Question

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 a 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.

Answer 1

(a) Since in a ferromagnetic substance the magnetic properties are due to alignment of domains, therefore on withdrawing the magnetizing field the original domain formation does not take place.

(b) Carbon steel piece, because heat lost per cycle is proportional to the area of hysteresis loop.

(c) Magnetization of a ferromagnet is not a single-valued function of magnetizing field. Its value for a particular field depends both on the field and also on the history of magnetization. In other words, the value of magnetization is a record of 'memory' of its cycle of magnetization. If information bits can be made to correspond to these cycles, the system with such a hysteresis loop can act as a device for storing information.

(d) Ceramic(specially treated barium iron-oxides) also called ferites.

(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.