Ideas to Implementation

Question 1

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

F=qvBsin(theta)

Answer 1

F=the magnetic force experienced by a charge moving with a component of its velocity perpendicular to the B-field.
q= the charge (c)
v= the velocity of the charge (m/s)
B= magnetic field strength (T)
Theta= angle between b-field and velocity (degrees)

The charge experience a force at a magnitude following this equation and in a direction following the right hand palm rule (i.e. Perpendicular to the velocity)

Question 2

Primary: Investigation observe different striation patterns for different pressures in discharge tubes

+risks and precautions

Answer 2

Draw diagram

Include power supply and induction coil

Procedure:
Turn on power supply and observe patterns produced in each tube

Results:
5% pressure = purple streamers between electrodes

1% = uniform pink glow fills tube

0. 5% = striation patterns anode and cathode glow, dark space top half
1. 01% = dark space fills tube green glow on the tube

High voltage causes high potential difference between -ve charge cathode snd +ve charged anode causing electrons in cathode to traverse tube to anode

-Bright regions are areas where electrons struck gas atoms and excited their electrons, giving off energy as EMR

Danger- electrocution due to high voltage
Precaution- do not touch equipment while operating

Danger- low energy x-rays produced
Precaution- stand at least 1 metre back from equipment

Question 3

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

Answer 3

Electric field strength at a point is defined as the force a +1 coulomb would experience if placed at that point.

For a point charge (singular charge):

Draw field around positive charge

Draw field around negative charge

Oppositely charged plates- 
Diagram 
And E=V/d
E=electric field strength (V/m)
V=voltage across the plates (volts)
d=distance between plates (m)

In all cases field strength is shown by density of field lines.

Question 4

Describe quantitatively the electric field due to oppositely charged plates:

Answer 4

Electric field is a vector field around a charged particle or object that exerts a force on any other charge particle or object placed in the field. Fields are shown as the force that a +1 coulomb of charge would experience if placed at that point.

Diagram

E = V/d
E = electric field strength (V/m)
V = voltage across the plates(volts)
d = distance between plates (metres)

I.e. The electric field is proportional to the voltage across the oppositely charged plates and inversely proportional to the distance between the plates

Question 5

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

Answer 5

1) set up apparatus (see diagram in book)

2) adjust the E-field strength (by adjusting voltage on electric plate) & B-field strength (by current through electromagnets) so that electric force and magnetic force on cathode ray is equal in magnitude but opposite in direction, thus the cathode rat passes through undeflected.
Thus: Fm=Fe
qvB=qE
v=E/B

3) turn off the power supply supplying the electric plates so is solely deflected by the magnetic field, as velocity of ray is perpendicular to B-field the force applied on it is centripetal
i.e. Fm = Fc
qvb = (mv^2)/r
qBr = mv
q/m = v/Br

Question 6

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

Answer 6

The electrodes (cathode and anode) allow for a potential difference (voltage) to be applied and thus cathode rays (electrons) can be emitted from the cathode and be accelerated and collimated at the anode.

The phosphor coated fluorescent screen absorbs the kinetic energy from the cathode ray and converts it to light energy, allowing visualisations of the beam.

The deflection plates or coils provide the electric or magnetic fields that exert a force on the electrons proportional to the voltage put across the plates/electromagnets guiding them to sweep and for the image on the screen.

Quick diagram if time

Question 7

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

Answer 7

Behaviour of cathode rays (electrons) caused debate as it seemed to exhibit properties of EM waves and charged particles

Observations suggesting charged particles:

• do not reflect at equal angles
• deflected by magnetic fields
• carry momentum and thus mass

Observations suggesting EM waves:

• pass through gold foil without leaving a hole suggests no mass (actually moves through gaps in the atoms and between them)
• does not appear to be deflected by electric fields (actually being blocked by particles in the cathode ray tube due to not being a good enough vacuum)
• effectively unaffected by gravity (due to extremely low mass)

Question 8

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

Answer 8

A cathode ray tube is a glass tube containing a near vacuum. It is connected in series where a negatively charged cathode passes a stream of charged particles (electrons) through the tube to the positively charged cathode due to the potential difference between the two electrodes.

Diagram

The near vacuum of the cathode ray tube with the stream of electrons allows them to be manipulated by electric and magnetic fields ( due to the electric and magnetic force exhibited due to the negative charge of the electrons) without interference from other particles.

