Module 4 Definitions and Statements

Question 1

Describe what is meant by the term mean drift velocity of the electrons in the wire

Answer 1

the average displacement/distance travelled of the electrons along the wire per second, they move slowly in one direction
through the metal/ lattice (when there is a p.d. across
the wire) because they collide constantly within the lattice

Question 2

State Kirchhoff’s first law

Answer 2

The sum of the currents entering a point / junction is equal to the sum of the currents leaving (the same point)

Question 3

State Kirchhoff’s second law.

Answer 3

e.m.f.s = sum /total of p.d.s/sum of voltages (in a loop)

Question 4

Define the kilowatt hour.

Answer 4

Energy (transformed by a device working) at 1 kW for 1 hour

Question 5

Define electrical resistivity.

Answer 5

ρ = RA/L

symbols defined: A = cross-sectional area, R = resistance, L = length

Question 6

Use energy considerations to distinguish between potential difference (p.d.) and electromotive force (e.m.f.).

Answer 6

p. d.: energy transferred per unit charge from electrical form (into other forms, e.g. light/heat)
e. m.f.: energy transferred per unit charge into electrical form (from other forms, e.g. chemical/mechanical)

Question 7

State Ohm’s law in words.

Answer 7

Current is (directly) proportional to potential difference (for a metal conductor) provided the temperature \ (all) physical condition(s) remains constant

Question 8

Define resistance.

Answer 8

p.d./voltage (across component) divided by current

Question 9

State the difference between the directions of conventional current and electron flow.

Answer 9

current moves from + to – (of battery in circuit) and electrons move from – to +

Question 10

Define potential difference.

Answer 10

energy per unit charge transferred from electrical to other forms

Question 11

Explain what is meant by internal resistance.

Answer 11

(some) energy is transferred into thermal energy /lost as heat
in (driving charge through) the battery. It behaves as if it has an
(internal) resistance

Question 12

Explain the term e.m.f

Answer 12

energy transferred from source/changed from some form

to electrical energy per unit charge

Question 13

Define the term intensity.

Answer 13

intensity is the (incident) energy per unit area per second

Question 14

Define the terms wavelength, frequency and speed used to describe a progressive wave

Answer 14

λ distance between (neighbouring) identical points/points with same
phase (on the wave)
f number of waves passing a point /cycles/vibrations (at a point) per unit time/second
v distance travelled by the wave (energy) per unit time/second

Question 15

Explain what is meant by infra-red radiation.

Answer 15

infra red is part of the e-m spectrum

lower f or longer λ than the visible region/light

Question 16

Explain what is meant by the principle of superposition of two waves

Answer 16

when two waves meet/overlap/interfere/collide/superpose the resultant displacement is the sum of the displacements

Question 17

State the meaning of amplitude

Answer 17

the maximum displacement from equilibrium or rest position

Question 18

State the meaning of phase difference

Answer 18

fraction of a cycle between the oscillations at the two points

Question 19

explain the meaning of interference

Answer 19

when two (or more) waves meet/superpose/overlap (at a point)
there is a change in overall displacement

Question 20

Explain the meaning of coherent

Answer 20

constant phase difference/relationship (between the waves)

Question 21

State two properties which distinguish electromagnetic waves from other transverse waves

Answer 21

all travel at speed of light through a vacuum
are oscillating E and B fields or are caused by accelerating
charges

Question 22

Describe what is meant by a plane polarised wave.

Answer 22

oscillations (of particles/e-m fields along the wave) are in one
direction only
perpendicular to the direction of wave propagation/of travel of the
wave/of energy transfer

Question 23

Describe how a stationary wave is different from a progressive wave.

Answer 23

progressive a wave which transfers energy
stationary a wave which traps/stores energy (in pockets)
OR
progressive : transfers shape/information from one place to
another
stationary where the shape does not move along/which has
nodes and antinodes

Question 24

Explain what is meant by a line spectrum.

Answer 24

light emitted from (excited isolated) atoms produces a line spectrum a series of (sharp/bright/coloured) lines against a dark background

Question 25

When used to describe stationary (standing) waves explain the terms antinode and node.

Answer 25

node occurs where the amplitude/displacement is (always) zero
antinode occurs where the amplitude (of the standing wave) takes the maximum (possible) value

Question 26

State the principle of superposition of waves

Answer 26

when two(or more) waves meet/cross/interact (at a point) the (resultant) displacement is the (vector) sum of the (individual) displacements

Question 27

Explain what is meant by a progressive wave

Answer 27

a transfer of energy as a result of oscillations (of the source/medium/particles through which energy is travelling)

Question 28

Explain what is meant by diffraction of a wave.

