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Question 3
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(a) Diagram
The diagram above illustrates a photocell connect to a battery, B and a muuameter G. In the experiment,
ultraviolet rays are shown on the photocathode and the resulting photocurrent is measured with the
milIiameter.
The intensity E of the incident rays is adjusted and the corresponding photocurrent, I is measured.
Fig. 3(a) shows the various values of Ei
Fig. 3(b) shows the corresponding values of the photocurrent Li where i = 1,2,4,5 and 6.
(i) Measure and record the real values Ei of the incident radiations.
(ii) Read and record the corresponding current Ii'
(iii) Evaluate I-1' and E.
(iv) Tabulate your readings
(v) Plot a graph with Ei on the vertical axis and t ion the horizontal axis.
(vi) Determine the slope, s of the graph.
(vii) State two precautions that are necessary to ensure accurate results when performing this
experiment.
(b) (i) Differentiate between a Red light and a Violet light in terms of
(A) wavelength;
(B) energy.
(ii) A red light has a wavelength of700 nm. Calculate its energy.
[h = 6.6 x 10-34 Js; c = 3.0 x 108ms-1]
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This question was very popular among the candidates and the performarice was above average. Some candidates did not convert the values of E, hence raw values of E was used to evaluate E; instead of the
converted values of E, some of the candidates that converted E could not evaluate KI correctly. Few
candidates approximated E-1 and I-1 to 1 s.f instead of a least 2 s.f.
The (b) part was poorly handled as many responding candidates could not differentiate between red light
and violet light in terms of wavelength and energy. The numerical part was poorly tackled because
candidates failed to convert correctly 700 mm to 700 x 10-9 m
In part (a), candidates were required among other things, to:
- measure and record six values of E
- read and record six values of I
- evaluate E-1 and I-1
- record data in composite table
- correctly distinguish the graph axes, select reasonable scales, plot six points correctly
and draw line of best fit.
State any two of the following precautions:
- Ensure terminals are clean and tight
- Avoid parallax error on the metre rule/ammeter
- Note and correct for zero error on metre rule/ammeter
- Open key when readings are not taken
- Repeated readings (stated)
The expected responses to part (b) are as follows:
(b )(i) Two correct differences stated
| Red light |
Violet light |
| long wavelength |
short wavelength |
| has low energy |
has high energy |
(ii) E = hf = hc/λ
= 6.6 X 10-34 x 3 X 108
700 X 10-0
= 2.83 x 1O-19J
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