Question 9

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

• maltese cross
• containing electric plate
• fluorescent display screen
• glass wheel
• determine sign of the charge on cathode rays

Answer 9

Draw a diagram of cathode ray tube setup (including power supply and induction coil) with space for object

Procedure:
Switch on power supply and record observations

-containing a maltese cross
cathode rays created a shadow of cross on anode therefore cathode rays travel in straight lines from cathode to anode.

-containing electric plates
Cathode rays were deflected by the electric field between the plates and to attract towards the positively charge plate therefore negatively charged

-with a fluorescent display screen
Displays the path of the cathode rays and able to observe the deflection of this path by magnetic or electric fields. Has energy to activate phosphors on collision

-containing a glass wheel
glass wheel pushed up the slope therefore cathode rays have momentum and therefore mass (particle)

-analyse information gathered:
we can conclude that cathode rays are negatively charged particles.

Question 10

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

Answer 10

Conductors such as metal lattices consist of an ordered array of positive metal ions bonded to each other. To maintain stability, valence electrons are delocalised and shared between atoms (a ‘sea’ of delocalised electrons).

Comparatively in insulators the atoms are held tightly together by strong covalent bonds where electrons are locked in their valence shells and cannot move.

Question 11

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

Answer 11

Difference in level of electrical resistance is due to comparative sizes in their forbidden energy gap i.e. How readily electrons can move from the valence band to conduction band and become charge carriers.

Valence band- range of energy levels possessed by electrons which are struck in their valence shells.

Conductivity band- range energy levels possessed by electrons delocalised from their valence shells.

Forbidden energy gap- the minimum amount of energy required to excite electrons from the valence band ti the conduction band.

diagrams for each

Question 12

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

Answer 12

The absence of electrons in a nearly full outer-valence shell is referred to as a positive hole. When a voltage is put across the material electrons flow to fill the hole, which in turn creates a new hole, however it is easier to think of the hole moving in the opposite direction to the flow of current (i.e. Towards the negative terminal). in the case of semiconductors, positive holes are created when electrons move to the conductivity band due to heat energy. In this way both free electrons (negative charge carriers) in the conductivity band and holes (positive charge carriers) in the valence band increase the flow of current i.e. increase conductivity.

Question 13

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

Answer 13

Conduction band- energy levels of free electrons, delocalised from valence shell

Valence band- energy levels of electrons bound to their valence shell

Forbidden Energy Gap (FEG)- minimum amount of energy to move an electron from the valence to conduction band

Conductor- high number of free electrons (negative charge carriers) in conduction band. Many electrons per atom can drift (therefore low resistance and high conductivity.)

Insulator- no free electrons in conduction band since electrons are stuck in valence band with a large FEG keeping them from being delocalised (therefore high resistance and low conductivity.)

Semiconductor- due to small FEG (approx. 1eV) electrons can be excited across the FEG by heat energy and as such number of free charge carriers varies depending on temperature (some free electrons at room temp) (Resistance and conductivity vary according to temperature.)

Conductivity:
Conductor>semiconductor>insulator
Except when heated the resistance of a conductor increases while in a semiconductor it decreases

Question 14

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

Answer 14

Transistors are a use of doped semiconductors in a p-n-p or n-p-n junction, used as an electronic switch and to amplify electrical signals. Transistors are primarily composed of group 4 elements. These elements consist of 4 electrons and thus form strong covalent bonds , but there is still movement between some of the electrons.

Germanium and silicon are both group 4 elements that can used in transistors however silicon is generally superior due to:

• abundance and hence cheaper
• retains semiconducting properties at higher temps
• forms an insulating oxide layer when heated in oxygen (critical to integrated circuits)

However in the earliest transistors germanium was used. This was because scientists of the time had a technique to produce a suitable purity of germanium for a transistor that they could not replicate with silicon as it is more reactive than germanium.

Purity is vital to semiconductors due to the delicate balance of an adequate sized FEG.

Question 15

Describe how doping a semiconductor can change its electrical properties

Answer 15

Doping- adding tiny amounts of impurity atoms to a pure semiconductor material. If the dopant atoms have a different number of valence electrons to the intrinsic semiconductor atoms it replaces, extra energy levels can be formed in the forbidden energy gap, meaning conductivity is increased as the FEG for these dopant atoms is less.

There are 2 types of extrinsic semiconductor:

P-type: made by adding dopant atoms from a group 3 element. The loss of electrons adds positive holes meaning they are the dominant mobile charge carriers.

N-type: made by adding dopant atoms from group 5. The extra electrons meaning mobile charge carriers are mostly negative free electrons.

