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`.A. The `vecH` field in the solenoid is (nearly) unchanged but the `vecB` field decreases drasticallyB. The `vecH` and `vecB` fields in the solenoid are nearly unchangedC. The magnetisation in the core reverse directionD. The magnetisation in the core diminishes by a factor of about `10^8`

A long solenoid has 1000 turns per metre and carries a current of `1A`

1.	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`.A. The `vecH` field in the solenoid is (nearly) unchanged but the `vecB` field decreases drasticallyB. The `vecH` and `vecB` fields in the solenoid are nearly unchangedC. The magnetisation in the core reverse directionD. The magnetisation in the core diminishes by a factor of about `10^8`
Answer» Correct Answer - A::D In the solenoid, `H=nI=a` constant and `B=mu_0mu_rnI` changes due to variation in `mu_r`. When temperature of the iron core of solenoid (which is ferromagnetic materials) is raised beyond curie temperature, then soft iron core behaves as para magnetic material. We know that `(chi_m)_(para)~~10^-5.(chi_m)_(Ferro)~~10^3` `:. ((chi_m)_(Ferro))/((chi_m)_(Para))=10^3/10^-5=10^8` Thus magnetisation of the core diminishes by a factor `10^8`. Hence, options (a) and (d) are correct.

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