Answer the following questions
a. Explain qualitatively on the basis of domain picture the irreversibility in the magnetisation curve of a ferromagnet.
b. The hysteresis loop of soft iron piece has a much smaller area than that of a carbon steel piece. If the material is to go through repeated cycles of magnetisation , which piece will dissipate greater heat energy ?
c. 'A system displaying a hysteresis loop such as a ferromagnet , is a device for storing memory ? ' Explain the meaning of this statement.
d. What kind of ferromagnetic material is used for coating magnetic tapes in a cassette player , or for building ' memory stores ' in a modern computer ?
e. A certain region of space is to be shielded from magnetic fields . Suggest a method.

Explanation

a. A ferromagnetic material has many small domains of magnetic field . But the magnetic moments of these domains are distributed in random so that there is no magnetism in the unmagnetised state. Under the increasing external magnetic field, the domains merge and magnetism increases until saturation is produced . On reducing the magnetising dield, the merged domains do not split at the rate they merged and hence the retentively . The domains are not completely randomised even if the external field is brought to zero. This shows te irreversibility in the magnetsation curve of the ferromagnetic material.
b. Carbon steel piece, because heat lost cycle is proportional to the are of hysteresis loop.
c. Magnetisation of ferromagnegnet is not a single - valued function of the magnetising field . Its value for a particular field depends field both on the field and also on history of magnetisation (i.e., how many cycles of magnetisation it has gone through etc.) In other words , the value of magnetisation is a record or memory of its cycles , of magnetisation, If information bits can be made to correspond to these cycles, the system displaying such a hysteresis loop can act as a device for storing information.
d. Ceramics (specially treated barium iron oxides) also called ferrites.
e. Surround the region by soft iron rings Magnetic field lines will be drawn into the rings and the enclosed space will be free of magnetic field . But this shielding is only approximate , unlike the perfect electric shielding of a cavity in a conductor placed in an external electric field.