If we double the radius of a coil keeping the current through it unchanged, then the magnetic field at any point at a large distance from the centre becomes approximately
A
double
B
three times
C
four times
D
one-fourth
AIIMS 2014, Medium
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C
four times
c $\mathrm{B}_{\text {axis }}=\left(\frac{\mu_{0} \mathrm{NI}}{2 \mathrm{x}^{3}}\right) \mathrm{R}^{2}$
$\mathrm{B} \propto \mathrm{R}^{2}$
So, when radius is doubled, magnetic field becomes four times.
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