Q. An $\alpha$ particle with a kinetic energy of $2.1\, eV$ makes a head on collision with a hydrogen atom moving towards it with a kinetic energy of $8.4\, eV$. The collision:

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A

must be perfectly elastic

B

may be perfectly inelastic

C

may be inelastic

D

must be perfectly inelastic

Solution:

Total momentum of system is $O$ hence maximum possible energy loss can be $10.5\, eV$ out of which $10.2\, eV$ can be absorbed hence option (C) is correct. For completely inelastic collision both come to rest after collision
and net energy of $4E + E = 10.5\, eV$ is lost. But electron in ground state of $H$-atom can accept only an energy of $10.2\, eV$. Hence the collision maybe inelastic but it can never be perfectly inelastic.

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