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Moving Charges And Magnetism

Question
CBSEENPH12037689

A current carrying coil of 200 turns has an area of cross-section 1 x 10–4 m2. When suspended freely through its centre, it can turn in a horizontal plane. What is the magnetic moment of the coil for a current of 5 A? Also calculate the net force and torque on coil if a uniform horizontal field of 10 x 10–2 T is set up at an angle of 30° with axis of coil when it is carrying the same current.

Solution

Here,
Number of turns, N = 200
Area of cross-section. A = 1 x 10–4 m2
Current passing through the coil, I = 5 A
M =?
Magnetic momemnt of the coil, M = N IA
                                               = 200 x 5 x 10–1 
                                               = 10–1 JT–1 

In a uniform horizontal field B = 10 x 10–2 T, the ends of coil experience equal and opposite forces. Therefore, net force on the coil = 0.
θ = 30o

Therefore, torque acting on the coil is, 

Torque, τ = MB sin θ                  = 10-1 × 10 × 10-2 sin 30°                 = 10-2 ×12                  = 0.5 × 10-2 Nm