Question

When torque is max of a dipole in magnetic field then is its nonstable position?

Answer 1

In Physics, the dipole moment is directed from -ve charge to + charge.

Now, in equilibrium, net force and torque on the system should be zero. In addition if PE is maximum, equilibrium will be unstable and for minimum PE it will be stable.

torque = p × E = pE sin (theta), theta is the angle between E and p

Potential energy U =- p.E = -pE cos theta

In uniform electric field net force on dipole will always be zero but torque is zero for theta =0 degree and 180 degree.

Now PE for theta =0, U=-pE (which is minimum), and for theta = 180 degree, U=+pE( maximum value)

Therefore, for theta = 0 degree, the equilibrium will be stable and if dipole is slightly displaced, it performs oscillations.

For theta = 180 degree it will be unstable equilibrium.

Always remember that toque and force are zero in both cases.