Dipole moment

The notion of dipole moment in physics and chemistry is based on the existence of electrostatic dipoles. This is an unequal distribution of electrical charges where the barycentre of the positive charges does not coincide with that of the negative charges. The simplest dipole is therefore a pair of two charges of opposite sign separated by a finite distance. A dipole moment is a vector generally noted P or D.

For two identical charges of opposite sign +q and -q, separated by a distance d, the dipole moment is described by the vector P=qd. The vector d is oriented from the negative charge to the positive charge. More generally, a dipole moment is the sum of n vectors P_i=q_ir_i where q_i and r_i are respectively the values of n charges q_i having n position vectors r_i.

Unit of dipole moment

The dipole moment is expressed in coulomb·metres (C·m) but it is often also expressed in debyes (D). The conversion factor is : 1 D = 3.33564×10^-30 C·m.

Application of the dipole moment

Usually used for atoms and molecules, the notion of dipole moment is used to describe the electric fields of such systems as well as Van der Waals forces. It is also found in particle physics in the measurement of possible dipole moments of electrons and neutrons for example.