HSC Physics – Ideas to Implementation – dot point notes

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HSC Physics – Ideas to Implementation syllabus dot point summary

This is a set of HSC Physics notes for each syllabus dot-point of Ideas to Implementation. HSC Physics tutoring at Dux College provides students with the right support to achieve a band 6 result in HSC Physics.

Contents

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

Many experiments were performed by scientists in the late 19th century. These experiments had results which indicated cathode rays were either charged particles or electromagnetic waves.

A number of experiments were used by both groups as evidence and support of the conflicting arguments. Their interpretation was often ambiguous and some findings were used to support both arguments.

An example is the sharp shadows cast from objects in the tube, suggesting the rays travelled in straight lines

Observations suggesting charged particle Observations suggesting wave
Deflected by both electric and magnetic fields (charged) Initially not deflected by electric and magnetic fields*
Slower than light and other EM waves Essentially unaffected by gravity
Emitted perpendicular to cathode surfaces (light is emitted in all directions) Could pass through certain thin metal sheets
Could turn a paddle wheel (had momentum and mass) Produced fluorescence

*Note: Cathode ray tubes initially had too much gas in them. These gas particles were ionised by the cathode rays and hence moved towards the deflection plates, neutralising them. When the technology was developed to evacuate the tubes more, deflection was observed.

Cathode Ray Tubes allow manipulation of Charged Particles 

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

Cathode ray tube: an evacuated tube which allows the unobstructed movement of cathode rays from a cathode to an anode.

Cathode ray: a stream of electrons from the surface of a cathode and accelerated towards anode by a high potential difference

  • It is a highly evacuated glass tube containing 2 electrodes (negative cathode, positive anode).
  • Structures built into or around tube allow manipulation of the rays, such as electric plates, magnetic fields,
    extra anodes and collimators
  • Solid objects may be placed in the path of the rays to block them.

cathode ray tube figure

Moving Charged Particles experience a force

  • Identify that moving charged particles in a magnetic field experiences a force
  • Describe quantitatively the force acting on a charge moving through a magnetic field: Latex formula
  • Solve problems and analyse information using: Latex formulaLatex formula and Latex formula

A charge moving through a B-field experiences a force

Latex formula

The force is a vector quantity so the direction can be determined using the R-hand palm rule.

  • Note that this force is perpendicular to the direction of motion (I.e. it is a centripetal force) and will cause the particle to undergo UCM (if no other forces are in play)
  • Also note that the “left” hand palm rule can be used on negatively charged particles (be careful)
  • The speed of the charged object remains constant even though its direction changes.
  • If the particle enters at an oblique angle, it will follow a helical path

direction of motion figure
different type of motion figure

Charge Plates, Electric Fields and Point Charges

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

Charged Plates

  • Charged plates produce an electric field
  • This electric field strength between oppositely charged plates is: Latex formula
  • The force on a charged particle due to an electric field is: Latex formula
  • Therefore, the force on a charged particle between two charged plates can be given by: Latex formula
  • Note: direction of force is not perpendicular, but more like force due to gravity, producing a parabolic path
Electric Fields of Charged Plates
  • Are represented by field lines perpendicular to the plates
  • Evenly spaced and parallel except the curved end lines (use ruler)
  • Always run from positive to negative (label).

figure of electric fields

Point Charges

  • Point charges produce radial electric fields which diminish in strength further from origin.
  • Field is pointed radially away from positive charges and radially towards negative charges.
  • Remember that line density represents field strength, so field weakens with distance
Electric Fields of Point Charges
  1. Like charges: their fields cancel out and create a zone void of electrostatic influence
  2. Opposite charges: their fields join up and the charges are attracted to each other

radial electric fields

Thomson’s Experiment

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

 

  • Thompson built a cathode ray tube, including a set of charged parallel plates to provide a uniform electric field and wire coils to provide a uniform magnetic field.
  • The fields were oriented at right angles, producing opposing forces on a moving particle.
  • Cathode ray particles were produced at the cathode, accelerated and focused by the double collimator anodes and directed through both the electric and magnetic fields.
  • It then struck a fluorescent screen on the other end of the cathode ray tube.
    Thomson’s experiment involved two stages:

cathode ray tube

1. Equating electric and magnetic field forces

  • The electric and magnetic field strengths were varied until their opposing forces acting on the cathode ray particles exactly cancelled, allowing the rays to pass undeflected.
  • Using calculations, Thomson was able to find the velocity of the cathode ray particles.
Latex formula Latex formula Latex formula

2. Equating magnetic and centripetal forces

  • The electric plates were then switched off so only the force due to the magnetic field acted on the cathode ray.
  • The radius of curvature of the cathode ray was found and Thompson was able to use calculations to find the charge : mass ratio of the particles by equating FLatex formula with centripetal force.

Latex formula
Latex formula
Latex formula
Latex formula

3. Conclusion

  • Thomson’s value for Latex formula for cathode ray particles was extremely large suggesting the existence of a subatomic particle with either:
    • A very small mass
    • A very large charge
  • Thomson concluded that the size of the particle must be very small as the charge would have been so large, it would not have been contained stably inside an atom.
  • The direction of deflection suggested that this particle was negatively charged and this led to discovery of the electron.
  • Thomson’s experiment put to rest the wave-particle debate on cathode rays.

