The libration of electrons from the surface of a substance is known as electron emission.
We know that there are a large number of free electrons in a metal that wander throughout the body of the metal. However, these electrons are not free to leave the surface of the metal. As they try to come out of the metal, the surface acquires a +ve charge and pulls the electrons back to the metal.
Minimum energy, equal to the work function (), must be given to an electron so as to bring it out of the metal surface.
“The minimum energy, required by an electron to just escape from the metal surface is called the work function of the metal “. It is denoted by
The work function depends on the properties of the metal and the nature of the surface.
WORK FUNCTION OF SOME METAL
(These values are approximate as they are very sensitive to surface impurities)
| Metal | Work function |
Metal | Work function |
|---|---|---|---|
| Cs | 2.14 | Al | 4.28 |
| K | 2.30 | Hg | 4.49 |
| Na | 2.75 | Cu | 4.65 |
| Ca | 3.20 | Ag | 4.70 |
| Mo | 4.17 | Ni | 5.15 |
| Pb | 4.25 | Pt | 5.65 |
The work function is generally measured in eV (electron volt). This unit of energy is commonly used in atomic and nuclear physics.
“One electron volt is the amount of energy when it is accelerated through a potential difference 1 Volt”
i.e
The minimum energy required for the electron emission from the metal surface can be supplied to the free electron by any of the following processes:-
(1) Thermionic emission:-
By suitably heating (~ ), sufficiently thermal energy can be imparted to the free electron to enable them to come out of the metal.
(2) Field emission:-
By applying a very strong electric field () to a metal, the electrons can be pulled out of the metal as in a spark plug.
(3) Photoelectric effect:-
When the light of suitable frequency falling upon the metal surface, electrons are emitted from the metal surface. These photon(light) generated electrons are called photoelectrons.
| Introduction | Electron Emission |
| Photoelectric Effect | Experimental Study of Photoelectric Effect |
| Photoelectric Effect and Wave Theory of Light | Einstein’s Photoelectric Equation: Energy Quantum of Radiation |
| Particle Nature of Light: The Photon | Wave Nature of Matter |
| Davisson and Germer Experiment | |