Question

The coercivity of a bar magnet is 140 A m\(^{-1}\). To demagnetize it, it is placed inside a solenoid of length 1.6 m and number of turns 112. What is the current flowing through the solenoid?

• A. 9 A
• B. 2.25 A
• C. 3.2 A
• D. 1.25 A

Hint:

To demagnetize a bar magnet using a solenoid, the magnetic field strength inside the solenoid must match the coercivity of the magnet.

Answer 1

The Correct Option is B

The coercivity is the magnetic field strength required to demagnetize the magnet. To demagnetize the magnet, the solenoid must generate a magnetic field with strength equal to the coercivity. The formula for the magnetic field inside a solenoid is: \[ B = \mu_0 \frac{N}{L} I \] Where: - \(B\) is the magnetic field strength, - \(\mu_0 = 4\pi \times 10^{-7} \, \text{T m/A}\) is the permeability of free space, - \(N = 112\) is the number of turns, - \(L = 1.6 \, \text{m}\) is the length of the solenoid, and - \(I\) is the current. We set the magnetic field \(B\) equal to the coercivity: \[ B = 140 \, \text{A/m} \] Solving for the current \(I\): \[ I = \frac{140 \times 1.6}{4\pi \times 10^{-7} \times 112} \approx 2.25 \, \text{A} \]