Conventional TVs and Oscilloscopes

  • Outline the role of:
    • electrodes in the electron gun
    • the deflection plates or coils
    • the fluorescent screen

in the cathode ray tube of conventional TV displays and oscilloscopes

Electron gun

  • Heating filament heats the cathode, releasing electrons by thermionic emission
  • The positively charged anode exerts a force on negatively charged electrons, accelerating them along the tube
    • There may be a few anodes, for focusing and accelerating the beam

Deflecting plates

  • A pair of parallel deflecting plates produce an electric field that can deflect the beam up or down
  • Another pair of deflecting plates deflect the beam left or right
  • Magnetic fields produced the electromagnets (copper windings) may also be used to deflect the electron beam
  • These electric and magnetic fields can be used to aim the electron beam anywhere onto the fluorescent screen

Fluorescent screen

  • Glass screen is coated with a fluorescent material, which emits light when high energy electrons strike it and the image is retained by the eyes
Television Displays
  • 3 electrons guns (one for each of the primary colours red, green and blue) produce rays
  • Electromagnets used to deflect the beams (cheaper than high precision electric plates)
  • Fluorescent screen consists of dots of fluorescent paint on the screen which are excited when electrons strike
  • These dots fluoresce and emit light.
    • Once the electron gun produces cathode rays corresponding to the correct colour, they are focused by a shadow mask then strike the fluorescent dots on the screen to give an image.
  • Each image is formed as the beam scans from left to right and top to bottom
  • The colour is determined by the relative intensities of the three beam colours

television displays figure

Cathode Ray Oscilloscopes (CRO):

  • Uses a single cathode ray tube to display an input signal voltage as a waveform
  • High precision electrostatic deflection is used
  • Horizontal deflection of the beam is provided by a time based voltage
    • The waveform of the voltage across the horizontal plates is known as a sawtooth waveform.
  • Vertical deflection is provided by input voltage, allowing voltage to be plotted as a function of time
    • If input voltage varies from zero, the cathode ray is deflected upwards for positive polarity or downwards for negative polarity
  • Controls on a CRO include:
    • Time base control to set the speed at which the line is drawn, to select ‘time per division’
    • Vertical control sets the scale of vertical deflection, controlling amplitude of the trace.
cathode ray oscilloscope figure one cathode ray oscilloscope figure two

Modelling Striation Patterns

  • Perform an investigation and gather first-hand information to observe the occurrence of different striation patterns for different pressures in discharge tubes

Striation patterns refer to light and dark areas inside a discharge tube. Electrons colliding with air particles release light dependant on the energy of the electrons, but also on the amount of gas inside the tube. As the pressure of gases change, so do the striation patterns.

In this experiment, we had 4 discharge tubes each with different pressures: (percentage of atmosphere).

  • 5% Atm – Glowing purple/pink streamers formed, extending from the cathode to the anode
  • 2% Atm – the pattern changed to a series of alternating light and dark bands running perpendicular to the length
  • 0.5% Atm – the dark gaps between the lines widened (there were fewer lines), with the pink/purple glow concentrated around the anode, and a blue glow forming at the cathode.
  • 0.01% Atm – There were no striations. Instead, the glass around the whole anode glowed yellow-green.

Brief Explanation

The exact nature of the striation patterns depends on what gas is used (more in Q2Q).

  • Bright regions are areas where electrons struck gas atoms and excited the shell electrons
  • This makes these electrons less stable and when they return to their stable state, they give off the energy as EMR (light in this case)
  • The dark bands form in areas where the cathode ray electrons did not have enough energy to cause this excitation

Modelling Properties of Cathode Rays

  • Perform an investigation to demonstrate and identify properties of cathode rays using discharge tubes:
    • containing a maltese cross
    • containing electric plates
    • with a fluorescent display screen
    • containing a glass wheel
    • analyse the information gathered to determine the sign of the charge on cathode rays

Maltese Cross: Particle

  • A maltese cross was placed near the end of the tube, opposite the cathode
  • When voltage was applied, the cathode ray, travelling straight and being obstructed by the Maltese cross, cast a proportional shadow, showing the cathode rays had been blocked and that they travel in straight lines.
  • The shadow had a very sharp edge, suggesting that diffraction was not occurring and therefore cathode rays could be particles, however waves also travel in straight lines.

Electric Plates: Wave then Particle

  • When electric plates were set up inside the tube and a potential difference was placed across them, the cathode ray beam was deflected.
  • To perform this experiment, the tube had a curved screen set up inside it so that the horizontal path of the beam was visible.
  • The beam was deflected towards the positive plate, so we concluded that cathode rays were negatively charged.
  • Initially, no bending was observed but this was corrected (see page 1)

Fluorescent Screen: Particle

  • A fluorescent screen in the path of the cathode ray was lit up
  • This suggested that the cathode ray carried enough energy to produce the reaction in the screen necessary to produce light.

