Question

State two ways of providing energy to a metal to emit electrons from its surface.

Answer 1

The electron emission from the surface of a metal is possible only if sufficient additional energy is supplied from some external source. This external energy may come from a variety of sources such as:
( i ) Thermionic emission. In this method, the metal is heated to sufficient temperature (about 2500ºC) to enable the free electrons to leave the metal surface. The number of electrons emitted depends upon the temperature. The higher the temperature, the greater is the emission of electrons. This type of emission is employed in vacuum tubes.
( ii ) Photo-electric emission. In this method, the energy of light falling upon the metal surface is transferred to the free electrons within the metal to enable them to leave the surface. The greater the intensity ( i.e. brightness) of light beam falling on the metal surface, the greater is the photo-electric emission.