Gce Physics Edexcel Unit 2 Model Answers

Front 1

Describe how infrasound travels through the air.

Back 1

Air particles oscillate parallel to the direction of propagation

producing compressions & rarefactions

Front 2

The temperature of a filament lamp increases as the current through it increases. Explain this in terms of the structure of a metal.

Back 2

Electrons gain kinetic energy as the current increases. (Greater current = greater temperature)

So more frequent collisions of electrons with the lattice ions

lattice ion vibrations increases

Front 3

Explain the difference between polarised and unpolarised light.

Back 3

Unpolarised – oscillations/vibrations in many directions

Polarised – oscillations/vibrations in single direction

oscillations/vibrations are perpendicular to direction of propagation

Front 4

Explain why there is a threshold frequency.

Back 4

One electrons absorbs one photon

Photon energy: E=hf/E=hv

There is a minimum energy needed for the emission of a (photo)electron

So the emission of electrons only occurs if the frequency of (e.g. light in Q) is higher than the threshold frequency OR

Threshold frequency is the minimum frequency for the emission of photo(electrons)

Front 5

Explain what is meant by a photon.

Back 5

(Discrete) quantum/packet of electromagnetic radiation

Front 6

How set of dark lines are produced

Back 6

photon absorbed by electron electron moves to higher energy level Or electron excited

where photon energy = difference in energy levels

only certain changes/differences possible between discrete energy levels

Front 7

Suggest why the energy levels all have a negative value

Back 7

energy value of level n = ∞ is 0

electrons need to gain energy to move to a higher energy level

Front 8

How are stationary/standing waves formed?

Back 8

Waves travelling in opposite/both directions

when the two waves meet they superpose/undergo superposition

producing nodes & antinodes

Front 9

Define frequency

Back 9

Number of oscillations/cycles per second

Front 10

Define wavelength

Back 10

Distance travelled during one complete oscillation/cycle

Front 11

State what is meant by critical angle

Back 11

angle of incidence (for light travelling from denser medium) has angle of refraction of 90 degrees

Front 12

Refractive index for light passing from air to glass experiment

Back 12

Measure angles of incidence & refraction

Plot sin i (y-axis) vs sin r (x-axis)

Find gradient

Front 13

Explain what is meant by plane polarised light.

Back 13

Reference to oscillations of electric / magnetic field (accept vibrations)

Oscillations/vibrations in one plane only

Plane includes direction of propagation

Front 14

Explain why the following observations may be understood by using a photon model of light, rather than a wave model.

Back 14

photon energy depends on frequency Or reference to E = hf (must be link to photons/light)

if photon energy greater than work function, electron emitted (immediately)

whereas for waves energy could build up Or with waves that the electron can absorb energy continuously or over time so any frequency should work

Front 15

Describe how the ultrasound travels through a metal.

Back 15

Particles/molecules of metal oscillate

along the direction of propagation/direction of wave travel

Making compressions and rarefactions Or as a longitudinal wave

Front 16

Explain why pulses are used rather than a continuous wave

Back 16

Need to measure time at which the echo arrives back

If continuous couldn’t tell when this was Or so pulse must return before next is emitted

so pulse must return

Shorter pulses means smaller thickness can be measured Or longer pulses means only larger thickness can be measured

Front 17

Sketch the graph of V against I and explain how it may be used to determine İ and r.

Back 17

Straight line (dependent on first marking point)

V = ε – Ir

Intercept on y axis = ε

Gradient = – r

Front 18

We usually assume that ammeters have negligible resistance and voltmeters have infinite resistance. The determination of İ and r is not affected by using an ammeter with non-negligible resistance but is affected by using a voltmeter with a low resistance.Explain why.

Back 18

If ammeter has resistance, current decreased but doesn’t affect the determination because current through cell/r is measured

The resistance of the ammeter contributes to the load/circuit/total resistance

If voltmeter has smaller resistance it would draw current measured current not current through cell/r

Front 19

Explain why an ammeter

must be placed in series to measure current through a component

must have a very low resistance

Back 19

current same in series

to ensure the total resistance in the circuit isn’t increased Or to ensure no pd lost

because that would reduce the current being measured

Front 20

Explain why coherence is necessary to observe the light and dark bands.

Back 20

coherent = constant phase relationship/difference

The idea that at a given point there would sometimes be constructive interference and sometimes destructive interference etc

Front 21

Explain what is meant by refraction

Back 21

change in direction / wavelength (of wave/ray/light/speed?)

when entering an optically denser medium/when entering a medium where the density is different

Front 22

Describe how a spectral line is produced by a hot gas, explaining why a particular element can only give rise to particular frequencies.

