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Units and Measurement
Question
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A closely wound solenoid of 2000 turns and area of cross -section 1.5 x 10-4 m2 carries a current of 2.0 A. It is suspended through its centre and perpendicular to its length, allowing it to turn in a horizontal plane in a uniform magnetic field 5 x 10-2 T making an angle of 30o with axis of  the solenoid. The torque on the solenoid will be

  • 3 x 10-3 N-m

  • 1.5 x 10-3 N-m

  • 1.5 x 10-2 N-m

  • 3 x 10-2 N-m

Answer

A.

3 x 10-3 N-m

Given, N = 2000,
A = 1.5 x 10-4 m2
i = 2.0 A B = 5 x 10-2 T
and
θ = 30o
Torque = NiBA sin θ
2000 x 2 x 5 x 10-2 x 1.5 x 10-14 x sin 30o
= 2000 x 50 x 10-6 x (1/2)
= 1.5 x 10-2 Nm

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