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Question

A rectangular coil of 250 turns having length 2.1 cm and width 1.25 cm carries a current of 85 μA. It is subjected to a magnetic field of strength 0.85 T. Work done for rotating the coil by 180°against the torque is approximately 
  1. 4.44 μJ
  2. 2.3 μJ
  3. 1.15 μJ
  4. 9.4 μJ


Solution

The correct option is D 9.4 μJ
Work done in rotating a coil
W=MB(cos(θ1)cos(θ2))
When it is rotated by angle 1800 , then
W=2MB=2(NiA)BGiven, N=250, i=85μAA=1.25×2.1×104m2A=2.625×104m2B=0.85 T   ...(1)
Putting the value in (1), we get
W=2×250×85×106       ×2.625×104×0.85W=9.4 μJ

 

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