(a) Majority of the responding candidates presented the correct explanation of work function but
fumbled with that of stopping potential.
(b) The question was found very tractable by most of the respondent candidates.
(c) Two uses of a photocell were correctly stated by majority of the candidates
(d) Few candidates responded satisfactorily.
The expected answers are:
(a) (i) Stopping potential is the (negative) minimum potential difference of the anode
which is required to stop the most energetic electron from reaching the anode.
Any valid additional information e.g Unit is volt.
(ii) Work function is the minimum amount of energy required to liberate/eject an
electron from the surface of a metal when irradiated by electromagnetic radiation.
Any valid additional information e.g. joule.
(b) Factors affecting kinetic energy of ejected electrons
- work function of surface/metal irradiated
- wavelength/frequency of incident radiation
(c) Uses of photocell
- burglar alarms
- switches for street lights
- automatic doors
- television cameras
- solar calculators
Accept any other valid example
(d) Work function = Wo = E - eVs or = hf/λ - eVs
= hc/ - eVs
= hc/5.893 x 10 -7 - 0.36e ..............(α)
= hc/5.969 x 10-7 -1.38e.............(β)
Equation (β) - Equation (α) gives
1.38e - 0.36e = hc/5.969 x 10-7
- hc/5.893 x 10 -7
1.02e = hc (0.2520 - 0.1697) x 107
Taking 1.6 x 10-19
as e and 3.0 x 108
1.02 x 1.6 x 10-19
= h x 3 X 10-8
x 0.0823 X 107
h = 6.61 X 10-34
From equation (α)
Wo = 6.61 X 10-34x 3.0 X 108
5.893 X 10-7 - 0.36 x 1.6 x 10-19
= 3.365 X 10-19 - 0.576 x 10-19
= 2.79 X 10-19 J
= 2.79 X 10-19
6.61 X 10-34
= 4.22 x 10-14 Hz