Question

A bar magnet of magnetic moment \(5\,\text{A m}^2\) is placed in a uniform magnetic induction \(3\times10^{-5}\,\text{T}\). If each pole of the magnet experiences a force of \(2.5\times10^{-4}\,\text{N}\), then the magnetic length of the magnet is

• A. \(0.8\,\text{m}\)
• B. \(0.2\,\text{m}\)
• C. \(0.6\,\text{m}\)
• D. \(0.4\,\text{m}\)

Hint:

Magnetic moment equals pole strength multiplied by magnetic length.

Answer 1

The Correct Option is C

Step 1: Relation between force and pole strength.
Force on a magnetic pole in a uniform magnetic field is \[ F = mB \]

Step 2: Find pole strength.
\[ m = \frac{F}{B} = \frac{2.5\times10^{-4}}{3\times10^{-5}} = \frac{25}{3} \]

Step 3: Use magnetic moment relation.
Magnetic moment is \[ M = m \times 2l \] where \(2l\) is magnetic length.
\[ 5 = \frac{25}{3} \times 2l \Rightarrow 2l = 0.6\,\text{m} \]