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YOUR PARTICIPATION FOR THE GROWTH OF PHYSICS REFERENCE BLOG

Sunday, October 7, 2018

Why, even when the incident light is monochromatic, the emitted electrons have a range of kinetic energy up to a maximum value


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.

The intensity thus determines the number of electrons emitted per unit time and not the energy of those electrons.

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