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,
$\tau=M B \sin \theta$
$\tau=(N I A) B \sin \theta$
$\tau=10 \times 1 \times 0.2 \times 6 \times 10^{-4} \times \sin 30^{\circ}$
$=10 \times 1 \times 0.2 \times 6 \times 10^{-4} \times \frac{1}{2}$
$=6 \times 10^{-4} N-m$