(1) A pair of opposite charges separated by a small distance is called dipole.
(2) Consider the charge of dipole are –q and +q coulomb and the distance between them is 2a.
(3) Then the electric dipole moment P is given by P = q × 2a = 2aq. It is a vector. It's direction is from –q to +q along the axis of dipole.
(4) It is placed in a uniform electric field E, making an angle θ with field direction as shown in fig.
(5) Due to electric field force on +q is F = +qE and force on –q is F = –qE.
(6) These two equal and opposite charges constitute torque or moment of couple.
i.e., torque, τ = ⊥r distance × magnitude of one of force
∴ τ = (2asinθ)qE = 2aqEsinθ = PEsinθ
In vector form, τ = P x E
