Q. 300 turns of a thin wire are uniformly wound on a permanent magnet 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
(a) 2 kAm-1
(b) 4 kAm-1
(c) 5 kAm-1
(d) 6 kAm-1
Ans : (d)
Sol: coercive force of the material $ \displaystyle H = n i = \frac{N}{l}\times i $
$ \displaystyle H = \frac{300}{0.15}\times 3 $
= 6000 A/m
= 6 kA/m