`N = 300` turns of thin wire are uniformly wound on a permanent magnet shaped as a cylinder whose length is equla to `l = 15 cm`. When a current `I = 3.0 A` was passed through the wring the field outside the magnet disappeared. Find the coercive force `H_(e)` of the materail from which the magnet was manufaucured.

Explanation

The corecive force `H_(e)` is just the magnitude field within the cylinder. This is by circulation theorem, `H_(e) = (N I)/(l) = 6 kA//m`
(from `oint vec(H).d.vec(r) = I`, total current considering a recantagular contour.)