Paddle Wheel: Particle

  • A glass paddle wheel was mounted on the inside of a cathode ray tube and allowed to spin
  • When the cathode ray struck the paddle wheel, it caused it to spin and rotate along the tube.
  • Through conservation of momentum, the fact that the cathode ray could move the wheel by colliding with it strongly suggested that they had mass, and were therefore particles.

The Reconceptualisation of the Model of Light led to an Understanding of the Photoelectric Effect and Blackbody Radiation

Hertz’s Experiment

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

Hertz demonstrated the production of radio waves and confirmed Maxwell’s prediction that there were EM waves with frequencies outside the visible light spectrum.

Aim

Hertz wanted to produce EM waves with frequencies and wavelengths other than visible light.

Setup and Method

An induction coil was used to create a rapidly oscillating B-field which caused a rapid sparking across a gap between spherical electrodes in a conducting circuit.

This circuit formed the transmitter and a receiving loop also with a gap in it, was placed some distance from the transmitter.

The high voltage induction coil connected to the transmitter was switched on and changes observed

Changes in the receiving electrodes were observed

induction coil figure

Observations and Explanations

When the power was on, sparking occurred between the electrodes at the transmitter and this also resulted in sparks at the receiver.

  • The high voltage AC produced sparking and a rapidly oscillating electric field which gave rise to a magnetic field and so on.
  • Thus, EM radiation (radio waves) were produced and traversed the distance to the receiver
  • The EM radiation travelling towards the receiver struck the electrodes of the receiver, energising electrons in the conducting surface and caused them to jump across the gap as a spark.
  • Note: there were no electrical connections between the transmitter and receiver

Other properties observed

  • Reflection – Hertz reflected waves off a zinc plate and they still reached receiver to cause sparking
  • Refraction – Radio waves were refracted through a prism
  • Polarisation – He rotated the receiver’s plane relative to the transmitter
    • The receiver’s intensity and length of sparking at the receiver was a maximum when the plane was parallel and a minimum when perpendicular.
  • Interference – he observed that waves reaching the receiver from 2 different paths interfered constructively and destructively to produce interference pattern of light and dark patches.
  • Distance – the length and intensity of sparking at the receiver was not affected by the distance between transmitter and receiver. Suggested that light waves are self-propagating
  • Speed – The speed was accurately measured to be “c” (see next dot point)

Conclusions

These observations strongly supported Maxwell’s prediction of EM radiation and model of light: self-propagating, transverse waves of alternating electric and magnetic fields that are perpendicular to one another. Hertz concluded that radio waves were able to cause sparking at the receiver.

Photoelectric Effect observation

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

The photo electric effect is the emission of electrons from the surface of a conductor when it is subject to EMR.

  • This was first observed by Hertz in 1887 but he did not create the above definition.
  • He enclosed the receiver in a dark box to create a dark environment for making observations.
  • In doing so, he observed that the length and intensity of sparking diminished.
  • On removing the various walls of the box in succession, he found that only the portion of the case which shielded the receiver from the transmitter had this effect on sparking.
  • Hertz knew that radio waves produced by the transmitter would not be blocked by the box so he reasoned that the box must be blocking other type of EM waves.

This led onto further investigations and observations.

  • The length and intensity of sparking at the receiver was diminished when glass (blocks UV) was used as a shield between transmitter and receiver.
  • Then quartz was used (quartz does not block UV) and no change to the sparking was observed
  • When a mercury vapour lamp (emits UV) was shone onto the receiver, sparking was increased.

He named these effects the photo electric effect but did not investigate any further.

Hertz’s experiment to measure the speed of radio waves

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

In order to prove that these radio waves were EM waves, he showed they had similar properties to light, namely its speed.

Measuring the Speed of Radiowaves

  • In order to determine velocity Latex formula, Hertz needed to measure frequency and wavelength.
  • The frequency of the waves was already known, as Hertz had used an RLC (resistor inductor capacitor) in his set up. This produced a sinusoidal current of constant frequency.
  • To measure the wavelength, radio waves were allowed to reach the receiver from 2 different paths – one directly and one following reflection off a metallic surface.
  • These two waves met at the receiver and created an interference pattern of light and dark patches corresponding to relative max and min.
  • By moving the receiver back and forth, the interference pattern of the waves could be analysed
  • The difference between successive maxima or successive minima gave Latex formula

Conclusion: Hertz then found the speed of radio waves using Latex formula and found Latex formula

speed of radio waves

Planck’s Hypothesis

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

Radiation is emitted and absorbed by a black body radiator in discrete amounts called quanta. Their total energy occurs in multiples of E=hf. .

A black body is an object that absorbs all incoming radiation (none is reflected or passes through). As it gets hotter, it will emit radiation with a wavelength dependent on its temperature.

