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.
Answers
a) in a specimen of ferromagnets, the atomic dipoles are grouped together in domains . all the dipoles of a domain are aligned in the same direction and have net magnetic moment. in an unmagnetised substance these domians are randomly distributed so that the resultant magnetisation is zero. when the substance is placed in an external magnetic field, thses domain align themselves in the direction of the field.some energy is spent in the process of alignment when the external field is removed, the substance retains some magnetisation. the energy spent in the process of magnetisation is not fully recovered.the balance of energy is lost as heat . this is the basic cause for irreversibility of the magnetisation curve of a ferromagnet substance.
(b) carbon steel piece, because the heat produced in complete cycle of magnetisation is directly proportional to the area under the hysteresis loop.
(c) magnetisation of a ferromagnet is not a single valued function of the magnetic field.
magnetisation of a ferromagnet is not a single valued function of the magnetic field.its value for a particular field depends both on the magnetising field and on the history of its magnetisation.e.g., how many cycles of magnetisation it has done through etc.so, the value of magnetisation is a record or memory of it's cycles of magnetisation. if the 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) ferrites or ceramics which is specially treated barium ion oxide.
(e) when the region is surrounded with soft iron rings , as magnetic field lines will be drawn into the rings and the enclosed space becomes free of magnetic field.