Question

An electron with kinetic energy $5eV$ 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

Answer: (A)

must be elastic

The hydrogen atom in ground state will only absorb energy greater than $13.6eV$. If energy of an electron is absorbed then collision will be inelastic. If there is no absorption of energy(i.e. $E<13.6eV$) then collision will be elastic. When electron with KE, $5eV$ collides with hydrogen atom in ground state i.e. $E<13.6eV$, hence, there is no absorption of energy and collision will be elastic.