A 250-turns rectangular coil of length 2.1 cm and width 1.25 cm carries a current of `85 muA` and subjected to magnetic field of strength `0.85 T`. Work done for rotating the coil by `180^(@)` against the torque is

Options

A. `9.48 mu J`
B. `4.55 mu J`
C. `2.3 mu J`
D. `1.15 mu J`

Answer

A

Explanation

When coil is rotated by angle from `0^(@)" to " 180^(@)` then
` W = MB ( cos 0^(@) - cos 180^(@))`
` W = 2 MB`
` W = 2 (NI A) B`
` = 2 xx 250 xx 85 xx 10^(-6)`
` [1.25 xx 2.1 xx 10^(-4)] xx 85 xx 10^(-2) `
` = 9.48 mu J`
Closest option is (a)