Answer 28

wavefronts/paths spread out after passing through a gap or around an obstacle

Question 29

State the condition necessary for electrons to produce observable diffraction when passing through matter, e.g. a thin sheet of graphite in an evacuated chamber.

Answer 29

wavelength of electrons must be comparable/of the order of magnitude of the atomic spacing

Question 30

Explain what is meant by the work function energy of the metal.

Answer 30

(Minimum ) energy needed to free an electron /an electron to escape (from the metal surface)

Question 31

Explain what is meant by the de Broglie wavelength of an electron.

Answer 31

electron wavelength depends on its speed/momentum

Question 32

Define the electronvolt. State its value in joule

Answer 32

an eV is the energy acquired by an electron accelerated/moves through a p.d. of 1 V
1 eV = 1.6 x 10-19 J

Question 33

Explain what is meant by the de Broglie wavelength of an electron

Answer 33

Electrons are observed to behave as waves/show wavelike properties where the electron wavelength depends on its speed/momentum

Question 34

Explain what is meant by a photon

Answer 34

a quantum/lump/unit/packet/particle of (e-m) energy/light

Question 35

Explain what is meant by a continous spectrum

Answer 35

all wavelengths/frequencies are present (in the radiation)

Question 36

In an experiment it is observed that when blue light is shone on a clean metal surface electrons are emitted, but with red light there is no electron emission. Describe Einstein’s theory to explain these observations.

Answer 36

1. Individual photons are absorbed by individual electrons ( in the metal surface)/ one to one interaction
2. Only photon with energy above the work function energy will cause photoelectron emission/idea of threshold frequency
3. Photon energy is proportional to frequency
4. (therefore) blue photons with higher f/shorter wavelength will cause photoemission but red photons will not.
5. hf – ϕ = KEmax is the equation resulting from conservation of energy or resulting from the meaning of each term
6. A wave model does not explain instantaneous emission

Question 37

In 1905 Einstein presented a theory to explain the photoelectric effect using the concept of quantisation of radiation proposed by Planck in 1900.
Show, with the aid of a suitably labelled diagram, the arrangement of apparatus that could be used to demonstrate the photoelectric effect. Describe how you would use the apparatus and what would be observed.

Answer 37

A (clean) zinc plate mounted on the cap of a gold-leaf electroscope.
Plate initially charged negatively
A u-v lamp shining on plate
The gold leaf collapses as the charge leaks away from the plate (when ultra-violet light is incident on the zinc plate) so experiment indicates the emission of negative charge/electrons

A simple photocell, eg two plates in a vacuum envelope
A (12 V) dc supply is connected to the photocell and (nano)ammeter.
A suitable frequency/u-v lamp shining on one plate
The presence of u-v /blue light causes a current in the circuit. so experiment indicates the emission of negative charge/electrons

A (potassium) photocell connected across a (high impedance) voltmeter.
Incident light of different frequencies;
produced either by white light source and colour filters of known spectral range or by using a diffraction grating or prism to produce a first order spectrum.
Different p.d.s are set up across the electrodes of the photocell (when the
photocathode is illuminated with light of different frequencies). so experiment indicates the emission of negative charge

Question 38

A physical quantity is also conserved in the photoelectric effect. Describe and explain the photoelectric effect.

Answer 38

a photon is absorbed by an electron (in a metal surface); causing electron to be emitted (from surface).
Energy is conserved (in the interaction).

Only photons with energy/frequency above the work function energy/threshold frequency will cause emission
(energy of photon) = (work function of metal) + (maximum possible kinetic energy of emitted electron)
work function energy is the minimum energy to release an electron from the surface
Number of electrons emitted also depends on light intensity
Emission is instantaneous

Question 39

Explain what is meant by a line spectrum

Answer 39

light emitted from (excited isolated) atoms produces a line spectrum
a series of (sharp/bright/coloured) lines against a dark background

Question 40

Describe how an absorption line spectrum differs from an emission line spectrum

Answer 40

in an absorption spectrum a series of dark lines (appears against a bright background/within a continuous spectrum)

Question 41

State what is meant by the photoelectric effect.

Answer 41

emission of electron(s) from a metal (surface) when photon(s)/ light/uv/em radiation are incident (on surface)