When an electric dipole of dipole moment p is placed in a uniform external electric field E , it experiences a torque τ and is given by τ=P×E
In magnitude, τ = PEsinθ, where θ is the angle between P and E
when P is perpendicular to E i.e. 90∘ then τ=PEsin90∘=pE =maximum, or τmax=PE