Topic 3: Ideas to Implementation

9.4.1.2.1 Explain why the apparent inconsistent behaviour of cathode rays caused debate as to whether they were charged particles or electromagnetic waves


Wave                                                                Particle

Travelled in straight lines

Rays left the cathode at right angles to the surface

Shadow of opaque objects were projected (Maltese Cross)

Deflected by magnetic fields

Could pass through thin foils without damaging them (suggesting they were massless)

Travelled considerably more slowly than light.

Small paddlewheels turned when placed in the path of the rays

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

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

And

9.4.1.2.5 Describe quantitatively the force acting on a charge through a magnetic field

9.4.1.2.4 Identify that charged plates produce an electric field

And

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

 (From f = eq and W= qV and W = fd)

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


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

9.4.1.2.9 Outline the role of: In the cathode ray tube of conventional TV displays and oscilloscopes

TVs                                                               CROs        

Televisions use magnetic deflection as more deflection can be created in a shorter space

CROs use electric deflection because the beam has to move quicker

Self-induction becomes a problem in magnetic coils if the field is changed rapidly

Cro screens are longer (deeper) to account for the smaller deflection

TVs only operate at 50Hz

9.4.1.3.1 Perform an investigation to observe the occurrence of different striation patterns for different pressures in discharge tubes.

9.4.1.3.2 Perform an investigation to demonstrate and identify properties of cathode rays using discharge tubes

9.4.1.3.3 Solve problems and analyse information using:

F = Eq

E = v/d
W = qV = ½ mv
2
To work out the velocity of a charge between plates qV = ½ mv2

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

Hertz noted that sparks could be produced across the gap for distances of several meters.

He noticed that the spark jumped more readily when it was eliminated by UV light. He called it the photoelectric effect but failed to investigate it.

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

He used the Lloyd's mirror experiment where:

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

A black body is any object that will absorb 100% of radiation that hits it and emits radiation at all wavelengths and therefore appears black.
This spectrum shows the amount of each colour being emitted at each temperature.

The classical wave theory predicted that as the wavelength of emission became shorter, the radiation intensity would increase approaching infinite amounts of radiation. This cannot be true as it violates the law of conservation of energy and was described as “the ultraviolet catastrophe”.

The experimental data (that the radiation intensity curve corresponding to a given temperature has a given peak and then declines) could not be explained.  

Planck suggested that the energy radiated and absorbed by a black body was not continuous but rather quantised - it should be treated as packets of energy (photons). The energy could be described by E = hf

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

9.4.2.3.2 Assess Einstein’s contribution to quantum theory and its relation to black body radiation

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

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

And

9.4.2.3.4 Solve problems using

https://qph.ec.quoracdn.net/main-qimg-d9743eeb2963b7c91fe0a519b210abc0

9.4.2.3.1 Perform an investigation to demonstrate the production and reception of radio waves

9.4.2.3.3 Summarise the use of photoelectric effect in photocells

A photocell is a device that uses the photoelectric effect (the interaction between a photon and an electron in a metal to produce an electron no longer bound by the metal) to operate.

These devices include Phototubes and photomultipliers. They could additionally be photoresistors, which are used to detect light.

Photocells operate by the photoelectric effect, which dictates that electromagnetic radiation (above a certain cut-off frequency - principally in the ultraviolet range for most materials) falling onto the surface of a material causes the emission of photoelectrons and a current to flow in the photocell.

Photoresistors and light detectors rely upon the change in resistance caused by the emission of electrons from materials when exposed to electromagnetic radiation.

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

Political Views of Planck and Einstein

9.4.3.2.1 Identify that some electrons in solids are shared between atoms and move freely

Some electrons in solids are shared between atoms and move freely

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

Electrically conductive

No energy gap

As temp increased, vibration of ions increases, hindering the straight movement of electrons

More electrically conductive than insulators BUT less electrically conductive than conductors

Not electrically conductive - high resistance

Large energy gap

Valence band: the highest energy band in which electrons exist in an unexcited state

Conduction band: band where excited electrons can move freely and conduct electricity.

As can be seen only when the electrons in a semiconductor's valence band have enough energy to move into the conduction band will it conduct electricity. This energy is provided by heat.

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

The net absence of an electron in a full valence band is known as a positive hole.Holes act as positive charge carriers and move in the direction of conventional current.

absence of electons as holes

Positive current flow relies on the movement of holes from one atom to another. i.e. the hole is filled by a neighbouring electron which simultaneously leaves behind a new positive hole.