Question 16

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

Answer 16

Both thermionic and solid state devices are used to force current to only flow in one direction (in the case of a diode) or act as an electronic switch or an amplifier of electric signals in a current (in the case of a triode or transistor).

Solid state devices are created via the junction between p and n type semiconductors. The electric field created across the depletion zone at p-n junctions is utilised fulfil these ends.

Thermionic devices utilise heat energy to excite electrons across the potential difference across oppositely charged electrodes (-ve cathode and +ve anonde) inside the vacuum of a vacuum tube to fulfil these ends.

Advantages of ssd’s:

• (size) thermionic devices bigger that ssd’s, cannot be miniaturised for complex circuits due to the size of vacuum tubes
• (robust) thermionic devices are fragile due to glass vacuum tubes, ssd’s used solid metals thus more robust
• (time) thermionic devices require the time to heat the cathode, ssd’s do not require this warmup period
• (energy consumption) thermionic devices require heating which is energy inefficient, ssd’s dont
• (cost) components of thermionic devices are more expensive (i.e vaccum tube) than ssd components

Question 17

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.

Answer 17

Students and seats model

Seniconductors are materials with a small forbidden energy gap between their valence band (range of energies possessed by electrons stuck in their valence shells) and conduction band (range of energies possessed by electrons delocalised from their valence shells).
As such electrons can be excited from the valence band to the conduction band.

Diagram

Procedure:

1) have one student sit up on the desk behind the chairs, telling him he has been excited
2) tell students a potential difference has been applied across the semiconductor and thus the closest student must move into a free chair

Findings: when an electron is excited from the valence to the conduction band due to thermal oscillations its atom is left a positive ion, a hole is left where the electron was before delocalisation. When a potential difference is applied across the semiconductor electrons in the conduction band flow towards the positive terminal and an electron in the valence band moves to fill the hole, lesving another hole behind. As such it can be thought of as the hole is moving towards the negative terminal, making it a positive charge carrier.

Danger- falling off desk
Precaution- due diligence or simply have the student stand behind the seats

Danger- attempting to sit down and missing the seat
Precaution- protective leather pants

Question 18

Collecting reliable and valid info in secondary investigations

Answer 18

1) researched topic on google looking for reputable and current websites.
2) Search library for texts outlining the topic.
3) use texts supplied by teacher
4) cross reference all collected sources to find a consensus answer

Validity: check dates published/reviewed to make sure not outdated

Reliability: cross referencing, multiple types of sources, reputable sources

Question 19

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

Answer 19

Shortcomings of the thermionic triode (predecessor of transistor):
-large and fragile
Radar systems using them were unreliable
-telecommunications were unreliable as signals needed to be amplified using triodes.

These shortcomings lead to:

• ability to dope a semiconductor to increase conductivity
• an effective method to purify silicon discovered (abundant intrinsic semiconductor)
• lead to discovery of the p-n junction this became first solid state diode.
• leads bardeen brattain and shockley who discovered that an n-p-n or p-n-p arrangement could act as an electronic switch and amplify electrical signals i.e. Transistor

Question 20

Secondary sources:

Assess the impact of the invention of transistors on society with particular reference to microchips and microprocessors

Answer 20

Transistor invented by shockley, bardeen and brattain in 1947.
p-n-p or n-p-n semiconductor combination acts as electronic switch and a way to amplify electrical signals

Advantages on society (over thermionic triode):
-can be miniaturised thus used complex circuits. Microchips and microprocessors being the ‘brains’ of modern devices such as laptops and home computers allow access to a broad range of information via the internet, benefitting the educational resources of society.

(-use of microprocessors in modern telecommunications resulted in reliable communications (no heating time needed as in the triode) benefitting societies ability to communicate and build strong relationships.)

Negative impacts:
-use in machinery in industry lead to immediate loss of jobs, negatively impacting workers quality of life
(
-use of microprocessors in tv and gaming consoles promote a sedentary lifestyle negatively impacting societys health.)

Overall the impact of the invention of the transistor has been positive as the sheer breadth of technology benefitting society outweighs the negatives.

Question 21

Secondary source:

Summarise the effect of light in semiconductors in a solar cell

Answer 21

(Diagram in book)

1) photons of light collide with valence electrons in the p-n junction, providing energy to excite then across the FEG into the conduction band. This produces an electron-hole pair i.e. mobile charge carriers in the depletion zone.
2) the electric field across the p-n junction accelerates the electrons and holes in opposite directions. This results in positive charges accumulating on the p type side and negative charges on the n type side, thus a voltage is produced i.e if connected in an external circuit current flow p to n.

