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.

Let the dipole is kept in two positions (a) and (b) in constant electric field Eo as shown in the figure.
Case (a) : Angle between dipole and the electric field θ=0o
Potential energy of dipole U=−PEocosθ
∴U=−PEocos0o=−PEo
Thus the dipole is said to be in stable equilibrium.
Torque τ=PEosinθ
∴τ=PEosin0o=0
Case (b) : Angle between dipole and the electric field θ=180o
Potential energy of dipole U=−PEocos180o=PEo
Thus the dipole is said to be in unstable equilibrium.
Torque τ=PEosin180o=0