Question

How can electrons be made to leave the metal surfaces? (state any two ways).

Answer 1

Hint: Electrons leaving the metal surface can be as a result of an energy being supplied into the metal one way or another. It can also be as a result of energy being absorbed directly by the electrons themselves.

Complete Step-by-Step solution:
Generally, there are several ways to eject electrons from the surface of a metal. However, it boils down to two ways, either the general metal is made to absorb some sort of energy which is then transferred to the electrons or the electrons are made to absorb some sort of energy directly. But the escape of the electrons themselves all boils down to one thing, that it possesses enough energy to overcome the work function of the metal.
I would discuss one way for each of the methods.
Photoelectric effect (Direct absorption of energy): this is a phenomenon in which electrons are ejected from the surface of a metal as a result of light being shone on the metal. During the photoelectric effect, one photon is absorbed by one electron and the electron is ejected by absorbing the energy of the photon. If the energy of the photon is not sufficient to overcome the work function of the metal, the electron will not be ejected.
Thermionic emission (Indirect absorption of energy): this is a phenomenon in which the electrons of metal are ejected as a result of the temperature of the metal. During this process, the metal itself is heated from, and the heat is conducted to all the particles in the metal including the electrons. Now if heating continues, the electrons continue to absorb heat energy until it is sufficient for the electron to overcome the work function of the metal.

Note: For clarity, we emphasize the sharp contrast between the first method and the second one. For the direct absorption, the energy is absorbed directly and the ejection takes place instantaneously. It is either the photon energy is sufficient or not. But for the second situation, the electrons gradually absorb the heat energy (increasing the general temperature), mostly from neighbouring atoms (due to the conduction), until it is able to overcome. Thermionic emission is gradual.
Another method of direct absorption is the field effect emission, where the electrons are emitted to an electrostatic field. The energy is given directly to the electrons by creating a potential difference hence placing the electrons at a higher potential energy. It is direct because the electrons do not receive the potential energy from the atom but directly from the source of the electric field.