Question

If you shine an ultraviolet light on the metal ball of a negatively charged electroscope, it will discharge. But if the electroscope is positivelely charged, it won't discharge. Why?

Answer 1

The photoelectric effect will liberate electrons from metals when ultraviolet light is used, because each photon provides at least 4 eV of energy to the electrons. That exceeds the work function of most metals, so that elctrons can be ejected. However, if the electroscope metal is positively charged, the freed photoelectrons will be attracted back to the surface.