A rectangular coil of 100 turns has length 5 cm and width 4 cm. It is placed with its plane parallel to a uniform magnetic field and a current of 2 A is sent through the coil. Find the magnitude of the magnetic field B, if the torque acting on the coil is 0.2 N m−1.
τ=nia×B
τ=niBa sin 90∘
⇒ 0.2=100×2×5×4×10−4×b
⇒ b=0.5 tesla