Question 22

Outline the methods used by the Braggs to determine crystal structure

Answer 22

Digram 1 (experiment setup)

As x-rays have a wavelength comparable to the interatomic spacing of the crystal when the rays hit the individual atoms they were scattered and interacted with each other resulting in constructive and destructive interference which could be seen as bright and dark spot respectively on the developed photographic film.

2nd digram
Therefore distance between atoms could be found by:

n lamda = 2dsin(theta)

n=order of diffraction (known)
Lamda= wavelength of x-rays (found experimentally by braggs snr)
Theta=angle between x-rays and crystal so constructive interference occurs.

By repeating this on different angled cuts of the crystal to produce many facets and on the ground powder of the crystal braggs discovered the lattice structure of crystalline solids.

Question 23

Identify that metals have a crystal lattice structure

Answer 23

Diagram

Discovered using bragg’s x-ray crystallography showing metals have a crystal lattice structure i.e. An orderly 3d lattice structure of positive ions in a sea of delocalised electrons in the conduction band.

Question 24

Describe conduction in metals as the free movement of electrons unimpeded by the lattice/ identify that resistance in metals is increased by the presence of impurities and scattering of electrons by lattice vibrations.

Answer 24

Conduction in metals is the ability for delocalised electrons to flow freely through the metal. Resistance in a metal is due to electrons scattering (colliding with and bouncing off) the lattice, kinetic energy lost as heat to the positive ion lattice.

The lattice of a metal can increase resistance in 3 ways:
1) thermal vibrations- increased movement of the lattice increases likelihood of collisions
2) lattice impurities
3) lattice defects
Both distort the lattice increasing collisions

Question 25

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

Answer 25

Superconductors are materials that, when cooled to or below a certain ‘critical’ temperature, have zero electrical resistance.

This can be explained via BCS Theory which states when below its critical temp thermal vibrations in the lattice of a superconductor cease. Thus when an electron moves through the lattice, the positively charged ions are actively attracted to it. This distortion of the lattice creates a region of +ve charge which moves through the lattice as a vibration called a phonon. A second electron is attracted to this +ve region, and thus causing a transient pairing to the first electron, this pair of electrons is called a cooper pair. This pairing allows the electrons to travel through the lattice without collisions i.e. Zero resistance

Diagram

Question 26

Discuss BCS Theory

Answer 26

BCS Theory states that when cooled below its critical temp thermal vibrations in the lattice cease. Thus when an electron moves through the lattice, the positively charged ions are attracted to it. This distortion of the lattice creates a region of +ve charge which moves through the lattice as a vibration called a phonon. A second electron is attracted to this +ve region, and thus causing a transient pairing to the first electron, this pair of electrons is called a cooper pair. This pairing allows the electrons to travel through the lattice without collisions i.e. Zero resistance

(Diagram)

Positive: BCS theory is very successful at explaining the phenomena in type 1 superconductors, predicting critical temps etc

Negative: BCS theory does not explain type 2 superconductors and their higher critical temps

Question 27

Discuss the advantages of using superconductors and identify limitations in their use

Answer 27

Advantages:

• no resistance would mean no loss of energy in long range transmission
• no resistance would mean no resistive heating (as electrons do not collide with the lattice) meaning superconductor switching devices could be miniaturised to minute sizes without melting
• zero resistance would mean electrical energy could be stored perfectly for any length of time on superconducting rings
• no power loss to resistive heating means much more powerful electromagnets useful for MRIs to maglev trains

Limitations:

• coolants such as liquid helium or nitrogen required to keep below critical temp. Technically difficult when designing a device using them and increases energy requirements and running costs.
• type 2 superconductors are brittle ceramics which limits their applicability (cant be made into wires etc)

Question 28

Secondary study:
Identify some metals, metal alloys and compounds that have been identified as exhibiting the property of superconductivity and their critical temps

Answer 28

Metal: mercury

• type 1
• critical temp= 4K
• liquid helium coolant

Metal Alloy: Niobium- Titanium (Nb-Ti)

• type 2
• critical temp= 10K
• liquid helium coolant

Compound: YBCO (yttarium barrium copper oxide)

• type 2
• critical temp= 92K
• liquid nitrogen coolant

Question 29

Perform an investigation to demonstrate magnetic levitation

Answer 29

Diagram of dish with liquid nitrogen in and superconductor disk in it, rare earth magnet levitating over it.)

