For a moving coil galvanometer, the deflection in the coil is $0.05\,rad$ when a current of $10\,mA$ is passed through it. If the torsional constant of suspension wire is $4.0 \times 10^{-5}\,Nm\,rad ^{-1}$, the magnetic field is $0.01\,T$ and the number of turns in the coil is $200$,the area of each turn (in $cm ^2$ ) is :
JEE MAIN 2023, Medium
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$\tau= K \theta$
$NiAB = K \theta$
$A =\frac{ K \theta}{ NiB }=\frac{4 \times 10^{-5} \times 0.05}{200 \times 10 \times 10^{-3} \times 0.01}$
On solving $A =10^{-4}\,m ^2=1\,cm ^2$
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