Question

An electron with kinetic energy 5 eV is incident on a hydrogen atom in its ground state. The collision

(a) must be elastic
(b) may be partially elastic
(c) must be completely inelastic
(d) may be completely inelastic

Answer 1

(a) must be elastic.
The minimum energy required to excite a hydrogen atom from its ground state to 1st excited state is approximately 10 eV. As the incident electron energy is not sufficient for excitation of the hydrogen atom so electron will not get absorbed in the hydrogen atom so it can not be an inelastic collision. Also this collision can not be partially elastic because in an partially elestic collision, there is a net loss on kinetic energy. If the energy is lost then corresponding amount of heat shlould have been produced but it is not so which implies that the collision is completely elastic.