Question

A short bar magnet of magnetic moment m = 0.32 JT^-1 is placed in a uniform external magnetic field of 0.15 T. If the bar is free to rotate in the plane of the field, which orientations would correspond to its

• stable and
• unstable equilibrium? What is the potential energy of the magnet in each case?

Answer 1

Given M = 0.32 JT^-1 B = 0.15 T, U = ?

• If \(\overrightarrow M||\overrightarrow B\) then we have stable equilibrium and U = -MB = -0.32 × 0.15 T = -4.8 × 10^-2J
• If is \(\overrightarrow M\) anti-parallel to \(\overrightarrow B\) we have unstable equilibrium and

U = MB = 0.32 × 0.15 J = 4.8 × 10^-2 J