Electron emission
Electron emission from metal
Electron emission definition
Generally, emission refers to a process by which any object becomes free. In the similar way, the process by which the free electrons becomes free from the metal and enters into the vacuum is called electron emission.
Atom
We know that every solid, liquid, and gas is made up of atoms. The size of these atoms is very small. These atoms consist of even smaller particles such as protons, electrons, and neutrons.
The strong nuclear force between the neutrons and protons makes them stick together to form a nucleus at the center of an atom. Protons have positive charge and neutrons do not have any charge. Hence, the overall charge of a nucleus is positive.
Electrons are the negatively charged particles, which continuously revolve around the nucleus of an atom. These electrons are revolving around the nucleus of an atom because of the electrostatic force of attraction between the electrons and the nucleus.
Electrons revolve around the nucleus in different orbits or shells. The electrons, which are revolving at the outermost shell of an atom, are calledvalence electrons. The valence electrons and the atomic nucleus are present at greater distance. Hence, the electrostatic force of attraction present between the valence electrons and nucleus is very small. Thus, the valence electrons are loosely attached to the nucleus.
What is electron emission?
When a small amount of external energy is applied to the valence electrons, they gain enough energy and break the bonding with the parent atom. The electron, which breaks the bonding with the parent atom, moves freely from one place to another place. These electrons, which move freely within the metal, are called free electrons. However, these free electrons cannot escape from the surface of a metal.
It is because the free electrons in the metals do not have enough energy to escape from metal. The free electrons, which try to escape from the metal, are pulled by positive electric force of atomic nucleus towards the metal. Hence, free electrons cannot escape from metal without sufficient energy. Thus, the positive atomic nucleus of the metal opposes the free electrons, which try to escape from the metal.
In order to make the free electrons to escape from the metal, an additional external energy is required. This additional external energy is applied in the form of heat, electric field, or light.
When the external energy in the form of heat, light, or electric field is applied to the metal, the free electrons gain enough energy and break the bonding with the metal. Free electrons, which break the bonding with metal, will jumps into the vacuum. The free electrons in the vacuum are not associated with the metal.
The process by which, these free electrons break the bonding with metals and jumps into the vacuum is called electron emission. The amount of external energy required to remove an electron from the surface of a metal is called work function. Work function is measured in joules.
The free electrons in the vacuum have higher energy than the electrons that are revolving around the nucleus because the free electrons in the vacuum had gained additional energy from the source of heat or light.
The process of electron emission is somewhat similar to the ionizationprocess. In ionization process, electrons are removed only from the atom and not from the metal whereas in electron emission process electrons are removed from the atom as well as from the metal.
Different methods of electron emission
The different methods to emit free electrons from the metal surface include: