A long solenoid has 1000 turns per metre and carries a current of `1A`. It has a soft iron core of `mu_r=1000`. The core is heated beyond the Curie temperature, `T_c`.

Options

A. The `vecH` field in the solenoid is (nearly) unchanged but the `vecB` field decreases drastically.
B. The `vecH` and `vecB` 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 `10^8`

Answer

A, D

Explanation

The B field inside the solenoid is given by `B = mu_0 mu_r nI`
Where `mu_r` = relative permeability of core
n = number of turns per unit length
I = current through the solenoid
Due to drastic decrease in the value on heating beyond Curie.s temperature, the B field will decrease drastically. On the other hand, the H field given by
H=nl
Since I is constant, the H field remains constant. When temperature of iron core is increased beyond the Curie temperature, it will start behaving as a paramagnetic material. On the basis of susceptibility ratio of the two types of materials, given by:
`(chi_(ferro))/(chi_(para) ) = (10^3)/(10^(-5) ) = 10^8`
We can say that the magnetization in the core decreases by a factor of about `10^8` as magnetization `I_m = chi` .