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NCERT Class XII Chapter
Magnetism and Matter
Questions With Solutions

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Question : 8 of 25
Marks: +1, -0
A closely wound solenoid of 2000 turns and area of cross-section 1.6 × 10–4m210^{–4} m^2, carrying a current of 4.0 A, is suspended through its centre allowing it to turn in a horizontal plane.
(a) What is the magnetic moment associated with the solenoid?
(b) What is the force and torque on the solenoid if a uniform horizontal magnetic field of 7.5 × 10–2 T is set up at an angle of 30° with the axis of the solenoid
Solution:  
Soln. (a) Magnetic moment associated with solenoid
M = NIA = 2000 × 4 × 1.6 × 10–410^{–4} = 1.28 A m2m^2
(b) Force on the solenoid will be zero in uniform magnetic field.
Torque τ = MB sinθ
= 1.28 × 7.5 × 10–210^{–2} × sin 30° or τ = 4.8 × 10–210^{–2} N m
The torque tends to align the solenoid in the direction of magnetic field.
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