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.