A rectangular wire-loop of width a is suspended from the insulated pan of a spring blance as shown in A current I exists in the anticlockwise direction in the loop. A magnetic field B exists in the lower region.Find the change in the tension of the spring if the current in the loop is reversed.

Explanation

Force in the lower side of rectingular loop due to current I in magnetic fied B Initially when current is flowing in anticlockwise direction .
Then ` F= iab in right of a
when current is reversed the tension is
` F = iaB in left of a `
So , change in tension
` = iaB - (-iaB) = 2 iaB`