The ratio of the magnetic field at the centre of a current carrying circular coil to its magnetic moment is $'\alpha '.$ If the current and radius both are doubled then new ratio will become
A$\frac {\alpha }{8}$
B$\frac {\alpha }{4}$
C$2\,\alpha $
D$4\,\alpha $
Medium
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A$\frac {\alpha }{8}$
a $\mathrm{B}=\frac{\mu_{0} \mathrm{NI}}{2 \mathrm{R}}, \mathrm{M}=\mathrm{NI}\left(\mathrm{pR}^{2}\right)$
$\frac{B}{M} \propto \frac{1}{R^{3}} \quad$ (not depend on $I$)
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