Results:
magnet levitates once superconductor it’s below its critical temp due to the meissner effect- total exclusion of magnetic fields from superconducting material. To achieve this a current is induced on the surface of the superconductor that flows to create a B-field which perfectly opposes that of the magnet i.e. Fm=Fw, therefore magnet levitates.

Danger-liquid nitrogen can cause frostbite
Precaution-wear proper protective gear

Question 30

Secondary Study:

Describe how superconductor and the effects of magnetic fields have been applied to develop a maglev train

Answer 30

(2 diagrams, front view and top view)

Superconductors used for the electromagnets connected to the train as they produce much stronger magnetic fields than conventional electromagnets as no electrical energy is lost to resistive heating.

Magnetic fields used for:
Levitation- Repulsion of like poles is used to levitate the train.

Propulsion: Repulsion of like poles and and attraction of unlike poles, alternating current through the tracks electromagnets switches polarity as train moves through to keep propelling the train.

Question 31

Secondary study:
Discuss the possible applications of superconductivity and the effects of those applications on computers, generators and motors and transmission of electricity through the power grid

Answer 31

Computers-
“joesphson junctions” superconductor switching devices work much faster than solid semiconductor transistors and can be miniaturised without melting bc no resistive heating. =smaller faster more powerful computers

Motors and generators-
due to all the electrical energy being used by the device and none lost to resistive heating (Ploss=I^2R) use of semiconductors would mean much more efficient generators and smaller, lighter, more powerful motors.

-electricity transmission
Superconducting transmission lines would mean perfectly efficient transmission of electrical energy, even over long distance, would also eliminate the need to transform electricity to high voltage for transmission (i.e. safer) Ac for transformation.
Superconducting rings would also enable electrical energy to be stored for any length of time without power loss.

Negatives:
-coolant is cost, time and energy expense to keep the superconducting material within devices below Tc

• coolant could cause frostbite if leaked from a faulty device
• type 2 semiconductors are brittle limiting applicability (wiring etc)

Question 32

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

Answer 32

Diagram

Induction coil used to generate a large potential difference between the 2 sides of the antennae. This produced an electric field cause electrons to oscillate. The accelerating charges produced changing electric and magnetic fields, radiating energy outwards as EM radiation.

Hertz was able to observe the spark transmitted electromagnetic waves, detected at a distance by his ring receiver via a spark produced across the gap in the receiver due to the changing electric fields of the EM wave oscillating electrons in the loop back and forth.

Hertz was also first to observe the photoelectric effect- emission of an photoelectron from the surface of a metal due to energy supplied by a photon of light (frequency above the threshold frequency) . Hertz recorded that the sensitivity of his receiver was increased by applying visible or UV light as more electrons were freed from the metal to jump the gap.

Question 33

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

Answer 33

Diagram (power supply, induction coil, receiving antenna, CRO)

Procedure: switch on power supply and observe CRO

Results:
Radio Waves produced by continuous sparking at induction coil consists of oscillating charges which produce radio waves.
Radiowaves detected by peaks on CRO display whenever induction coil is switched on, but not when switched off

Danger- high voltage
Precaution- dont touch equipment while operating

Question 34

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

Answer 34

Hertz showed radio waves could reflect, refract, interfere and be polarised like light waves.
Thus he inferred velocity of the wave would equal frequency times wavelength (v=f lamda).

Frequency of the radiowaves was found via finding the corresponding frequency of the oscillating electrons in the transmitting antenna.

Wavelength was found via: (diagram, standing radiowave reflecting off zinc plate, spark at antinodes and no spark at nodes)

Thus velocity was found to be 3x10^8 m/s, speed of light.

Question 35

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

Answer 35

A black body is an object that perfectly absorbs incident radiation and reflects none, thus only emitting thermal radiation.

Classical models predicted that the intensity of the radiation emitted by a black body would increase dramatically at shorter wavelengths but experimentally this was not the case:

Diagram of blackbody radiation curve

This was known as the ultraviolet catastrophe

Using empirical methods from these results Planck derived his hypothesis:

EM radiation absorbed and emitted by walls of a blackbody cavity are quantised in units given by
E=hf

E=energy of quanta absorbed or emitted (J)
f=frequency of EM radiation absorbed or emitted (Hz)
h= plancks constant = 6.626x10^-34 (Js)

Question 36

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

Answer 36

Einstein used Planck’s theory of the emission of energy from black bodies as quanta to more accurately solve the problem of black body radiation curves (ultraviolet catastrophe) and explain the photoelectric effect (in conjuncture with the law of conservation of energy) by hypothesising:

