Electronic Emission Test is started
What is Electron Emission ?
Electron Emission:- As we know the valence electrons of the conductor atoms are loosely attached to the atomic nuclei. At room temperature, the thermal energy in the conductor is enough to break the bonds of the last valence electrons and leave them free to any one nucleus. These unbound electrons move at random within the conductor and are known as free electrons.
when an electric field is applied across the conductor, these unbound (free) electrons move through the conductor in an orderly manner, hence constituting the electric current. In this way, these free electrons move through the conductor or electric current flows through a wire.
As the operation of many electronic devices depends on the movement of electrons in an evacuated space. For this reason, the free electrons must be evicted from the surface of the metallic conductor by supplying enough energy from some external source. This is known as electron emission. The emitted electrons can be made to move in a vacuum under the influence of an electric field, thus constituting electric current in a vacuum.
Explanation of electron emission
The release of electrons from the surface of a substance is known as electron emission. Metals are used For electron emission because they have many free electrons. The random motion of free electrons is observed If a piece of metal is investigated at room temperature. though these electrons are free only to the extent that they may transfer from one atom to another within the metal they cannot depart from the metal surface to provide electron emission As shown in the below figure 2.1
It is because the free electrons that start at the surface of metal the nuclei positive charge pulling them back and none pulling forward. Hence at the surface of a metal, a free electron encounters forces that prevent it to leave the metal. In other words, the metallic surface offers a barrier to free electrons and is known as a surface barrier.
However, when sufficient external energy is applied to the free electron, its kinetic energy is increased and as a result, the electron will cross over the surface barrier to depart the metal. This additional energy needed by an electron to overcome the surface barrier of the metal is called the work function of the metal. The amount of additional energy needed to emit an electron from a metallic surface is known as the work function of that metal. Hence, if the total energy needed to release an electron from metal is 4 eV* and the energy already possessed by the electron is 0.5 eV, then additional energy needed (i.e., work function) is 4.0− 0.5 = 3.5 eV. The work function of pure metals changes roughly from 2 to 6 eV. It depends upon the nature of metal, the conditions of its surface, and its purity. The work function of metal used for electron emission should have low so that a small amount of energy is needed to cause emission of electrons.
Types of Electron Emission:
The electron emission from the surface of a metal is possible only if enough additional energy (equal to the work function of the metal) is supplied from some external source. This external energy may come from a different sources such as the energy stored in the electric field, heat energy, light energy or kinetic energy of the electric charges bombarding the metal surface. Accordingly, there are following four principal methods are used to get electron emission from the surface of a metal.
In this method, the metal is heated to sufficient temperature (about 2500ºC) to enable the free electrons to depart the metal surface. These emitted free electrons depend upon the temperature. when the temperature is higher than the emission of electrons will be greater. This kind of emission is used in vacuum tubes.
In this method, a strong electric field (i.e. a high positive voltage) is supplied at the metal surface which attracts the free electrons out of metal because of the attraction of a positive field. This emission depends upon the field supplied, the stronger the electric field, the greater is the electron 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 depart the surface. This emission depends upon the intensity, the greater the intensity (i.e. brightness) of light ray falling on the metal surface, the greater is the photo-electric.