AS Daily A Level Physics question
A photoelectric effect experiment is set up with a photocell. Monochromatic light of a fixed frequency, f, and intensity is incident on the cathode. The frequency f is greater than the threshold frequency of the cathode material. An accelerating potential difference, V, is applied between the cathode and the anode. Which statement best describes how the photoelectric current, I, in the circuit changes as the accelerating potential difference, V, is increased from zero?
Answer
The correct answer is D.
The intensity of the light determines the number of photons arriving at the cathode per second. Since the frequency is above the threshold, each photon liberates one electron. Therefore, electrons are emitted from the cathode at a constant rate. At V = 0, electrons are emitted with a range of kinetic energies and in random directions. Some will have enough kinetic energy in the correct direction to reach the anode, resulting in a non-zero current. As the accelerating potential difference V is increased, the electric field it creates directs more of the emitted electrons towards the anode, including those emitted at wider angles. This increases the number of electrons collected per second, so the current increases. However, once V is large enough to collect every single electron emitted by the cathode, the current cannot increase any further. This maximum current is called the saturation current. Increasing V beyond this point will increase the kinetic energy of the electrons upon arrival at the anode, but it cannot increase the rate of arrival of charge, as this is limited by the constant rate of emission. Therefore, the current starts at a non-zero value, increases, and then saturates (becomes constant).