Question

A moving coil galvanometer has a coil with $175$ turns and area $1{\text{cm}}^2.$ It uses a torsion band of torsion constant ${10}^{-6}{\text{Nm rad}}^{-1}.$ The coil is placed in a magnetic field $B$ parallel to its plane. The coil deflects by $1^{\circ }$ for a current of $1\text{mA}.$ The value of $B\text{(in T)}$ is approximately-

• A. ${10}^{-1}$
• B. ${10}^{-2}$
• C. ${10}^{-3}$
• D. ${10}^{-4}$

Answer 1

The correct option is C ${10}^{-3}$

Torque produced in the coil is,

$\tau =MB\sin \theta$

$C\theta =NiAB\sin {90}^{\circ }$
${10}^{-6}\times \left(\frac{\pi }{180}\right)=175\times B\times {10}^{-3}\times {10}^{-4}$
$\therefore B\approx {10}^{-3}\text{T}$
<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> Hence, $\left(D\right)$ is the correct answer.