to be transmitted in DC which would require significant changes to our electricity systems from AC.