Problem 3.280

$N=300$ turns of thin wire are uniformly wound on a permanent magnet shaped as a cylinder whose length is equal to $l=$ $=15 \mathrm{~cm} .$ When a current $I=3.0 \mathrm{~A}$ was passed through the wiring the field outside the magnet disappeared. Find the coercive force $H_{c}$ of the material from which the magnet was manufactured.