`300` turns of a thin wire are uniiformly wound on a permanent manet shaped as a cylinder of length `15cm`. When a current `3A` is passed through the wire, the field outside the magnet disappears. Then the coercive force of the material is

Options

A. `2 kNm^(-1)`
B. `4 kNm^(-1)`
C. `5kNm^(-1)`
D. `6 kNm^(-1)`

Answer

D

Explanation

The magnetic field at the centre of a long straight conductor, whose cross section is in the form of a thin half ring is ltbRgt `B=(mu_(0)I)/(pi^(2)R)rarr(1)`
So the force per unit length of the wire carrying current `I` and placed on the axis of the first conductor is
`F=BIxxl`
`=(mu_(0)I^(2))/(pi^(2)R)` `("for unit length")rarr(2)`