First slide

Magnetism

Question

A bar magnet has a length of 8 cm. The magnet field at a point at a distance 3 cm from the center in the broadside on position (equatorial) is formed to be $4 \times 10^{- 6} T$ . Find the pole strength of the magnet.

Moderate

A

$6.25 \times 10^{- 2} A m$
B

$125 \times 10^{- 5} A m$
C

$62.5 \times 10^{- 4} A m$
D

$125 \times 10^{- 4} A m$

Solution

Magnetic field at the broad side on position (equatorial point).
$B = \frac{μ_{0}}{4 π} \frac{M}{{(d^{2} + l^{2})}^{3 / 2}}$ [M=2ml where m = pole strength]
Given : $2 l = 8 c m \Rightarrow l = 4 c m; d = 3 c m$ ; $B = 4 \times 10^{- 6} T$
$∴ 4 \times 10^{- 6} = \frac{10^{- 7} (m \times 2 l)}{{(9 \times 10^{- 4} + 16 \times 10^{- 4})}^{3 / 2}}$
$\Rightarrow m = 6.25 \times 10^{- 2} A m$ .

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