Question

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

• A. $2A$
• B. $4A$
• C. $6A$
• D. $8A$

Answer 1

Answer: (D)

The bar magnet coercivity $4\times 10^{3} \, A \, m^{- 1}$ i.e., it requires a magnetic intensity $H=4\times 10^{3} \, A \, m^{- 1}$ to get demagnetised. Let $i$ be the current carried by solenoid having n number of turns per metre length, then by definition $H \, = \, ni$ . Here $ \, H=4\times 10^{3} \, A \, m^{- 1}$
$n=\frac{N}{l}=\frac{60}{0.12}=500 \, m^{- 1}$
$\Rightarrow i=\frac{H}{n}=\frac{4 \times 1 0^{3}}{500}=8.0 \, A$