CBSE Class 12 Physics 2019 Outside Delhi Set 1 Paper

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Question : 16
Total: 27
Prove that the magnetic moment of the electron revolving around a nucleus in an orbit of radius r with orbital speed v is equal to evr∕2. Hence using Bohr's postulate of quantization of angular momentum, deduce the expression for the magnetic moment of hydrogen atom in the ground state.
Solution:  
Proving magnetic moment as ‌
evr
2

Deducing expression of the magnetic moment of hydrogen atom
The magnetic moment is
m=IA
But current is I=‌
e
T
=‌
ev
2Ï€r

where, T=‌
2Ï€r
v
and the area, A=Ï€r2
m=‌
ev
2Ï€r
Ï€
r2
=‌
evr
2

But from Bohr's second postulate
‌mevr=‌
nh
2Ï€
=‌
h
2Ï€
‌ for ‌n=1
‌vr=‌
nh
2Ï€me

Hence the magnetic moment is
m=‌
e
2
‌
h
2Ï€me
=‌
eh
4Ï€me
(Here n=1 )
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