Thus electron current is much faster as they are able to flow through the conduction band, whereas positive current flow is slower (relies on “leapfrogging”).

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

Conductors have many electrons in the conduction band able to act as free charge carriers

Semiconductors have a few free electrons

Insulators have no free electrons

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

Germanium was the first element to be used in early transistors as it was the only element which could be purified to a sufficiently high level.

Germanium Disadvantages

Silicon Advantages

9.4.3.2.6 Describe how ‘doping’ a semiconductors can change its electrical properties

Doping: Addition of a group III or group V elements to group IV semiconductors to change its electrical properties

Addition of a group V dopant: N-type

n-type figure

Addition of a group III dopant: P-type

p-type figure
Group III atoms have one less electron in valence shell than group IV.

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

N-type

P-type

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

Thermionic Diode

thermionic diode figure

Solid State Devices

p-n junction figure

The application of current can be either “forward” or “reverse” biased:

Forward Biased

forwrd baised figure

Reverse Biased

reverse based figure

Thermionic vs solid state pros and cons:

Pros                                                                Cons

Amazing breakthrough

Bulky

Used in communication systems like wireless radio to act as an amplifier

Hot

Glass is fragile

Warming up time

Needs replacing

Huge rooms to hold a small computer

9.4.3.3.1 Perform an investigation to model the behaviour of semiconductors

Limitation

9.4.3.3.2 Discuss how shortcomings in available communication technology lead to an increased knowledge of the properties of materials with particular reference to the invention of the transistor

During WWII, an increased need for more efficient and sensitive radar and radio communications sparked research into transistors.

Germanium – Experimentation yielded transistors using Germanium crystals.

Silicon – this led onto research which enhances our understanding of methods of silicon production, eventually allowing the creation of Si transistors which overcame shortcomings of Ge use.

9.4.3.3.3 Assess the impact of the invention of transistors on society with particular reference to their use in microchips and microprocessors

The invention of the transistor paved the way for the development of miniature electronic circuits - the most profound being integrated circuits (IC). Devices could now be made that required less space and less power and had far more reliability than existing devices. With the introduction of the transistor, electronic systems have continued to become smaller, more sophisticated and cheaper. It led to the so called computer revolution which has changed our lives greatly. Today, IC’s are used in a huge variety of microprocessor based equipment, ranging from mobile phones and calculators to ATM’s. The first microprocessors appeared in 1971 and have been reducing in size ever since.

Fast computers and tiny electronics have connected the world in an incredible amount of ways. For example, communication technologies have increased in efficiency and reliability, and are so advanced that through GPS, they are able to pinpoint any location on Earth to within a few metres. This has had a profound impact on missile and war technology, satellite technology and communication technology. It is now possible to speak and communicate with others all the way around the world, wirelessly and cheaply.

Computers no longer require a full cooling system and a storeroom full of computer circuits. They can now process huge volumes of information which has reduced repetitive manual labour. Transistors are particularly useful in memory chips. They control the flow of charges to a tiny capacitor. Other transistors act as amplifiers when information is retrieved.

In terms of environmental impact, there are positives and negatives. It has led to the development of solar cells in an attempt to battle scarce resources and reduce our dependence on fossil fuels. There is less pollution because transistors require less energy to run. Furthermore, there is no EMR produced from transistors. The main ingredient of semiconductors, silicon, is also an extremely abundant material.

On the other hand, while transistors are small in size, they have contributed to waste disposal problems. The process of producing transistors uses more toxic chemical and creates pollution, especially on non-degradable plastics which disturb the ecosystem when they are thrown away.

Essentially impacts of the transistor have occurred in communication, technology, daily lives, and of course the environment. These impacts have on the whole been positive to society as they have greatly improved living standards and encouraged rapid growth in many areas of science.

9.4.3.3.4 Summarise the effect of light on semiconductors in solar cells.

Solar cells convert the sun’s light energy into electrical (chemical) energy using p-n junctions

semiconductors in solar cells

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

braggs lwa.jpg

9.4.4.2.2 Identify that metals possess a crystal lattice structure.

Metals possess a crystal lattice structure. Metals are three dimensional lattices of ions in a sea of delocalised electrons (which come from the valence shell) which hold the ions in an array.

  

9.4.4.2.3 Describe conduction in metals as a free movement of electrons unimpeded by the lattice. (Include a comparison of conduction in superconducting materials above and below their critical temperature)

The sea of electrons in the lattice (made up of valence electrons) are loosely called a ‘sea’ due to the fact that they are loosely bound to the metal ion and so can drift between ions. As they can move, they can transfer charge and thus conduct electricity.