  • Classical physics predicted that as the wavelength of the radiation emitted by a black body decreased, the radiation’s energy would increase.
  • This would mean that as the black body got hotter, a graph of energy/intensity against wavelength would rise infinitely as wavelength approaches zero.
  • This is a violation of the conservation of energy and thus experimental results did not match theoretical predictions – hence the “UV catastrophe”.
  • In reality, as the temperature increased, the peak wavelength of radiation decreased AND the intensity of the shorter wavelengths decreased and are hence quantifiable (obey C.O.E.).

peak wavelength of radiation

Einstein’s contribution to Quantum Theory

  • Identify Einstein’s contribution to quantum theory and its relation to black body radiation
  • Identify data sources, gather, process and analyse information and use available evidence to assess Einstein’s contribution to quantum theory and its relation to black body radiation

Einstein proposed that EMR is made of concentrated bundles of energy known as photons.

  • Einstein extended Planck’s idea from black body radiators and applied it to all light, suggesting that light occurred in discrete packets/bundles called photons With this quantised particle model of light, Einstein was able to explain the photoelectric effect as:
  • The emission of photoelectrons from the surface of a conductor when light is incident on its surface.
  • When more experiments were conducted, it was observed that:
  • A small current was observed when photons struck the cathode (i.e. electrons struck the collector)
  • As the intensity of the light increases, there was an increase in current
  • When the frequency of light was increased, the kinetic energy of the electrons increased

photoelectric effect

Other terms

  • Photoelectrons: electrons emitted from the conductor surface by the photoelectric effect)
  • Stopping voltage: the minimum voltage required to to stop the flow of photo electrons.
  • Work function: the work which must be done to overcome the attraction between the conductor surface and the electrons.
  • Critical frequency: The minimum frequency of light required to just overcome the work function of the metal.
  • Intensity: the amount of energy imparted by the radiation on to the conductor, determined by the number of photons incident per unit time

maximum kinetic energy vs frequency

Einstein’s Explanation of the Photoelectric Effect

  • One to one: Energy was only transferred from one photon to one electron.
  • Latex formula: Photons had energy dependent upon frequency, thus KE of photoelectron and its stopping voltage depends on frequency of incident wave.
  • Latex formula: Energy of incident photons must be sufficient to overcome the work function. Below that energy, no electrons would be emitted, explaining the threshold frequency.
  • Intensity α Current: The number of electrons emitted and the photocurrent magnitude would depend on the intensity of the light (the number of photons incident per unit of time).
  • All or none: Photons could transfer all or none (not part) of their energy to electrons, meaning there is no energy build up, nor time delay.

Classical understanding vs experimental findings

1. The intensity problem

Classical: Energy of light increases as the intensity of the beam increases.

  • This suggests that kinetic energy of the photoelectrons would be determined by the beam’s intensity
  • Therefore the energy required to stop the voltage would also increase with higher intensity.

Experimental: KE of photoelectrons was a function of the frequency of the light and the stopping voltage was independent of light intensity.

2. Time delay problem

  • Classical: Electrons require some time to gain enough energy from low intensity light to be emitted.
  • Experimental: There was no delay between the light being shone and the electrons being emitted, no matter how dim the light source. i.e. All or none of the energy is transferred.

3. The frequency problem

  • Classical: Photoelectric effect should occur at all frequencies of light if the energy of waves was determined by their intensity (provided intensity was enough).
  • Experimental: No electrons were ejected from the metal below the critical frequency regardless of intensity

Equations

  • Light exists as photons, each with energy given by Latex formula
  • To produce the photoelectric effect, the energy contained in a photon must be greater than or equal to the work function
  • Latex formula to emit photoelectrons
  • If the energy of the photon is greater than the work function, the additional energy of the photon over the work function will provide the kinetic energy of the photoelectron
    • Latex formula
    • Latex formula
    • Latex formula
    • Latex formula – The work that needs to be done by the field to halt the electron is: W = qV

Assessment

Einstein’s theory explains what classical physics could not, lending significant support to quantum theory. The wave and particle properties of light described by Einstein also led to de Broqlie’s hypothesis of the wave-particle duality of matter.

Therefore, Einstein’s predictions were an important step towards the development of Quantum theory.

Particle model of light

  • Explain the particle model of light in terms of photons with particular energy and frequency
  • Identify the relationships between photon energy, frequency, speed of light and wavelength: Latex formula and Latex formula
  • Solve problems and analyse information using: Latex formula and Latex formula

Light is considered to consist of a stream of particles or discrete bundles of energy called photons. Photons carry an amount of energy that is proportional to the frequency of radiation.

  • Latex formula
  • Latex formula
  • Latex formula

The Photoelectric Effect in Photocells

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

A photocell is a device which uses a light source to produce a photocurrent

  • A light source (laser) is directed towards a metal plate and excites electrons in the metallic surface of the emitter, emitting them as photoelectrons.
  • The metallic surface is curved so that the photoelectrons are focused towards a collecting electrode.
  • It then passes through an external circuit as current.
  • If the light beam is interrupted (i.e. something gets in the way), the lack of photocurrent can trigger an alarm.
  • Photocells are commonly seen in motion detectors, light detectors and security camera

Einstein and Planck’s different views

  • 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 nationalist and used his research to support the political Nazi movement.
  • Einstein was a Pacifist and refused to use his research to support social or political movements.
  • In Sept. 1914, Planck signed the “Manifesto of 93 German intellectuals to the civilised world” in support of the growing militancy of Germany.
  • Einstein with 3 others signed a counter-manifesto which did not support the use of scientific research for the German regime.
  • Planck supported the Nazi regime and was appointed president of the Kaiser-Wilheim Institute for scientific research which contributed to the war effect.
  • Einstein fled Germany refusing to support the Nazis as he was Jewish.
  • Einstein wrote to President Roosevelt and encouraged the development of the Manhattan Project, an American research effort aimed at preventing the scientific development of German atomic bombs which would be misused in order to advance the Nazi Political Goals.

