Question

A short bar magnet of magnetic moment $0.4{\text{JT}}^{-1}$ is placed in a uniform magnetic field of $0.16\text{T}.$ The magnet is in stable equilibrium. Then the potential energy is,

• A.
  $-0.64\text{J}$
• B.
  Zero
• C.
  $-0.082\text{J}$
• D.
  
  $-0.064\text{J}$
  

Answer 1

The correct option is D


$-0.064\text{J}$

Given $M=0.4{\text{JT}}^{-1},B=0.16\text{T}$

Stable equilibrium means $\theta$ between $\overrightarrow{M}$ & $\overrightarrow{B}=0^{\circ }$

So Potential energy $U=-\overrightarrow{M}.\overrightarrow{B}=-MB\cos 0^{\circ }U=-MBU=-0.4\times 0.16=-0.064\text{J}$

Hence, option $\left(D\right)$ is the correct answer.