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NTA Abhyas JEE Main2020PhysicsMagnetic Effects of CurrentPractice

A metallic wire is folded to form a square loop of side a . It carries a current i and is kept perpendicular to a uniform magnetic field. If the shape of the loop is changed from square to a circle without changing the length of the wire and current, the amount of work done in doing so is

Options

  1. AiBa 2 π + 2
  2. BiBa 2 π - 2
  3. CiBa 2 4 / π + 1
  4. DiBa 2 1 - 4 π

Correct answer

D. iBa 2 1 - 4 π

Step-by-step solution

W = U f – U i = - M f B cos 0 o - - M i B cos 0 o = i B A i - A f where A i = a 2 ,A f = π R 2 = 4 a 2 π as 2 π R = 4 a or R = 2 a π ∴ W = i B a 2 1 - 4 π

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