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

Options

A. 4.55 `mu J`
B. `2.3 mu J`
C. `1.15 mu J`
D. `9.1 mu j`

Answer

D

Explanation

Work done in a coil
`W = mB (cos theta_1 - cos theta_2)`
When it is rotated by angle `180^@` then
W = 2mB = 2(NIA)B
Given : `N = 250 ,I = 85 mu A = 85 xx 10^(-6) A`
`A = 1.25 xx 2.1 xx 10^(-4) m^(2)""^(cong)2.5 xx 10^(-4) m^2`
B = 0.85 T
Putting these values in eqn. (i), we get
`W = 2 xx 250 xx 85 xx 10^(-6) xx 2.5 xx 10^(-4) xx 0.85`
`~~ 9.1 xx 10^(-6) J = 9.1 mu J`