Electromagnetic radiation is quantised by its nature, existing as discrete energy packets (photons) given by E=hf

E= energy of photon (J or eV)
f=frequency of light
h= plancks constant= 6.626x10^-34 (Js)

This reconceptualised maxwells classical wave model of light which predicts energy of a light wave depends only on its intensity (resulting in ultraviolet catastrophe of black body radiation)

Einsteins quantum model of light predicts lights behaviour as a particle and a wave with energy dependant solely on frequency, solving the discrepancy between prediction and results which was the ultraviolet catastrophe

Blackbody radiation: thermal radiation from an object which absorbs all incident radiation and reflects none.

Intensity of light is directly proportional to size of the photocurrent

(quick graph if theres time)

If asked as a secondary source question:
Einstein used Planck’s theory of the emission of energy from black bodies as quanta to solve the problems of black body radiation and the photoelectric effect by hypothesising:

Electromagnetic radiation is quantised by its nature, existing as discrete energy packets (photons) given by E=hf

E= energy of photon (J or eV)
f=frequency of light
h= plancks constant= 6.626x10^-34 (Js)

Meaning energy of light (photons) was dependent on frequency not intensity, solving the disparity between expected blackbody radiation curves and experimental results.

Question 37

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

Answer 37

The particle model of light, introduced by einstein, proposes that electromagnetic radiation is quantised by its nature, existing in discrete energy packets (photons) given by E=hf

E= energy of a photon (J or eV)
f= frequency of radiation (Hz)
h=plancks constant=6.626x10^-34 (Js)

In this way einstein reconceptualised maxwell’s classical model of light that predicted light behaved only by a wave by introducing lights dual nature that it can also behave as a particle.

Einsteins equation also showed that the energy of EM radiation is solely dependent on its frequency not intensity as maxwell predicted.

Question 38

Identify the relationship between photon energy, frequency, speed of light and wavelength:

E=hf
c=f lamda

Answer 38

E=hf
E=energy of a photon (discrete energy packet of EM radiation) (J or eV)
f= frequency of the radiation
h= plancks constant= 6.626x10^-34 (Js)
Shows energy of a photon is proportional to its frequency

c=f lamda
c= speed of light (velocity of the EM wave) (m/s)
f= frequency of EM wave (Hz)
Lamda= wavelength of EM wave (m)
Shows the speed of light is proportional to an EM waves frequency and wavelength

As c is constant and f=c/lamda the equations can be combined as
E=(hc)/lamda
Shows energy is inversely proportional to wavelength

Question 39

Secondary study:

Summarise the use if the photoelectric effect in photocells

Answer 39

Photoelectric effect- the emission of an electron from the surface of a metal due to the energy supplied by a photon above the threshold frequency

Photocell diagram

The photoelectric effect is utilised in a photocell as when the photocathode is exposed to EM radiation it emits more photoelectrons from cathode to anode and thus increases the photocurrent and when the photocathode is exposed to less EM radiation the opposite occurs. This change in photocurrent (positive or negative change) will trigger another function.

Question 40

Secondary study:
Discuss Einstein and Planck’s differing views about whether science research is removed from social and political practice.

Answer 40

Einstein:

• Believed scientific research should be conducted separately from any social and political agenda.
• research should be conducted regardless of debate and controversy to eventually benefit society
• however compromised view to push the US to create the first atom bomb before Nazi Germany

Planck:

• supported science being used for social and political purposes
• if there was controversy die to a scientific experiment it should be stopped due to no clear benefits to society
• compromised his view after realising the atrocities of the nazi political regime his research was supporting.

Question 41

1. Increased understanding of cathode rays led to the development of television:
   - describe historical development
   - outline key physics concepts
   - explain how technology utilises the physics
   - outline impact of technology on society and environment

Answer 41

-with the invention of glass vacuum tubes experiments were done on them, one being putting a voltage across the vacuum. The result was different patterns of rays at different atmospheric pressures (By Plucker). This spurred debate over these rays as to whether they were EM radiation or charged particles, with experiments leading evidence to either side due to gaps in current knowledge. JJ Thomson eventually proved they were negatively charged particles (electrons) due to moving below the speed of light and being bent towards a positively charged plate. This was the first recognition of electrons.

