18.2: Electric Dipole
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A dielectric supports charge by acquiring a polarisation in an electric field, whereby one surface develops a net positive charge while the opposite surface develops a net negative charge. This is made possible by the presence of electric dipoles – two opposite charges separated by a certain distance – on a microscopic scale.
A mathematical treatment of dipole moment can be found in the TLP on ferroelectrics. For the purposes of this TLP it is worth noting that a dipole can be considered in two ways:
1. If two discrete charged particles of opposite charges are separated by a certain distance, a dipole moment μ arises.
2. If the centre of positive charge within a given region and the centre of negative charge within the same region are not in the same position, a dipole moment μ arises. For example, in the diagram below the centre of positive charge from the 8 cations shown is at X, while the centre of negative charge is located some distance away on the anion.
The second view of dipole moment is more useful, since it can be applied over a large area containing many charges in order to find the net dipole moment of the material, and can also be used in situations where it is inappropriate to consider the charges as belonging to discrete particles – e.g. in the case of the electron cloud that surrounds the nucleus in an atom, which must be described by a wavefunction.
Note that in the equation for dipole moment, r is a vector (the sign convention is that r points from negative to positive charge) therefore the dipole moment μ is also a vector. The polarisation of a material is simply the total dipole moment for a unit volume.
where V is the overall volume of the sample.
Since Σμ is a vector sum, a material may contain dipoles without having any net polarisation, since dipole moments can cancel out.