A dipole moment
may either be electric
The electric dipole moment is an expression of the charge distribution. It is a vector
quantity, calculated by
p = | r rho dv
where p is the dipole moment. It has the units of charge x distance.
r is a vector from the origin to the charge
dv denotes the integration of the charge distribution (rho) over space
For the simplest case, a separation of two point charges
+q and -q, separated by a distance s, the magnitude of the dipole moment is qs, and the direction is from the negative charge to the positive.
Some molecules may have permanent dipole moments, or can be induced to form a dipole in an electric field
. The propensity for an induced dipole to be formed is known as the polarizability
of a molecule (or an atom). A spherically symmetric atomic orbital
, like that of the hydrogen
atom, will have no net dipole moment (time averaged) because the direction of the proton
dipole is spherically averaged out. However, in a strong electric field, the electron density can be forced in the direction of the field, causing a temporary bias in the dipole distribution.
Permanent dipole moments occur in molecules that are asymmetric such as HCl
or carbon monoxide
. The difference in electron distribution across the two atoms creates a permanent charge separation, resulting in a dipole.
The magnetic dipole moment is a different quantity. It is classically characterized by the magnetic field
observed at a distance from a current loop
with current I and area a
m = Ia/c
In a molecule, it is related to the spin
, or angular momentum
of an electron. In a similar fashion to polarizability, diamagnetic
materials can exhibit a magnetic moment which is proportional to the magnetic field applied. The relationship between field strength and moment is known as the magnetic susceptibility
. Magnetic fields in iron
or other diamagnetic substances is caused by uniform orientation of magnetic dipole domains within the material.