A dipole is a pair of electric charges or magnetic poles of equal magnitude but opposite polarity, separated by some distance. Dipoles can be characterized by their dipole moment, a vector quantity with a magnitude equal to the product of the charge or magnetic strength of one of the poles and the distance separating the two poles. The direction of the dipole moment corresponds to the direction from the negative to the positive charge or from the south to the north pole.
When placed in an electric (E) or magnetic (B) field, equal but opposite forces arise on each side of the dipole creating a torque τ:
- τ = p X E (Electric dipole moment p)
- τ = μ X B (Magnetic dipole moment μ)
- (note: X corresponds to a vector cross product)
which will tend to align the dipole with the field.
Strictly speaking a dipole contains only two point charges (or magnetic poles), however various arrangements of multiple charges have dipole moments and may be treated as an effective dipole. In these cases, the positive end of the dipole corresponds to the effective center of positive charge. Likewise, the negative end corresponds to the effective center of negative charge.
Many molecules have such dipole moments due to non-uniform distributions of positive and negative charges on the various atoms. For example:
(+ve) H-Cl (-ve)
A molecule with a permanent dipole moment is said to be polarised.
With respect to molecules there are three types of dipoles:
- Permanent dipoles: These occur when 2 atoms in a molecule have substantially different electronegativity - one atom attracts electrons more than another becoming more negative, while the other atom becomes more positive. See dipole-dipole attractions.
- Instantaneous dipoles: These occur due to chance when electrons happen to be more concentrated in one place than another in a molecule, creating a temporary dipole. See Instantaneous dipole attraction.
- Induced-dipoles These occur when one molecule with a permanent dipole repels another molecule's electrons, "inducing" a dipole moment in that molecule. See induced-dipole attraction.