Question 11.C-A.68: In a vacuum diode, some of the electrons possessing suffi ci...
In a vacuum diode, some of the electrons possessing suffi cient kinetic energy can reach the plate even if the plate potential is negative. For a certain diode, the plate current becomes zero when plate potential is –10 V. What is the kinetic energy of the electrons in eV, if the plate potential is −4 V?
The plate current is zero when the electrons are completely stopped by the negative potential on the plate. The work done to stop electrons is equal to the change in their kinetic energy. Let the kinetic energy of the electrons when emitted from cathode is K2. The kinetic energy of the electrons become zero when the plate potential is –10 V
If ΔKE = v × q
K_{2}-K_{1}=v \times e
K_{2}=v \times e
K_{2}=10 e (1)
Let the kinetic energy of the electrons on reaching the anode be K_{1}, when the potential on anode is −4 v.
∴ K_{2}-K_{1}=4 \times e (2)
Substituting K_{2} from equation (1) into (2)
10 e-K_{1}=4 e \Rightarrow K_{1}=10 e-4 e
K_{1}=6 e Joule