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Question

A moving hydrogen atom makes a head on collision with a stationary hydrogen atom. Before collision both atoms are in ground state and after collision they move together. What is the minimum value of the kinetic energy of the moving hydrogen atom, such that one of the atoms reaches one of the excitation state.

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Solution

Let u be velocity of hydrogen atom before collision and V the ve locity of two atoms moving together after collision By principal of conservation of momentum , we have
Mu+M×0=2MV
V=u2
The loss in kinetic energy Δϵ due to collision is
Δϵ=12Mu212(2M)V2
As V=u2, we have Δϵ=12Mu212(2M)(u2)2
=12Mu214Mu2=14Mu2
This loss in energy is due to excitation of one of hydrogen atoms . The ground state (n=1) energy of a hydrogen atom is ϵ1=13.6eV
The energy of first excited level (n=2) is :ϵ2=3.4eV
Thus minimum energy required to excite a hydrogen atom from ground state to first excited state is :
ϵ2ϵ1=[3.4(13.6)]eV=10.2eV
=10.2×1.6×1019J=16.32J
As per problem with the loss in kinetic energy in collision is due to energy used up in exciting one of atoms
Thus, Δϵ=ϵ2ϵ1
or 14Mu2=16.32×1019
or u2=4×16.32×1019M
The mass of hydrogen atom is 1.0078amu or 1.0078×1.66×1027kg
u2=4×16.32×10191.0078×1.66×1027=39.02×108kg
or u=39.02×108=6.24×104ms1

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