State what is meant by work function.

The diagram shows part of an experimental arrangement used to investigate the photoelectric effect.

(i) Explain how the maximum kinetic energy of the emitted electrons is determined experimentally.

(ii) On the diagram, draw the power supply and other necessary components needed in order to carry out the experiment in (b)(i).

Using results obtained with the apparatus in (b), the following graph was drawn. The graph shows how the maximum kinetic energy of the photoelectrons varies with the frequency of the incident radiation.

State how the graph can be used to determine

(i) a value for the Planck constant.

(ii) the work function of the material.

(iii) the threshold wavelength of the material.

In an experiment, light at a particular frequency is incident on a surface and electrons are emitted. Explain what happens to the number of electrons emitted per second when the intensity of this light is increased.

minimum (photon) energy needed to eject electrons (from a surface);

(i) apply stopping potential / OWTTE;
maximum kinetic energy =eV;

(ii)

power supply negative connected to detector;
ammeter and voltmeter correctly connected;
Allow voltmeter connected directly across power supply.

(i) Planck constant = gradient;

(ii) work function = (−) intercept on maximum kinetic energy (allow ‘y’) axis;

(iii) \({\lambda _0} = \frac{c}{{{\rm{frequency intercept}}}}\) or \(\frac{{hc}}{{{\rm{max ke intercept}}}}\);

more intense light means more photons per second;
so more electrons are ejected (per second);

Most candidates choosing this option were able to answer this well although a minority forgot to mention that this was the minimum energy needed for a photon to eject an electron from a metal surface.

(i) The technique for measuring the maximum kinetic energy of the emitted electrons was poorly known. Centres would be well advised to use a simulation if they do not have the opportunity to actually perform this experiment with a photocell.

(ii) Given that is such a simple circuit this was very badly answered with many ‘circuits’ not being circuits at all – in essence, ignoring the polarity needed for the cell, this is a simple resistance measuring circuit. Few realised that the detector must have a negative voltage to prevent the electrons from reaching it.

(i) In recognising that the gradient gives a value for the Planck constant, most candidates answered this correctly.

(ii) Again, this was well answered although few candidates mentioned that the work function was the negative of the value of the maximum kinetic energy intercept.

(iii) This was more involved and less well answered. Although many realised that the threshold frequency was the intercept on the frequency axis, few went on to say that the threshold wavelength was the speed of light divided by this value.

Only those candidates approaching this by relating the intensity of light to the number of incident photons per second tended to be successful here. By stating this it was a simple matter or recognising that there is a one to one correspondence between photons and electrons so more intense light inevitably meant more electrons emitted per second. Many wasted time in explaining why emission of electrons meant that the incident light had a frequency higher than the threshold frequency.