Limitations of Past Technologies and Increased Research in the Structure of the Atom resulted in the Invention of Transistors

Shared Electrons in Solids

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

Some electrons in solids are shared between atoms and move freely

In metals, a lattice of positive ions is surrounded by a sea of delocalised valence electrons which are free to move between atoms.

  • These electrons drift randomly with essentially no net movement
  • When an electric field is applied, they drift against it and are able to conduct electricity
  • In insulators, atoms are held in a lattice structure by strong covalently bonded valence electron pairs.
  • These electrons are not free to move and so cannot conduct electricity

Difference between Conductors, Insulators and Semiconductors

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

 

  • In atoms, electrons exist in well-defined energy levels or “shells” in which they have a certain well defined amount of energy.
  • Electrons in each of the electron shells possesses a certain amount of energy represented by a “band”
  • These energy bands are separated by zones where electrons with those energies cannot exist.
  • They are named forbidden “gaps” or forbidden energy zones

Two important bands

  • 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.

difference between conductors-insulators and semiconductors

Conductors

  • Electrically conductive – very low electrical resistance
  • The valence and conduction bands are touching or overlap – no forbidden energy zone
  • Electrons can thus move into the conduction band with very little additional energy or excitement.
  • Electrons therefore do not remain attached to individual atoms but form a sea of delocalised electrons which may drift and facilitate current flow when an electrical potential is applied.
  • Resistance increases with increasing temp.
  • If the temperature is increased, the vibration of ions about the equilibrium positions increases.
  • This hinders the straight movement of electrons and increases the resistance of the conductor.

Insulators

  • Not electrically conductive – high electrical resistance
  • Insulators are often covalently bonded substances.
  • There is a large forbidden gap between full valence bands and empty conduction bands.
  • It is very difficult for electrons to obtain sufficient energy to jump over this forbidden energy zone and enter this conduction band.
  • Thus, there are no free charge carriers to facilitate current flow

Semi-Conductors

  • More electrically conductive than insulators BUT less electrically conductive than conductors
  • Semiconductors are usually covalently bonded group IV atoms:
  • They have smaller forbidden energy zones then insulators but larger than conductors.
  • Electrical resistance decreases with increasing temperature.
  • At low temperatures, valence band is completely occupied and the conduction band is empty. So conductivity is low and resistance is high.
  • At high temperatures, oscillations of the lattice provides electrons with sufficient energy to jump across the energy gap, increasing the number of charge carriers in the conduction band
  • The net absence of an electron in the valence band also creates a positive hole which is able to carry positive current in the opposite direction to electrons.

Absence of Electrons as Holes

  • Identify absences of electrons in a nearly full band as holes, and recognise that both electrons and holes help to carry current
  • Perform an investigation to model the behaviour of semiconductors including the creation of a hole or positive charge on the atom that has lost the electron and the movement of electrons and holes in opposite directions when an electric field is applied across the semiconductor 

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”).

Relative Number of Free Electrons

  • 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
  • Semi-conductors have a few free electrons
  • Insulators have no free electrons

The use of Germanium in Early Transistors

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

A transistor is a semiconducting device used to amplify and/or switch current

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

  • At higher temperatures, it becomes too good a conductor – allows excessive and dangerous currents
  • Germanium is rare and costly.
  • Silicon eventually replaced germanium as the semi-conducting material of choice in transistors

Silicon Advantages

  • It is the second most abundant element on earth – widely available and cheap to obtain.
  • It retained its semiconducting properties at relatively high temperatures
  • It forms oxide wafers which can be doped with impurities.
  • Processing techniques were developed to produce very pure crystals.

Doping a Semiconductor

  • Describe how ‘doping’ a semiconductor can change its electrical properties
  • Identify differences in p and n-type semiconductors in terms of the relative number of negative charge carriers and positive holes

Doping is the addition of impurities into a semiconductor to enhance its electrical conductivity.

