Question

If this dipole were kept in a uniform external electric field e0, diagrammatically represent the position of the dipole in stable and unstable equilibrium and write the expressions for the torque acting on the dipole in both the cases.

Answer 1

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Answer 2

The torque acting on dipole is PE sin0 = 0° and  - PEsin 180 =0

Let the dipole two positions where dipole is kept in constant electric field be = a and b  

Case 1

Angle between electric field and dipole = Ф = 0°

Potential energy of dipole = U = - PEcosФ

U = - PEcosФ = -PE

Thus, the dipole is in stable equilibrium

Torque = T = PE sinФ

T = PE sin0 = 0°

Case 2  

Angle between electric field and dipole = Ф = 180°

Potential energy of dipole = U = - PEcos 180°

Thsu, the dipole is in unstable equilibrium

Torque= T = - PEsin 180 = 0