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Question

A bar magnet is hung by a thin cotton thread in a uniform horizontal magnetic field and is in equilibrium state. the energy required to rotate it by 60o is W, Now the torque required to keep the magnet in this new position is.
  1. 3W
  2. 32W
  3. W3
  4. 2W3

A
2W3
B
W3
C
3W
D
32W
Solution
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Torque acting on a bar magnet kept in a magnetic field B is τ=m×B=mBsinθ

Hence work done in moving from θ=0 to θ=60 is W=mBcos60=12mB

Hence torque acting when magnet is placed at 60=32mB=3W

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