(Diagram of JJ’s experiment if time)

-(diagram of conventional tv)

Electron gun- when high voltage (from an induction coil) electrical current is run towards the cathode it creates a -ve charged region at the cathode and a +ve charged region at the anode due to the flow of -ve electrons. When the cathode is heated via a heating element, thermal energy excites the electrons to escape the cathode (thermionic emission) and traverse the cathode rays tube (unimpeded by stray particles due to the vacuum created within the tube) accelerating towards the anode due to the potential difference between the cathode and anode, applying electrical force to the electrons towards the anode (F=qE).
The electrons are collimated and accelerated past the anode.

Deflection plates/coils- the cathode ray beam can be bent due to the -ve electrons by electric fields towards a +ve charged plate or by magnetic fields (in perpendicular direction to the plate via right hand palm rule). By having a pair of horizontal and vertical plates/electromagnets with an input signal used to vary the voltage through them, thus affecting their field strength (E=V/d or change in B) and thus the force applied on the particle in the vertical or horizontal direction (F=qE for plates and F=qvBsin(theta) for magnets) the cathode ray can precisely be swept across the area fluorescent screen.

Fluorescent screen- as electrons have mass and thus at speed have momentum (p=mv) when they collide with the phosphor coated fluorescent screen the electron will impart its energy to activate the phosphor, converting kinetic energy to light energy. Thus with the ray being manipulated by the input signal through the deflectors, images can be displayed on the screen.

impacts of TV on:
society:
+ visual educational programming can be TVs in a wide range of homes at once with one input signal, positively impacting the education of society.
- visual entertainment on TV’s promotes a sedentary lifestyle, negatively impacting the health of society

Environment:
+documentaries and advertising broadcast on TV promote awareness of damage being done to the environment, promoting activism to positively impact the environment
- great electrical consumption used by tv’s worldwide required burning of fossil fuels at power-plants producing greenhouse gases, enhancing global warming to negatively impact the enviornment

Overall the impact of TV on society has been positive due to the broad range of entertaining and education programming increasing quality of life. Impact on environment has been negative due to widespread power consumption outweighing positives

Question 42

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

Answer 42

Shortcomings of the thermionic triode (predecessor of transistor):
-large and fragile
Radar systems using them were unreliable
-telecommunications were unreliable as signals needed to be amplified using triodes.

These shortcomings lead to:

• ability to dope a semiconductor to increase conductivity
• an effective method to purify silicon discovered (abundant intrinsic semiconductor)
• lead to discovery of the p-n junction this became first solid state diode.
• leads bardeen brattain and shockley who in 1947 discovered that an n-p-n or p-n-p arrangement could act as an electronic switch and amplify electrical signals i.e. Transistor

How it works:
Semiconductors are materials with a small (around 1eV) (‘forbidden’) energy gap between the valence band (energy levels possessed by electrons stuck in their valence shells) and the conduction band (energy levels of delocalised electrons). As such electrons can be excited via thermal energy from the valence band to the conduction band to act as negative charge carriers and leaving behind a holes in the valence shells of their atoms that act as positive charge carriers (as valence electrons can move to fill these holes).

A intrinsic semiconductor has an equal number of positive and negative charge carriers but to decrease the FEG and up conductivity a semiconductor can be ‘doped’ by adding small amounts of impurity atoms to the lattice. There are 2 types of extrinsic semiconductor:
P-type: made by adding dopant atoms from group 3 making positive holes (positive charge carriers) the dominant charge carrier
N-type: adding dopant atoms from group 5 making electrons (negative charge carriers) the dominant charge carrier

When a p and n type semiconductor are brought together they form a p-n junction. Positive holes from the p-type and electrons in the n-type flow across the junction to equalise the number of free charge carriers in each, creating a zone deplete of free charge carriers at the junction, positively charging the n-type and negatively charging the p-type, thus creating an electric field at the p-n junction.

Diagram

The transistor uses p-type and n-type semiconductors brought together in a p-n-p or n-p-n formation i.e forming 2 p-n junctions. Using the electric fields across each a transistor is able to not only only allow current to flow in one direction through it but also amplify an electric input signal by flowing through the transistor in the emitter-base-collector direction.

Diagram

Transistors are used in complex circuits including microprocessors and microchips:

Impacts on society:
+microchips in computers allow us access to a wide range of information via the internet, positively impacting the education of society
-microchips in video game consoles promote a sedentary lifestyle, negatively impacting the health of society

Environment:
+transistors are more energy efficient than alternatives (e.g. Thermionic triode) meaning less fuel is burnt, producing less greenhouse gases having a smaller impact on global warming
-the cheap and effectiveness of transistors in a wide range of devices has promoted a net increase in electricity use, meaning more fuel is burnt producing more greenhouse gases and increasing global warming.