In a superconductor that is below critical temperature, the electrons pass almost unobstructed through the lattice (explained through the BCS theory for Type 1 superconductors). The lack of collision means that very little energy is lost. Above their critical temperature, the superconductors behave ‘normally’, that is like a metal or a compound or an alloy depending on what substance they are.  

9.4.4.2.4 Describe the effect, on the resistance of a metal, of scattering of electrons by lattice vibrations and by the presence of impurities in the lattice. Include a description of the link between lattice vibrations and temperature.

Essentially, resistance measures to what degree a substance opposes the passage of an electric current.

As the delocalised electrons drift around the lattice, any impurities and imperfections in the lattice slows down their flow. This leads to an increased resistance.

An increase in temperature leads to the atoms that make up the lattice shaking. The atoms that make up the lattice structure of conductors start to vibrate more vigorously (over greater distances). As a result of this, mobile electrons in the conductor may collide more often with phonons (packets of energy). Hence an increase in electrical resistance.

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

9.4.4.2.6 Discuss the BCS theory with reference to Cooper Pairs and Phonons - use diagrams.

The BCS theory (named after US physicists John Bardeen, Leon Cooper and John Schrieffer) explains superconductivity in terms of electron pairs and packets of sound waves related to lattice vibrations (called phonons). It only explains type I superconductors.

Formation of Cooper Pairs

At temperatures below the critical temperature for particular superconductors, the movement of electrons is enhanced by lattice vibrations (phonons - the lattice vibration equivalent of a photon (the quantum of light), i.e. the quantum of vibrational mechanical energy.) which cause electric field effects resulting in electron pairing (by overcoming what would normally be strong repulsive forces between like charges) and an assisted passage through the lattice with negligible energy loss.
Why they stay together
When an electron part of a cooper pair passes close to an ion in the lattice, the attraction between the negative electron and the positive ion causes a vibration to pass from ion to ion until the other electron of the pair absorbs the vibration. The net result is that the first electron has emitted a phonon and the second one has absorbed it. This is what keeps the cooper pair together.

Why is the negligible energy loss.  
The reason for the negligible energy loss can be thought of a line of skaters who are linked. If one experiences a bump, they group won’t slow down as he is supported by the rest. Because resistance is effectively zero, very narrow wires can carry very large currents. The lower the temperature, below the critical temperature, the higher that current can be. That current produces a magnetic field around the conductor. The strength of the magnetic field will reach a point where it will cause the loss of the superconducting state thus putting an effective limit on the current that can flow in any particular superconductor.

9.4.4.2.7 Discuss advantages of using superconductors and identify limitations to their use

Advantages:

Limitations:

9.4.4.3.1 Process information to identify some of the materials that have been identified as exhibiting superconductivity

Material                                    Critical Temperature Tc (K)                     Metal, alloy or compound

Hg

4

Metal

Al

1.2

Metal

Nb3Sn

18

Alloy

NbTi

10

Alloy

Bi2Sr2CaCu2O8

85

Compound

YBa2Cu3O7

90

Compound

9.4.4.3.2 Perform an investigation to demonstrate magnetic levitation

9.4.4.3.3 Explain why a magnet is able to hover above a superconducting material that has reached the temperature at which it is superconducting (see Meissner Effect)

When a superconductor is placed in a weak external magnetic field, and cooled below its Critical Temperature, the magnetic field is ejected (the superconductor becomes a perfect dimagnet). This is because the induced currents generate a magnetic field inside the superconductor that just balances the field that would have otherwise penetrated the material.

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

9.4.4.3.5 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  

Reducing the size of silicon chips is limited to heat build-up from the resistance in connections to the chip.  Present connections in the chip are also much slower in conducting electrical signals than superconductors. Using superconductors will allow more densely packed chips and much higher processing which can result in smaller and faster computers.

Generators & Motors  

Motors & Generators using superconductor magnets would not require an iron core. They would also become smaller & lighter. This could result in lower energy input from fossil fuels which results in them being cheaper to operate (less power used) for consumers & better for the environment (reduction in greenhouse gases if the energy was generated using fossil fuels.)

Transmission of electricity through power grids  

Traditionally electricity is transmitted at high voltages and through thick copper wires to minimise energy ‘loss’ in the form of heat from resistance. By using superconductors to transmit power there would not be minimal resistance and hence lower voltages, larger currents and thinner wires could be used. This would reduce cost to the power companies, increasing their profit or creating savings for consumers. This could also reduce greenhouse gas emission as not as much power is needed to be generated in the first place.