A Level Physics Practice Exam

When ultraviolet light is shone on a positively charged plate, what happens to the charge on the plate?

1. The charge increases

2. The charge decreases

3. The charge remains the same

4. The charge fluctuates

The correct answer is: The charge increases

When ultraviolet light is shone on a positively charged plate, the charge on the plate decreases. This occurs because ultraviolet light has enough energy to ionize atoms in the material of the plate or in the surrounding medium. When photons from the UV light hit the surface, they can eject electrons from the material. Since the plate is initially positively charged, the removal of negatively charged electrons results in a decrease in the overall positive charge of the plate. In this context, the other options don't accurately describe the situation. The charge does not stay the same because electrons are being lost, and it certainly does not increase, as that would imply gaining more positive charge which contradicts the action of emitting electrons. A fluctuation is also not a relevant outcome; the effect is more of a clear decrease in charge due to the loss of electrons.