Photoelectric Effect (CIE A Level Physics)

When electromagnetic radiation of frequency f is incident on a particular metal surface, photoelectrons are emitted. 

Fig. 1.1 shows how the maximum kinetic energy  of these electrons varies with frequency f.

Fig. 1.1

Sketch on Fig. 1.1 the shape of the graph that would be obtained if the experiment is repeated using metal with a greater work function.

A certain metal with a work function of 2.2 eV is selected to create two parallel plates. These are connected in a circuit to a sensitive ammeter and a power source, such that a uniform electric field of 140 N C^–1 exists between them. Fig. 1.2 shows the two plates, labelled A and B, separated by 25 mm in a vacuum. 

Electromagnetic radiation of wavelength 255 nm is incident on plate B, such that photoelectrons are emitted from B and move towards A. The ammeter detects a photocurrent in the circuit.

Fig. 1.2

State the direction of the electric field between the plates, given the photocurrent is at a maximum.

Explain your answer.

Show that the maximum possible speed of the photoelectrons as they reach plate A is approximately 1.5 × 10⁶ m s^–1.

Suggest and explain an experimental change to the circuit, part of which is shown in Fig. 1.2, that would reduce the photocurrent measured.

Describe two phenomena associated with the photoelectric effect that cannot be explained using a wave theory of light.

1 ....................................................................................................................

2 ....................................................................................................................

The maximum energy E_max of electrons emitted from a metal surface when illuminated by light of wavelength λ is given by the expression

where h is the Planck constant and c is the speed of light.

Identify the symbol .

[1]

The variation with  of E_max for the metal surface is shown in Fig. 8.1.

Fig. 8.1

Use Fig. 8.1 to determine the magnitude of λ_o.

λ_o = ....................................... m [1]

h = ................................. J s [3]

The metal surface in (b) becomes oxidised.

Photoelectric emission is still observed but the work function energy is increased.

On Fig. 8.1, draw a line to show the variation with  of E_max for the oxidized surface.