Question

A $250-$turn rectangular coil of length $2.1 \, cm$ and width $1.25 \, cm$ carries a current of $85 \, \mu A$ and subjected to magnetic field of strength $0.85 \, T$. Work done for rotating the coil by $180^{\circ}$ against the torque is:-

• A. $4.55 \, \mu J$
• B. $2.3 \, \mu J$
• C. $1.15 \, \mu J$
• D. $9.1 \, \mu J$

Answer 1

Answer: (D)

$W=M B\left(\cos \theta_{1}-\cos \theta_{2}\right)$
When it is rotated by angle $180^{\circ}$ then
$W =2 M B$
$W =2(N I A) B$
$=2 \times 250 \times 85 \times 10^{-6}\left[1.25 \times 2.1 \times 10^{-4}\right] \times 85$
$=9.1\, \mu J$