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have relatively few free electrons and are known as nonconductors or insulators. Materials that have an intermediate number of available free electrons are classed as semi-conductors. Work function :
(d)
The atoms and electrons in any material are ordinarily in rapid vibratory motion, the velocity of their motion, being a function of temperature. At ordinary temperatures the free electrons in the metallic emitter cannot leave its surface because of certain acts as a potential barrier. To escape from the surface of the emitter the electrons must perform a certain amount of work to overcome the surface potential barrier. At absolute zero, the minimum amount of energy required to enable an electron to escape from the metal surface is known as work function. This depends slightly on the temperature. It is possible to increase the energy of the free electrons in a conductor until they are able to pass through the potential barrier into space. Satish K. Gupta, op.cit. p. 1072— *FREE ELECTRONS IN METALS "Electron is a fundamental constituent of the atom. A metal contains free electrons, which move about freely through the atomic spaces in a random fashion. But as soon as an electron leaves the metal immediately an equal positive charge is produced on the surface of the metal. As a result the electrons pulled back into the metal and hence remains confined to it. The pull on the electrons at the surface is found to depend on the nature of metal surface and is described by a characteristic of the metal, called work function. The minimum energy which must be supplied to the electron so that it can just come out of the metal surface is called the work function of the metal. It is denoted by the symbol w and is measured in electron volt (eV). In order to make the free electrons escape the metal surface, an additional energy must be given to them. This process is called electron emission and may be achieved in the following ways: (i) Thermoionic emission: In this process of electron emission,
the additional energy is supplied in the form of heat. The emitted electrons are known as thermo-electrons.
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