Question 3
(a) By reference to the photoelectric effect, explain
(i) what is meant by work function energy, [2]
(ii) why, even when the incident light is
monochromatic, the emitted electrons have a
range of kinetic energy up to a maximum value. [2]
(b) Electromagnetic radiation of frequency f is
incident on a metal surface. The variation
with frequency f of the maximum kinetic energy EMAX of electrons emitted from the
surface is shown in Fig. 7.1.
Fig. 7.1
(i) Use Fig. 7.1 to determine the work function energy of the
metal surface. [3]
(ii) A second metal has a greater work
function energy than that in (i).
On Fig. 7.1, draw a line to show the variation with f of
EMAX
for this metal. [2]
(iii) Explain why the graphs in (i) and
(ii) do not depend on the intensity of the incident
radiation. [2]
Reference: Past Exam Paper – November 2013 Paper 43 Q7
Solution:
(a)
(i) Work function energy is the minimum
photon energy required to remove an electron (from the surface).
(ii)
The emitted electrons will have maximum kinetic
energy when they are emitted directly from the surface.
{max KE = photon energy – work function energy}
Other ejected electrons have energies lower
than this maximum value because some energy is required to bring the electrons
to the surface.
(b)
(i)
threshold frequency = 1.0 × 1015 Hz
{The threshold frequency can
be determined by extrapolating the graph line up to EMAX = 0.}
work function energy = hf0
= 6.63×10–34 × 1.0×1015
= 6.63 × 10–19 J
{The value of the Planck constant can be taken
from the data sheet.}
(ii)
sketch: straight
line with same gradient
displaced to right
(iii)
The intensity determines the number of photons
arriving at the surface per unit time.
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