Quantum Physics v2.0

Question 1

Define the ‘photoelectric effect’

Answer 1

The ejection of surface electrons from a metal sample when exposed to electromagnetic radiation of a frequency greater than the threshold frequency.
One photon interacts with one surface electron.

Question 2

Define ‘work function’

Answer 2

Minimum photon energy required to eject a surface electron from a metal.
(Hence work functions are measured in J or eV and are different for different metals)

Question 3

Define ‘threshold frequency’

Answer 3

The minimum frequency of light that will eject electrons from the surface of a metal.
The frequency of a photon with an energy equal to the work function.

Question 4

What is a ‘photon’?

Answer 4

A ‘packet’ or ‘quantum’ of electromagnetic (light) energy

Question 5

Einstein’s photoelectric equation

Answer 5

hf = Φ + K.E.max

photon energy of incident light = the work function of the metal + the maximum kinetic energy of ejected electrons

Question 6

State the equation for electrical potential energy that we use when modelling the photoelectric effect

Answer 6

E = QV

Where ‘Q’ is the charge on the ejected electron and ‘V’ the potential difference through which it moves.

Question 7

Define the ‘electronvolt’

Answer 7

The work done when 1 electron moves through a potential difference of 1 volt.

Question 8

Define ‘photon energy’

Answer 8

hf, where ‘h’ is the Planck constant and ‘f’ is the frequency of the incident light

Question 9

Define ‘stopping potential’

Answer 9

The potential difference which, when applied across a photocell, will just reduce the photocurrent to zero.

Question 10

What is a galvanometer?

Answer 10

A highly sensitive ammeter.

Question 11

Describe 6 observations you might make in investigating the photoelectric effect

Answer 11

Whether or not electron emission occurs is independent of light intensity.
Light below a certain frequency (the threshold frequency) will not cause electron emission.
For light above the threshold frequency, electron emission rate is dependent upon light intensity.
For light above the threshold frequency, the maximum K.E. of ejected electrons is independent of light intensity.
For light above the threshold frequency, the maximum K.E. of ejected electrons increases with frequency.
Electron emission (where it occurs) is instantaneous (occurs the moment the metal surface is illuminated)

Question 12

Give 4 predictions arising from the wave model of light in the case of the photoelectric effect

Answer 12

haven’t got round to this yet…

Question 13

In the photoelectric effect, electrons are observed to be ejected with a range of kinetic energies. Why?

Answer 13

Some electrons slightly further below the surface/more tightly bound are being ejected.

Question 14

What equation would model the photoelectric effect, where the incident light has a frequency equal to the work fuction?

Answer 14

hf(threshold) = Φ

Question 15

Dispersion

Answer 15

The variation of refractive index with frequency.

The phenomenon whereby light of different frequencies is refracted by different amounts when entering a material.

Question 16

Emission line spectrum

Answer 16

A spectrum composed of lines of unique wavelengths.

Corresponding to a specific element, emitted by diffuse gases which can be modelled as composed of isolated atoms.

Question 17

Continuous spectrum

Answer 17

A spectrum composed of all wavelengths of light (within certain bounds)
Typical of dense, thermally excited sources

Question 18

Absorption spectrum

Answer 18

A spectrum composed of dark lines of unique wavelengths overlying a continuous spectrum.

Question 19

A transition

Answer 19

The process by which an electron moves between permitted energy levels within an atom, resulting in either the absorption or emission of a photon. The energy of the photon corresponds to the difference in energy between the two levels.

Question 20

Ground state

Answer 20

For a given atom, the ground state is the lowest electron energy level