Question

A bar magnet has a coercively $ 4\times {{10}^{3}}A{{m}^{-1}} $ . It is desired to demagnetize it by inserting it inside a solenoid $12\, cm$ long and having $60$ turns. The current carried by the solenoid should be:

• A. 8 A
• B. 6 A
• C. 4.5 A
• D. 2 A

Answer 1

Answer: (A)

The reversed magnetic field needed to demagnetise the specimen is known as coercivity of the material.
$ B={{\mu }_{0}}H $
or $ {{\mu }_{0}}ni={{\mu }_{0}}H $
or $ \frac{Ni}{L}=H $
$ \left( \because n=\frac{N}{L} \right) $
or $ i=\frac{HL}{N} $
Given, $ H=4\times {{10}^{3}}A{{m}^{-1}}, $
$ L=12\text{ }cm=12\times {{10}^{-2}}m,\text{ }N=60\text{ }turns $
. Substituting the values, we get
$ i=\frac{4\times {{10}^{3}}\times 12\times {{10}^{-2}}}{60}=8A $