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 `vec(H)` field in the solenoid is (nearly) unchanged but the `vec(B)` field decreases drastically.
B. The `vec(H)` and `vec(B)` field in the solenoid are nearly unchanged.
C. The magnetisation in the core reverses direction.
D. The magnetisation in the core reverses direction.

Answer

A, D

Explanation

1) As number of turn/metre of the solenoid `(n)` and current through the solenoid `(I)` practically remain constant when the core in the solenoid is heated, `H(=nI)` remains unchanged.
But as there is a large decreases in the susceptibulity `(chi_(m))` of the core on heating it beyond critical temperature, `B` decreases drastically with temperature because `B=mu_(0)(1+chi_(m))H`.
4) The soft iron core which is ferromagnetic, changes to an ordinary paramagnetic when heated beyond Curie temperature. Susceptibility of soft iron core,
`chi_(m)=mu_(r )-1=1000-1approx1000=10^(3)`
Suceptibility of a paramagnetic, `chi_(m)approx10^(-5)` (known)
Since mangetisation in the core, i.e., `M=chi_(m)H`,