Back 22

Electrons/atoms move to higher energy levels / get excited

They then move to lower energy levels (accept ground state)

The energy from the move is given out in the form of a photon

The energy levels are discrete Or only certain energy levels are possible

The energy of the photon must be equal to the difference in energy levels Or hf = E2 − E1

There are only a limited number of energy differences and only a corresponding number of frequencies

Front 23

Suggest why the dolphins emit a series of clicks rather than a continuous sound

OR saying why pulses are more often used/better than continuous

Back 23

so one returns before one emitted

Front 24

Talking about emission of photoelectrons

With light (illuminated)

Darkness

Back 24

photons/light cause emission of (photo)electrons

Idea that (photo) electrons form a current

photon energy greater than or equal to work function

No photons so no photoelectrons released

Front 25

Define amplitude

Back 25

Maximum displacement from the equilibrium position

Front 26

State and explain the effect on the emitted electrons if the frequency of the light is increased

Back 26

Max kinetic energy/speed/velocity increases/electrons move faster

because Increasing frequency increases energy of photon

E=hf/E=hv

Front 27

State and explain the effect on the emitted electrons if the intensity of the light is increased.

Back 27

Number of electrons emitted per second increases/rate of electron emission increases

because there are more photons per second

Front 28

Difference between p.d. & EMF

Back 28

Potential difference:

- Electrical energy transferred between 2 points/ P.d. = work/charge

EMF

- Energy (conversion) per unit charge

Front 29

What is meant by when atoms become excited?

Back 29

Electrons gain energy

& so move to higher energy levels/states/state above the ground state

Front 30

Explain how the excited atoms emit radiation.

Back 30

Electrons move down energy levels

& emit electromagnetic radiation

Front 31

Explain why only certain frequencies of radiation are emitted

Back 31

Electrons exist in certain/discrete/specific energy levels

only certain energy changes are possible/ only certain/discrete amounts of energy are released

E=hf/E=hv

Front 32

Suggest why ____ emits different wavelengths than _____

Back 32

______ has different energy level spacings than ______

OR different elements have different energy level spacings

Front 33

Ultrasound PULSES example:

Explain how ultrasound pulses can be used to build up the image of the fetus in the uterus.

Back 33

Many/large number of pulses

(Ultrasound) travels as pulses so that one pulse is detected before the next pulse is sent.

Time for (echo/reflected) pulse to return measured

Front 34

Define diffraction

Back 34

Diffraction is the spreading out of a wave

as it passes through a gap/slit OR

as it passes around an obstacle

Front 35

State the principle of superposition of waves.

Back 35

When two or more waves meet

the total displacement at a point is the sum of the individual displacements

Front 36

Define critical angle

Back 36

The angle of incidence (in the denser medium) for which angle of refraction is 90º

Front 37

Describe the difference between a transverse wave and a longitudinal wave.

Back 37

Transverse waves oscillate perpendicular to the direction of the wave/direction of propagation/energy travel

Whereas

Longitudinal waves oscillate parallel to the direction of the wave/direction of propagation/energy travel

Front 38

How stationary/standing waves are formed

Back 38

2 waves of the same frequency/wavelength travelling in opposite directions

superpose/undergo superposition

nodes & antinodes produced

OR

where in phase, constructive interference/antinodes

where in antiphase, destructive interference/nodes

Front 39

Define polarised light

Back 39

when the oscillations (associated with the wave) travel in one direction only

OR you could describe it as a transverse wave as only transverse waves can be polarised

Front 40

How to measure angle of rotation for plane polarisation

Back 40

Mention of polarising filter/Polaroid/polariser

Rotation (of filter) until minimum/ maximum intensity (notrotation of solution)

(Rotation) done with and without the sugar solution

identifies correct difference in angles

use of protractor/polarimeter

Front 41

)In a demonstration, ultraviolet light is incident on a zinc plate and electrons are emitted. The intensity of the ultraviolet light is increased. Explain the following observations:

• the number of electrons emitted per second increases

• the maximum kinetic energy of an electron does not change.

Back 41

Mention of photons OR photoelectric (NOT photoelectrons)

• Idea of one to one relationship from photon to electron

• Intensity of light relates to number of photons/sec

wavelength/frequency is constant • photon energy depends on frequency /reference to E=hf

• Reference to hf = Φ + ଵଶmv௠௔௫ଶ and Φ constant

Front 42

e.g. Explain why the metal plate is made of caesium rather than zinc.

