Question

# A circular coil of 70 turns and radius 5 cm carrying a current of 8 A is suspended vertically in a uniform horizontal magnetic field of magnitude 1.5 T. The field lines make an angle of 30 with the normal of the coil then the magnitude of the counter torque that must be applied to prevent the coil from turning is :

A
33 N m
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B
3 N m
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C
3.3 x 102N m
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D
3.3 x 104N m
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Solution

## The correct option is C 3.3 x 102N mNumber of turns on the circular coil, n=70Radius of the coil, r=5.0cm=0.05 mArea of the coil , A=πr2=3.14×(0.05)2=Current flowing in the coil, I=8.0 AMagnetic field strength, B=1.5 TAngle between the field lines and normal with the coil surface,θ=30oThe coil experiences a torque in the magnetic field. Hence, it turns. The counter torque applied to prevent the coil from turning is given by the relation,τ=nIBAsinθ … (i)τ=70×8×1.5×0.00785×sin30oτ=3.3 Nm

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