Electron Emission - Practice Questions & MCQ

Electron Emission

Electron Emission-

As we have learned in Chemistry (Atomic structure) that the electrons in the outermost orbit of an atom are at maximum distance from the nucleus and hence most loosely bound to it. These type of electron is called free electrons. The free electrons in metals are free to move within the volume of metal even though they do not get ejected out of the surface of metal on their own. The main reason behind this is -  whenever an electron tries to leave the surface, the surface acquires a positive charge which pulls back the electron. So for escaping from the surface, an electron has to do a definite amount of work to overcome the force exerted by the opposite charges. To do this work, an external source imaparts minimum energy. This minimum energy is called the work function of the metal and is denoted by $\phi_0$.

Work function of any particular material is defined as the minimum energy which is required to liberate the most weakly bound surface electrons from that material without giving them any velocity. Since it is energy, but it is generally denoted in electron volt (eV). 

$$
\mathrm{leV}=\mathrm{le} \times 1 \mathrm{~V}=\left(1.6 \times 10^{-19} \mathrm{C}\right)(1 \mathrm{~V})=1.6 \times 10^{-19} \mathrm{~J}
$$

Since the energy of photon is given by - $h \nu$, So the minimum energy i.e., Work function is given by -

$$
\phi=h \nu_o=\frac{h c}{\lambda_o}
$$
 

When a free electron gets extra energy i.e., imparted energy $\geq$  work function from an external agent, then it is able to overcome potential barrier and the electron gets ejected out. There are a number of ways in which energy from outside can be supplied and based on these different way, there are different ways in which electron emission can take place. These ways are listed below:

1. Photoelectric emission: When electromagnetic radiations of suitable frequency (or wavelength) are incident on a metallic
surface, then electrons will be emitted, this phenomenon is known as photoelectric effect.

2. Thermionic emission: In this case, additional energy is given to the electrons to overcome potential barrier in the form of heat by
passing current through a filament.

3. Field emission: In this case, metal is placed in a strong electric field due to which the electrons are accelerated to such a speed
that the corresponding kinetic energy is sufficient to overcome potential barrier.

4. Secondary emission: It is a process in which the work function is supplied to the free electrons of a metal surface by collisions with fast moving secondary particles like neutrons, beta particles, etc