It involves the addition of a group III or group V elements to group IV semiconductors

Addition of a group V dopant: N-type

n-type figure

  • Group V atoms have one more valence electron than group IV atoms.
  • When added, 4 of its 5 electrons form covalent bonds with adjacent atoms but one remains unbounded.
  • This electron can be easily promoted to the conduction band and act as a free charge carrier, increasing negative charge carrier density.
  • This increases conductivity and reduces resistance.
  • The majority of charge carriers in semi conductors doped with group 5 elements are negative, thus these semiconductors are N-type

Addition of a group III dopant: P-type

p-type figure

  • Group III atoms have one less electron in valence shell than group IV.
  • When added into a semiconducting lattice, the 3 valence electrons form covalent bonds with surrounding atoms.
  • The absence of a 4th electron leaves a positive hole in the lattice which is able to act as a positive charge carrier in the valence band.
  • This increases positive charge carrier density, increasing conductivity.
  • The majority of charge carriers are positive, so it is called “P-type”

Extrinsic vs Intrinsic Semi-conductor

  • Extrinsic semiconductor- if conduction is dominated by donor or acceptor impurities
  • Intrinsic semiconductor- if conduction properties occur naturally without doping.

energy-band diagram of p-type and n-type

Difference in number of Charge Carriers in P-N type Semiconductors

N-type

  • Majority of charge carriers are electrons due to addition of dopant. (extrinsic)
  • Minority of charge carriers are positive holes due to natural excitement of an electron into the conduction band. (intrinsic)

P-type

  • Majority of charge carriers are positive holes due to doping. (extrinsic)
  • Minority of charge carriers are negative electrons.

Difference between Solid State and Thermionic Devices

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

Thermionic devices use the thermionic emission of electrons to rectify or amplify current.

Thermionic Diode

  • A negative filament is heated sufficiently to allow thermionic emission of electrons from the surface.
  • These electrons move towards a positively charged electrode at other end of the vacuum.
  • In diodes, this is the process by which current is rectified (AC to DC)
    • If the filament is negative and the electrode is positive, e_ will flow.
    • If supplied current is reversed, then e_ will not travel to the electrode and no current will flow.
  • The supply current is the current to be rectified.

thermionic diode figure

Thermionic Triode

  • In triodes, electrons are accelerated through a metallic grid.
  • This grid allows the amplification of current.

Solid State Devices

Solid state devices use semi-conductors and properties of p-n junctions to direct the flow of Latex formula

The junction between a p-type semiconductor and an n-type semiconductor acts as a diode, allowing current to only flow in a single direction.

Operation of a p-n junction

  • Electrons close to the p-n junction tend to diffuse from the n-type across the interface and combine with positive holes in the p-type semiconductor.
  • This movement of charge creates a net potential difference across the junction, setting up an E-field that goes from nàp .
  • This E-field acts as a potential stopper against further movement of charge across the junction and is called the “depletion zone
  • If a voltage is applied to a p-n junction, it will act as a diode, allowing current to flow from p to n

p-n junction figure

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

Forward Biased

  • This involves positive current being applied pàn and hence negative current from nàp.
  • The flow of electrons into the n-type repels electrons already present in the n-type semiconductor.
  • They are forced towards the depletion zone where they neutralise the positive holes at the junction.
  • Similarly, the positive holes in the p-type will move towards the depletion zones and neutralise negative charges at the junction.
  • This decreases the potential difference across the p-n junction. (narrows the depletion zone)
  • Eventually the movement of charge across the junction will no longer be opposed by the potential across the depletion zone, allowing the flow of conventional current from p-type to n-type.

forwrd baised figure

Reverse Biased

  • If voltage is applied so that positive current is directed from nàp and the negative current from pàn, the influx of positive charge to the n-type will attract electrons in the semiconductor away from the p-n junction.
  • This increases the potential difference across the junction (widens the depletion zone)
  • It is very difficult for charge carriers to attain sufficient energy to overcome the strong electric field and cross the depletion zone so no current will flow.

reverse based figure

Thermionic Devices Solid State Devices
High voltage requirements to accelerate electrons Have much lower voltage requirements – they use the properties of p-n junction to switch
A great deal of heat energy is produced in thermionic emissionà inefficient and prone to failure due to high temperatures Significantly less heat produced à more reliable and much more suitable in sensitive electronics
Needs a vacuum + glass tubes, wires and soldered components à very fragile and expensive Much simpler design à less fragile and much cheaper
Protective packaging and heat dissipation needed These are not needed and so the devices can be made much smaller à portable
Very noisy and insensitive to weak or high frequency radio signals Very quiet and sensitive to weak signals
Very slow switching due to large voltages involved Much faster current flow with rapid switching

Assessment: Overall, solid state devices are more reliable, efficient, portable and cheap, making them much more suitable for sensitive electronic devices.

Improved Transistor Technology

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

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

Thermionic devices used at the time were fragile, bulky, inefficient, noisy, unreliable and insensitive

Germanium – Experimentation yielded transistors using Germanium crystals.

  • This enhanced our scientific understanding of the semi-conducting properties of Ge and its potential for use in transistor devices.
  • Development in methods of Ge purification, doping and the new understanding of electron flow through semi-conductors allowed the creation of transistors which utilised pnp/npn junctions.
    • This allowed more reliable and efficient amplification of weak electrical signals. i.e. radio waves in long distance comms.
  • Advances in knowledge about Ge crystal production allowed them to be used for more sensitive transistors capable of handling weaker higher f. signals.
  • However, Ge lost its semi conducting properties at high temperatures and is rare and expensive so it became necessary to develop more reliable semiconductors.

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.

