A long solenoid has 1000 turns per metre and carries a current of 1 A. It has a soft iron core of μr = 1000. The core is heated beyond the curie temperature, TC.

Q: A long solenoid has 1000 turns per metre and carries a current of 1 A. It has a soft iron core of μ_r = 1000 . The core is heated beyond the curie temperature, T_C.

(a) The H field in the solenoid (nearly) unchanged but the B field changes .

(b) The H and B fields in the solenoid are nearly unchanged

(c) The magnetization in the core reverses direction

(d) The magnetization in the core diminishes by a factor of about 10⁸

Ans: (a) , (d)

Solution : Since H = n I ; Where n is the number of turns per unit length .

Hence H is independent of μ_r .

$\displaystyle B = \mu_0 \mu_r n I $

$\displaystyle B \propto \mu_r $

As core is heated beyond the curie temperature T_C , μ_r changes , B changes .

χ_para ≈ 10^-5

χ_Ferro ≈ 10³

χ_Ferro /χ_para = 10³/10^-5 = 10⁸