Force on Current-Carrying Conductors
A massless square loop of wire of resistance ' ' supporting a mass ' M ' hangs vertically with one of its sides in a uniform magnetic field ' B ' directed outwards in the shaded region. A d.c. voltage ' V ' is applied to the loop. For what value of ' ' the magnetic force will exactly balance the weight of the supporting mass ' M '? (side of loop = L, acceleration due to gravity)

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Answer: (c)
- Only the top side (length ) is in the field; forces on the vertical sides cancel.
- Balance: with
- So the answer is (c) .