(a) Explain the term photoelectric effect. (b) The diagram above represents a photocell with its associated electric circuit. Identify each of the physical ...

(a) Photoelectric effect

The photoelectric effect is the emission of electrons from the surface of a metal when electromagnetic radiation (light) of sufficiently high frequency falls on it.

(b) The physical quantities

• A - the incident light (beam of photons/radiation) falling on the emitting surface.
• B - the photoelectrons (stream of electrons) emitted from the surface and travelling across the tube.
• C - the cathode, i.e. the photo-emissive metal surface (emitter).
• D - the microammeter (galvanometer) that measures the photoelectric current.
• E - the anode (collector electrode) which attracts the emitted electrons.
• F - the evacuated glass envelope (vacuum tube) enclosing the electrodes.

(The battery labelled 100 V supplies the accelerating p.d. between anode and cathode.)

(c)(i) The current produced by the photocell is determined by the intensity (brightness) of the incident light.

(c)(ii) The maximum kinetic energy of the photoelectrons is determined by the frequency of the incident light (together with the work function of the metal).

(d) Photoemission and evaporation

Similarity: both are surface phenomena in which particles are given enough energy to escape from the surface of a material.

Difference: in photoemission the energy is supplied by light (photons) and the particles emitted are electrons; in evaporation the energy is supplied as heat and the particles emitted are molecules (or atoms) of the liquid.

(e) Other ways of producing free electrons

• Thermionic emission (heating a metal filament).
• Field (cold) emission using a very strong electric field.

(f) Applications of the photoelectric effect

• Automatic (photoelectric) switches for doors, burglar alarms and street lights.
• Light/exposure meters in cameras and the reproduction of sound from film sound-tracks.

(g) Calculations

Given \(\lambda = 5.0 \times 10^{-7}\,\text{m}\), \(W = 3.04 \times 10^{-19}\,\text{J}\), \(c = 3.00 \times 10^{8}\,\text{ms}^{-1}\), \(h = 6.6 \times 10^{-34}\,\text{Js}\).

(i) Frequency of the light

\[ f = \frac{c}{\lambda} = \frac{3.00 \times 10^{8}}{5.0 \times 10^{-7}} = 6.0 \times 10^{14}\,\text{Hz}. \]

(ii) Energy of the incident photon

\[ E = hf = 6.6 \times 10^{-34} \times 6.0 \times 10^{14} = 3.96 \times 10^{-19}\,\text{J}. \]

(iii) Maximum kinetic energy of the photoelectrons

\[ K_{max} = hf - W = 3.96 \times 10^{-19} - 3.04 \times 10^{-19} = 9.2 \times 10^{-20}\,\text{J}. \]