Question

Whenever a stream of electrons collides with a stream of photons, in this collision, which of the following is not conserved?
(A) Linear Momentum
(B) Total Energy
(C) No. of photons
(D) No. of electrons

Answer 1

Hint
Here we analyze all the four options to see whether these quantities are conserved or not. The photoelectric effect is a process that involves interaction between electrons and photons, this question talks about a similar phenomenon.

Complete step by step answer
Analyzing all four options-
Linear Momentum: According to the law of conservation of linear momentum, in any isolated collision the total momentum is conserved.
This shall be true for the electron and the stream of photons too, as light has momentum which is $p = \dfrac{h}{\lambda }$
Total energy- According to the law of conservation of energy, energy can never be created nor destroyed, it can only be transformed from one form to the other.
Thus, in a collision $T.E. = K.E. + P.E. + U$
This equation means that the total energy of a system is used to change the kinetic, potential, and internal energy in an isolated system.
No. of photons- In the photoelectric effect, it is observed that photons are absorbed by the electrons of an alkali metal, due to this the electron gains energy and can break free from its crystal lattice.
Thus it’s possible that the number of photons is not conserved.
No. of electrons- Electrons are a type of matter, all matter has mass and mass is conserved. The number of electrons is conserved. Electrons can only decay or annihilate in very special and rare conditions, none of which is possible due to collision with a stream of photons.
So, option (C) is the only quantity that is not conserved in the collision.

Note
Whenever the stream of photons collides with electrons, they are most likely absorbed, and the electron gains some energy. It jumps from ground state to excited state. In the photoelectric effect, the electrons gain enough energy to leave the shell and still have some amount of kinetic energy.