Questions from papers

Question 1

Critical temperature of a material

Answer 1

It is when it becomes cool enough that it has zero resistivity and below the critical temperature the resistance is 0

Question 2

Draw a graph which shows how power is dissipated with current in an ohmic component

Question 3

State two conditions necessary for light to undergo TIR

Answer 3

the angle of incidence must be greater than the critical angle

light ray must be coming from a higher refractive index to a lower refractive index

Question 4

What happens to light when it enters a denser medium

Answer 4

• refracts in
• the light has the same frequency
• and a smaller wavelength (the wave fronts bunch up)

Question 5

Destructive interference (path difference)

Answer 5

(n + 1/2)λ

Question 6

Standing waves

Question 7

Electrons are now fired at the graphite target with greater speed, what happens?

Answer 7

λ = h/mv therefore greater velocity means a smaller wavelength

Greater velocity means momentum of electrons is greater

les diffraction because shorter wavelength relative to the spacing between layers in graphite

maximum diffraction occurs when λ is the same/similar to the slit

Question 8

A cycling chooses to take a zig zag route p a hill deduce whether this is effective as opposed to a straight line

Answer 8

• less useful power output
• it is the same gain in potential energy no matter the route
• gains less potential energy per second

Question 9

A quark change for example in B+ or B- is an indication of?

Answer 9

• weak nuclear force
• W boson exchange particle

Question 10

Explain the difference between an emission spectra and absorption

Answer 10

• photons are the energy carriers in both emission and absorption
• An absorption spectra is when an atom absorbs a photon in a one to one interaction and the atom becomes excited and moves to a higher energy level
• An emission spectra is when an atom de-excites and moves down to the ground state and releases photons

Question 11

Increasing the resistance of a resistor in parallel to a bulb –>

Answer 11

• Reduces the current on the circuit as load resistance increases
• Therefore reduces pd dropped across the internal resistance of the cell
• This makes the lamp brighter

Question 12

Structure of optical fibres

Answer 12

• the core has a high refractive index surrounded by cladding with a low refractive index (to promote TIR)
• the cladding also protects the fibre from scratches which could allow light to escape
• the core is narrow so that the light always hits the boundary at an angle greater than the critical angle

Question 13

Signal degradation - Absorption

Answer 13

• this is when some of the signals energy is absorbed by the material the fibre is made from. This energy loss results in a smaller amplitude

Question 14

Modal Dispersion

Answer 14

• this is caused by light rays entering the optical fibre at different angles
• this causes them to take different paths down the fibre causing light to reach the other end in different times
• this can be reduced by using a single-mode fibre where light is only allowed to follow a narrow path

Question 15

Material Dispersion

Answer 15

• this is caused by the different amounts of refraction experienced by different wavelengths
• this forms varying times for waves to reach the end of the fibre
• using monochromatic light can help stop material dispersion

Question 16

Both Dispersions cause Pulse broadening

Answer 16

When signal in an optical fibre gets wider (broader) as it is transmitted due to dispersion

Question 17

Relationship between resistance and current in parallel circuits

Answer 17

R1 / R2 = I2 / I1