Question

The final torque on a coil having magnetic moment $25\, A\, m^{2}$ in a $5 \,T$ uniform external magnetic field, if the coil rotates through an angle of $60^{\circ}$ under the influence of the magnetic field is

• A. $216.5\, N\,m$
• B. $108.25\ N\,m$
• C. $102.5\, N\,m$
• D. $258.1\, N\,m$

Answer 1

Answer: (B)

$\left|\vec{\tau}\right|=\left|\vec{m}\times\vec{B }\right|=mB\,sin\,\theta$
Here, $m=25\, A\, m^{2} ; \theta=60^{\circ}; B=5\, T$
$\therefore \tau=25 \times 5 \times sin \, 60^{\circ}$ or $\tau =125 \times \frac{\sqrt{3}}{2}$
$=108.25\, N\, m$