The intensity of magnetisation $I$ is plotted against the magnetising field $H$ for different substances. The curves $A$ and $B$ shown in the adjoining figure are associated with

$A$ bar magnet has a coercivity of $4 \times 10^3 \, A/m$. It is desired to demagnetize it by inserting it inside a solenoid $12 \, cm$ long and having $60$ turns. The current that should be sent through the solenoid is... (in $, A$)

In the hysteresis cycle,the value of $H$ needed to make the intensity of magnetisation zero is called

In the following graph of flux density $(B)$ versus magnetizing force $(H)$,coercivity and retentivity are respectively represented by the points

Answer the following questions:
$(a)$ Explain qualitatively on the basis of the 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.

The coercivity of a magnet,where the ferromagnet gets completely demagnetized,is $3 \times 10^{3} \text{ A m}^{-1}$. The minimum current required to be passed in a solenoid having $1000 \text{ turns m}^{-1}$,so that the magnet gets completely demagnetized when placed inside the solenoid,is: