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. 2 A
• B. 4 A
• C. 6 A
• D. 8 A

Answer 1

The correct option is D 8 A

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
​$H=4\times {10}^{3}Amp$ turn $metr{e}^{-1}$
$n=\frac{N}{l}=\frac{60}{0.12}=500turnmetr{e}^{-1}\Rightarrow i=\frac{H}{n}=\frac{4\times {10}^3}{500}=8.0A$