First slide

Magnetism

Question

A short bar magnet of magnetic moment ${0.8 JT}^{-1}$ is placed in a uniform magnetic field of $0.32 T$ . The magnet is in stable equilibrium. When the potential energy is

Moderate

A

0.256 J
B

-0.256 J
C

zero
D

-0.512 J

Solution

Here magnetic moment ${M=0.8 JT}^{- 1}$
Magnetic field $B=0.32 T$
When a bar magnet is placed in a uniform magnetic field; its potential energy
$U = - \vec{M} . \vec{B}$
$⇒U=-MBcos θ$
For stable equilibrium $θ = 0$
$⇒U=-MB$
$⇒U=-0.8 \times 0.32$
$⇒U=-0.256 J$ .

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