Question

In using Ampere's law to find the magnetic field of a straight, long solenoid, the loop (Amperian loop) that is taken is

• A. a circular loop, coaxial with the solenoid
• B. a rectangular loop in a plane perpendicular to the axis of the solenoid
• C. a rectangular loop in a plane containing the axis of the solenoid, the loop being totally within the solenoid
• D. a rectangular loop in a plane containing the axis of the solenoid, the loop being partly inside the solenoid and partly outside it

Answer 1

Answer: (D)

When a current carrying coil is placed in a uniform magnetic field $B$, then torque,
$\tau =MB\sin \theta$
$\tau =(NIA)B\sin \theta$
$\tau =10\times 1\times 0.2\times 6\times 10^{-4}\times \sin 30^{\circ }$
$=10\times 1\times 0.2\times 6\times 10^{-4}\times \frac{1}{2}$
$=6\times 10^{-4}N-m$