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?
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.
The phenomenon you are referring to is called the photoelectric effect. When an ultraviolet light shines on the metal ball of a negatively charged electroscope, it causes the discharge of the electroscope. This occurs because the ultraviolet light carries sufficient energy to ionize the surrounding air molecules.
To understand why the electroscope discharges when negatively charged, but not when positively charged, we need to consider the behavior of positively and negatively charged ions in an electric field.
In an electroscope, the metal ball is connected to a metal rod, and the rod is connected to a metal leaf. When the electroscope is charged negatively, excess electrons accumulate on the metal ball, creating a negative charge. These excess electrons try to move away from each other due to their like charges, causing the metal leaf to spread apart.
Now, when an ultraviolet light shines on the negatively charged metal ball, it ionizes air molecules in its vicinity, releasing electrons. These free electrons are attracted to the positively charged metal ball, neutralizing the negative charge. As the negative charge on the metal ball reduces, the repulsive force between the excess electrons decreases, causing the metal leaf to collapse.
On the other hand, when the electroscope is charged positively, excess positive charge accumulates on the metal ball. In this case, the positively charged metal ball already has a deficit of electrons, so when the ultraviolet light shines on it, the released electrons are not attracted to the positively charged metal ball. As a result, the electroscope does not discharge, and the metal leaf remains unaffected.
In summary, negatively charged electrosopes discharge when exposed to ultraviolet light because the released electrons are attracted to the positively charged metal ball and neutralize the excess negative charge. However, positively charged electrosopes do not discharge because the ultraviolet light does not attract the electrons needed to neutralize the positive charge.