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)

8 A

Answer 2

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
$i=\frac{4\times {{10}^{3}}\times 12\times {{10}^{-2}}}{60}=8A$