Back 42

Zinc requires higher frequency /Zinc requires UV/UV dangerous (for students)

Caesium works with visible light

Front 43

Dopper effect

Back 43

Moving towards:

Higher frequency/pitch

So therefore wavelengths are shorter

Moving away:

Lower frequency/pitch

So therefore wavelengths increases/spread out

TALK ABOUT CHANGE IN FREQUENCY ALSO

Front 44

Define energy level

Back 44

Energy an electron can have/Discrete energy of an electron

Front 45

Define wavefront

Back 45

Line/surface joining points in phase

Front 46

Define displacement

Back 46

The distance any part of the wave has moved from its rest position

Front 47

Define nodes

Back 47

No displacement

0 amplitude

Destructive interference links with nodes

Where 2 waves cancel out/

2 waves exactly out of phase

Waves with more nodes represent higher energies because:

More nodes = higher frequency & shorter wavelengths, momentum larger, E=hf

Front 48

Define antinodes

Back 48

Position of maximum amplitude

OR

Position where waves are (always) in phase

So constructive interference links with antinodes

Front 49

Define monochromatic

Back 49

Single wavelength/frequency/colour

Front 50

Total internal reflection

Back 50

When the angle of incidence is greater than the critical angle

i > c

Front 51

Refractive index

Back 51

(of a material) is a measure of the change in speed of light as it passes from a vacuum (or air as an approximation) into the material

Front 52

Difference between plane polarised light & unpolarised light

Back 52

Plane polarised - Oscillations are in one plane only which travel perpendicular to the direction of the wave/direction of propagation

Unpolarised - Oscillations may occur in more than one plan

Front 53

Why transverse waves can be polarised but longitudinal waves cannot

Back 53

Transverse waves oscillate perpendicular to the direction of the wave/direction of propagation

So in transverse waves, oscillations can be in different planes

but polarisation restricts it to only one plane

Longitudinal waves however oscillate parallel to the direction of the wave/direction of propagation so it already oscillates in one plane (only)?

Front 54

Why does the ideal voltmeter have infinite resistance?

Back 54

So that the current through the voltmeter is 0/negligible/

So it doesn't reduce the resistance of the circuit

Or reduce the p.d. it is meant to be measuring

Front 55

Photon model rather than wave model

TALKING ABOUT EMISSION OF ELECTRONS FROM THE SURFACE OF A METAL

Back 55

Electrons in atoms can only occupy certain discrete energy levels

ground state = lowest energy level an electron can occupy

energy is transferred in the collisions

electrons can move/jump to higher energy levels & become excited when they gain energy

These electrons return later back down to the ground state by emitting energy in the form of photons

photons have a specific energy or and frequency

E=hf/E=hv

Front 56

Define internal resistance

Back 56

Resistance of a battery or cell

Front 57

I = nqvA

Back 57

I = Electric current/A

n = Number of charge carriers per unit volume/m^-3

q = Charge carried by each carrier/C

v = Drift velocity which is the average velocity of the electrons/chargecarriers / ms^-1

A = Cross-sectional area / m^2

Front 58

Define

Back 58

Quantum - A discrete/indivisible quantity

Particle - Photon/electron

Photoelectron - Electrons released by the interaction with a photon. 1 photon releases 1 electron

electronvolt, eV - The amount of energy needed to move 1 electron through a potential difference of 1 volt

Front 59

Define threshold frequency

Back 59

Minimum frequency of radiation that will produce a photoelectric effect. +

Cause electrons to be emitted as photoelectrons

Threshold frequency is needed because:

- Electrons require a certain amount of energy to escape from a surface

This energy comes from 1 photon of light

E=hf

Front 60

Define work function

Back 60

The minimum amount of energy required for an electron to escape the surface/break free

Front 61

Why it supports the particle theory but not the wave theory of light:

Back 61

Particle theory:

-E=hf/E=hv

-Increased frequency means more energy of a photon

-Increased frequency = greater energy of photon = greater kinetic energy of the electrons

-1 photon releases 1 electron

More intense light means more photons therefore more electrons

Front 62

Conditions for (observable) interference

Back 62

- Same type of wave

- (Approx) same amplitude/intensity

- Same frequency/OR wavelength

- Coherent, so constant phase relationship/difference?

Front 63

Define potential divider

Back 63

A type of circuit containing 2 components designed to divide up the p.d. in proportion to the resistance of the components

Front 64

I =nqvA

Back 64

If R increases due to stretching:- Current will decrease- I = nqvA- Drift velocity v decreases-nAe constant

If R decreases:- Current will increase- I = nqvA- Drift velocity v increases- nAe constant

Front 65

EXTRA

Back 65

-Frequency has no effect on the number of electrons emitted-

-Greater light intensity = more photons = greater number of electrons emitted (as photoelectrons)

Front 66

Wave theory

Back 66

Wave theory:

-Wave energy depends on intensity-More intense light = greater kinetic energy of the electrons-Energy is spread over the whole wave-If exposed for long photons are eventually released- For a particular frequency of light. the energy carried is proportional to the intensity