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The dipole moment vector  making an angle ? with the field . Let -q and +q be the charges forming the dipole and 2l the distance between them. Due to the electric field , the charge +q experiences a force q  (in the direction of the field) and the charge -q experiences an equal and opposite force q  (opposite to the field). Since the two forces are equal and opposite, the net translatory force on the dipole in 'uniform' electric field is zero; therefore there will be to translatory motion of the dipole in a uniform electric field. 

However, the forces q , q act at different points and form a couple which tens to set the dipole parallel to the field . The moment of this restoring couple is known as the 'torque'  on the dipole. The magnitude of the torque (= force × perpendicular distance) is given by 

     = qE (2l sin?) = 2ql E sin ? 

or  = pE sin ? newton - metre, 

where p(= 2ql) is the magnitude of the dipole moment. 

In vector form: 

Thus, in a uniform electric field, a dipole feels a torque (but no net force). In the torque is perpendicular to the page, pointing downwards (right-hand screw rule)

If the dipole be placed perpendicular to the elelectric field (? = 90^o or sin ? = 1), then the torque acting on it will be maximum. If this be  then, 

or 

If E = 1 newton/coulomb, then   coulomb-metre. Hence, the moment of an electric dipole is the torque acting on the dipole placed perpendicular to the direction of a uniform electric field of intensity 1 newton/coulomb.