An electric dipole is situated in an electric field of uniform intensity E whose dipole moment is p and moment of inertia is I. If the dipole is displaced slightly from the equilibrium position, then the angular frequency of its oscillations is 

(1) pEI1/2

(2) pEI3/2

(3) IpE1/2

(4) pIE1/2

(1) When dipole is given a small angular displacement θ about it's equilibrium position, the restoring torque will be

τ=pEsinθ=pEθ   (as sinθ = θ)

or Id2θdt2=pEθ (as τ=Iα=Id2θdt2)

or d2θdt2=ω2θ with ω2=pEIω=pEI

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