question_answer

A circular coil of 20 turns and radius 10 cm is placed in uniform magnetic field of 0.10 T normal to the plane of the coil. If the current in coil is 5 A, then the torque acting on the coil will be:

A)  31.4 Nm                             

B)  3.14 Nm

C)  0.314 Nm                           

D)  zero

Correct Answer: D

Solution :

                 Torque\[(\tau )\]acting on a loop placed in a magnetic field B is given by \[\tau =nVIA\,\sin \theta \]where A is area of loop, I the current through it, n the number of turns, and the angle which axis of loop makes with magnetic field B. Since, magnetic field  of coil is parallel to the field applied, hence \[\theta ={{0}^{o}}\]and\[{{0}^{o}}=0\] \[\tau =0\]