  • Si could also be made into thin oxide wafers allowing the miniaturisation of transistors and increasing the portability of communication technology.

Transistors in Microchips and Microprocessors

The invention of transistors led to rapid advances in computer based technologies.

Advantages Disadvantages
Increased our ability to store and transfer information via the world wide web More instances of copyright violation (pirating) and invasion of privacy
Increased the ease and convenience of communication. Reduced personal contact
Creates new leisure activities such as computer online and mobile phone games Reduced physical activity
They have also vastly improved home appliances and medical devices (hearing aids, pacemakers )
Automated technology has improved efficiency of many industries, allowing faster production and reducing costs. Automation of industry has reduced the need for unskilled labour, causing loss of jobs.
Allows new safety features in transport (GPS, ABS)

Assessment: Overall, use of transistors have improved standard of living for many

Semiconductors in Solar Cells

  • Identify data sources, gather, process and present information to 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

 

  • For a solar cell to work, the n-type is exposed to light while the p-type is not
  • At the junction, electrons flow from n -> p, setting up a voltage gradient across the depletion zone, which eventually prevents the further flow of electrons.
  • When a photon strikes an electron in the depletion zone, it is liberated and pushed towards the n-type layer by the electric field.
  • This creates a voltage difference between n&p type layers.
  • As a result, the excess e- in the n type flows into electrical contacts created by a fine metal grid and through an external circuit to reach the p-type layer to replenish the overall e- balance

semiconductors in solar cells

Superconductivity and the Exploration of Possible Applications

Braggs determines the Crystal Structure

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

Braggs proposed that x-rays could diffract off a crystal lattice surface due to their short wavelength.

 

  • Sir William Henry Bragg and his son developed a x-ray spectrometer to systematically study the diffraction of x-rays from crystal surfaces in order to ascertain crystal lattice structure.
  • Light from a monochromatic light source was shone onto the surface of a crystal mounted on a goniometer (allows rotation of crystal).
  • A detector is used to determine the angle θ between plane of the lattice and the incident light ray.
  • Photographic film is placed behind the crystal
x-ray spectrometer incident and reflection angle
  • Diffraction of x-rays by the crystal lattice structure led to an interference pattern of light and dark patches which could be visualised by the film
  • Based on the shifting interference pattern, Braggs determined that a crystal has 3 dimensional lattice-like arrangements of atoms.
  • A relationship between the wavelength of x-rays , θ and the interplanar space d could also be found. For waves to interfere constructively, then: [/latex]n\lambda=2d\sin\theta[/latex]

Metals possess a Crystal Lattice and how Conduction in Metals occur

  • Identify that metals possess a crystal lattice structure
  • Describe conduction in metals as a free movement of electrons unimpeded by the lattice

 

  • Metals have a crystal lattice of positive ions surrounded by a sea of delocalised electrons.
  • These delocalised valence electrons are shared by all positive ions of the lattice and are therefore free to move throughout the lattice
  • Conduction in metals can occur through the movement of these delocalised electrons unimpeded by attachment to individual atoms of the lattice structure.

Resistance in Metals due to Impurities and Scattering

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

 

  • Chemical impurities in the metal lattice disrupt lattice integrity and increase the likelihood of the electrons colliding with it and increases resistance.
  • Vibrations in the lattice cause the nuclei to collide with free moving electrons à deflecting them from their linear movement through the crystal and causing resistance.
  • Lattice vibrations increase with increasing temperature

Note: Remember that the electrons always “drift” with negligible net displacement and only when a voltage is applied will they drift in a net direction (towards the positive terminal)

BCS theory and Cooper pairs

  • Describe the occurrence in superconductors below their critical temperature of a population of electron pairs unaffected by electrical resistance
  • Discuss BCS theory

 

  • In superconductors above the critical temperature, Tc (or in normal conductors), the thermal vibration of the lattice structure causes collisions between which increase their resistance.
  • BCS (Bardeen, Cooper, Shnieffer) described the occurrence of electron pairs and phonons in superconductors below their Tc allowing the movement of e- without collisions or resistance.
  • Below the Tc, lattice vibrations in a superconductor become negligible so the predominant interaction between e- and the lattice structure is electrostatic attraction.

cooper pairs

Formation of a Cooper Pair

  • An electron moving through the lattice will attract positive ions towards itself to create a “trough” of positive charge density.
  • Before the lattice returns to normal shape, a second e- is drawn in the trough, creating a Cooper pair.
  • The distortion of the lattice by the leading electron creates/emits a packet of vibrational energy (a phonon) which is absorbed by the trailing e-.
  • This transfer of energy overcomes the electrostatic repulsion between negative electrons in the formation of the Cooper pairs.
  • Cooper pairs are continually formed, broken and then reformed between different electrons allowing them to move through the lattice coherently without collision and without resistance.

formation of the Cooper pairs

Advantages and Limitations of Superconductors

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

Advantages

  • Superconductors are able to carry larger currents with no resistance and therefore no heat.
  • They may also be used to produce very strong magnetic fields.

