Question

Write short note on thermionic emission and their applications.

Answer 1

Metals have free electrons that can move from one atom to the other within the metal. In fact, this factor is responsible for their excellent electrical conductivity. But if they try to escape the metal surface, they are unable to do so. This is because when these negatively charged particles (electrons) try to leave the metal, the surface of the metal acquires a positive charge. Due to the attraction between the negative and the positive charges, the electrons are pulled back into the metal. And there are no forces to pull them forward. The electrons are thus forced to stay inside the metal due to the attractive forces. This barrier provided by the metal surface to prevent escaping of free electrons is called the surface barrier.

However, the surface barrier can be broken by providing a certain minimum amount of energyto the free electrons which increases their kinetic energy and consequently help them escape the metal surface. This minimum amount of energy is known as the work function of the metal. And when the work function is provided to the metal, the consequent liberation of electrons from the metal surface is known as electron emission.