CBSE Class 12-science Answered
(a) Two characteristics of materials used for making permanent magnets:
(i) high retentivity so that the magnet is strong and
(ii)high coercivity so that the magnetisation is not erased by stray magnetic fields, temperature fluctuations or minor mechanical damage.
(b) Core of electromagnets are made of ferromagnetic materials because they have high permeability and low retentivity. That is to ensure that heating losses are minimum as they will have narrow Hystersis curve.
Or
Magnetic field lines when a diamagnetic material is placed in external magnetic field:
Magnetic field lines when a paramagnetic material is placed in external magnetic field:
Property that is responsible for the above behaviour is magnetic dipole moment.
Diamagnetic material has zero magnetic moment
Paramagnetic material has non zero magnetic moment