The impact of transistors on society is an overall positive due to its broad range of uses from entertainment to education and health. The net impact of transistors on the environment is negative due to the amount of fuel consumed by devices in which transistors are a vital component outweighs all positives.

Question 43

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

Answer 43

• Technology to create coolants near 0K enabled Onnes to discover mercury’s superconducting properties below 4K.
• Braggs experiment to determined the lattice structure of metals vital for understanding type 1 superconductors
• BCS theory developed by Bardeen, Cooper and Schrieffer as a model of type 1 superconductivity, able to predict critical temperatures (temperature below which a certain metal demonstrates superconducting properties)
• However did not account for type 2 superconductors discovered later

How it works:
BCS Theory states that when cooled below its critical temp thermal vibrations in the lattice cease. Thus when an electron moves through the lattice, the positively charged ions are attracted to it. This distortion of the lattice creates a region of +ve charge which moves through the lattice as a vibration called a phonon. A second electron is attracted to this +ve region, and thus causing a transient pairing to the first electron, this pair of electrons is called a cooper pair. This pairing allows the electrons to travel through the lattice without collisions i.e. Zero resistance

Diagram

Technology:

• Could be used in “joesphson junctions” superconducting devices working to amplify signals like the transistor but with zero power loss to resistive heating and this not melting at any size, allowing for smaller, more powerful computers.
• could be used to created more powerful electromagnets and more efficient motors as no input energy would be lost to resistive heating.
• could be used for perfectly efficient long and short range power transmission due to 0 resistance, allowing powerlines to use lower voltages and DC current (safer than high voltage AC used now) as current would not need to be transformed for efficiency.

Impacts on Society:
+strength of superconducting electromagnets allows for MRI scans, improving health of society
-coolants required to keep superconducting technology below its critical temperature would be an added expense and hassle to society, negatively impacting quality of life

Impacts on Environment:
+superconductors would allow for perfect transmission of electricity meaning less fuel would need to be burnt, less greenhouse gases, lesser than current impact on global warming
-superconductors require coolants which require energy to create, more fuel burnt, more greenhouses gases, greater than current impact on global warming

Overall the impact of superconductors on society has and will be positive due to the broad range of life improving technology it allows for/improves.
Impact on environment is/will also be positive as the amount of power saved by using a 0 resistance material outweighs all negatives.

Question 44

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

Answer 44

• Maxwell developed classical theory if light as a wave with energy solely dependent on intensity
• Hertz discovers radio waves and proving them as electromagnetic radiation using maxwells equations, unknowingly discovers the photoelectric effect as his receiver sparked more in presence of uv or visible light.
• to understand the blackbody radiation curve planck hypothesises that light absorbed and emitted by blackbodies is quantised
• Einstein solves the photoelectric effect and better explains the blackbody radiation curve by hypothesising that all light is, by its nature, quantised

How it works:
Einsteins quantum model of light reconceptualises maxwell’s classic model of light by stating light acts as both a wave and a particle. The energy of a quanta of electromagnetic radiation (photon) is shown by:
E=hf
E=energy of a photon
h=plancks constant (6.626x10^-34)
f= frequency of electromagnetic radiation (Hz)

Thus the energy of EM radiation to be dictated by its frequency and not its intensity as maxwell hypothesised.

This successfully solved the ultraviolet catastrophe and explained the photoelectric effect: when a photon of light above a certain (threshold) frequency supplies the energy to excite an electron to escape the surface of a metal.
It also solves the fact that this happens near instantly (as soon as the first photon of light reaches the metal) which maxwells model would expect the need of a buildup of energy from sustained exposure to the EM wave.

(Diagram if time)

Photocell:
Diagram

The photoelectric effect is utilised in a photocell as when the photocathode is exposed to EM radiation it emits more photoelectrons from cathode to anode and thus increases the photocurrent and when the photocathode is exposed to less EM radiation the opposite occurs. This change in photocurrent (positive or negative change) will trigger another function.

Impact on Society- photocells have made a positive impact on society as the ability to detect the presence of an object is used in burglar alarms keeping society safe, and to detect the present brightness, used to know when to turn streetlights on and off.

Impact on Environment has been negative as the photocell is used in useful devices that require current (particularly as it is a resistor) promoting greater global energy usage, requiring more fuel to be burnt, greenhouse gases, global warming.