Worked Out Example

A neutral water molecule (H₂O) in its vapour state has an electric dipole moment of magnitude 6.2 × 10^–30 C.m. How far apart are the molecule's centers of positive and negative charge? If the molecule is placed in an electric field of 1.5 × 10⁴ N/C, what maximum torque can the field exert on it ?

sol: A molecule's dipole moment depends on the magnitude q of the molecule's positive or negative charge and the charge separation d.

There are 10 electrons and 10 protons in a neutral water molecule, so the magnitude of its dipole moment is p = qd =(10e)(d) in which d is the separation we are seeking and e is the elementary charge.

The key idea here is that the torque on a dipole is maximum when the angle θ between and is 90°.

τ = pE sinθ
τ = (6.2 × 10^{– 30} C.m)(1.5 × 10⁴ N/C) (sin 90°)
τ = 9.3 × 10^{– 26} N.m