Question

The B-H curve for a ferromagnet is shown in the figure. The ferromagnet is placed inside a long solenoid with 1000 turns/cm. The current that should be passed in the solenoid to demagnetise the ferromagnet completely is :

• A. $1\, mA$
• B. $2\, mA$
• C. $20 \, \mu A$
• D. $40 \, \mu A$

Answer 1

Answer: (A)

$1\, mA$

Answer 2

Given: Number of turns in solenoid $=1000$ turns $/ cm ;$ Coercivity $=+100 A / m$ (from graph) Now since coercivity is given by $H=\frac{N I}{l}$ $100 A / m =\frac{100 turns / cm \times I}{10^{-2}}$ $I =\frac{100 \times 10^{-2}}{1999}=10^{-3} A$ $I=1\, mA$