These properties are particularly useful in:

  • Power transmission: no resistance removes the need for transformers and allows longer distance, cheaper transmission to remote areas
  • Magnetic levitation: can greatly increase speed by removing friction between moving parts
  • Generators: far more efficient and smaller as no iron core is required
  • Medical imaging; may be able to deflect extremely weak electrical signals in the brain, allowing disorders to be imaged.
  • Computer chips allow miniaturisation and increased speed
  • Electrical current storage using superconducting loops into which current is fed.

Limitations

  • Technical and economic difficulties with achieving and sustaining the low temperatures required.
  • Cost: Higher temp. ceramic superconductors are very expensive, limiting widespread use.
  • Superconducting materials often become very brittle at sub-critical temperatures limiting their widespread use in wiring and electrical transmission.

Examples of Superconductors

  • 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

 

Class Name Critical Temp. (K) Critical Temp. (C)
Type I (Elements) Mercury 4 -269
Type II (Alloys) Tin- Niobium 18 -255
Type II (Ceramics) YBCO (YBa2Cu3O7) 90 -183

Type Is exhibit a sudden transition from normal to superconductivity once Tc has been reached

Type IIs exhibit a gradual transition from normal to superconductivity through a “mixed state”

Hovering Magnet

  • Perform an investigation to demonstrate magnetic levitation
  • Analyse information to explain why a magnet is able to hover above a superconducting material that has reached the temperature at which it is superconducting

 

  • When a superconductor is exposed to a magnetic field, currents are induced in the surface so as to produce a B-field which opposes the external field. (Lenz’s law)
  • If the temperature is below critical, there is no resistance, hence the induced current gives rise to a magnetic field which is equal and opposite to the external field.
magnetic field figure one magnetic field figure two magnetic field figure three

Magnetic field has been excluded from within the superconductor.

 

  • Magnetic levitation occurs if the external magnet is brought down towards a superconductor below its Tc.
  • Under thes conditions, the repulsive force due to the internal/induced and external magnetic field is sufficient to balance out the weight force of the magnet.

Superconductors in Maglev Trains

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

Magnetic levitation in trains allows frictionless movement, making it useful in high speed trains.

Electromagnetic Suspension System

Levitation

EMS uses conventional electromagnets mounted under the train on C-shaped structures that wrap around the T-shaped guideway/track

  • They interact with magnets on the bottom of the guideway, so that if there is vertical displacement, a change in flux will be detected
  • The active feedback system then varies the current in the electromagnets (and hence magnetic field strength), bringing the train back to its equilibrium position.
Lateral guidance

Electromagnetic induction in the side of the guide way and the train prevent lateral displacement of the train.

  • The same active feedback system is used but now acting horizontally to keep the train at equilibrium position in the lateral plain
Propulsion
  • Uses a synchronous long stator linear motor in the guideway.
  • This is supplied with AC and produces an alternating magnetic field that always attracts the train from the front and repels it from the back.
vehicle figure magnetic field figure

 

Advantages Disadvantages
Compared to conventional trains, non-contact function decreases maintenance needed Requires feedback control to maintain equilibrium position
It is able to function well at low speeds without conventional suspension

Electrodynamic Suspension System

This (usually) uses electromagnets in the guideway and superconductors in the train.

Levitation and lateral guidance
  • Figure 8 coils are installed in the side of the guideway.
  • When superconductor electro magnets in the sides of the train are displaced vertically, current is induced in the coil in a direction such that the top and bottom portions of the “8” will attract and repel the train in order to return it to equilibrium.
  • When it displaces laterally, the change in flux threading the guidance loop causes the induction of a current which gives rise to a B-field which opposes this change.
  • It will thus return to equilibrium from either vertical or horizontal displacement
Propulsion
  • Propulsion coils are located in the sides of the guideway are powered by three phase AC.
  • This creates a shifting magnetic field in the guideway that continually moves forward alogn the track and attracts and repels the train forwards.

propulsion coils
shifting magnetic field

 

Advantages Disadvantages
Dynamic stability removes the need for feedback control Dynamic stability is ineffective at low speeds – track must support conventional wheels/rails for low-speed function
The propulsion system is largely held in the rails, making the train much lighter and faster Stronger magnetic fields used also preclude occupants with pacemakers and sensitive electronics
The track is more expensive to lay, so long distance travel is still impractical

Applications of Superconductivity

  • 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 speed and miniaturisation of computer chips are limited by the generation of heat and speed of signal conduction.
  • Superconductors have the potential to create very fast switches for use in transistor devices.

Generators

  • Using superconductors in generators would mean no iron core is required. (There is no need to concentrate the B-field)
  • This would reduce the energy requirements of the generators and also improve its efficiency à lower cost to run the generators
  • Lower fossil fuels reducing air pollution and greenhouse effect.

Motors

  • Superconducting electromagnets and wires could create a much more compact lightweight motor
  • Reduced energy requirements and improved energy efficiency would reduce cost and Env. Impact.

Transmission of Electricity

  • Electricity transmission lines lose a large amount of energy due to resistance.
  • If superconducting wiring was used, there will be no power loss.
  • There would be more efficient distribution to rural areas without using transformers.
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