Polarisation

In electromagnetism, polarisation is a measure of the electric dipole moment per unit volume.

Within dielectrics in an external electric field $\vec{E}$, the field will induce a dipole moment that lines up the dipoles with the field direction, i.e., it polarises some of the material. The external field will produce an induced field (the polarisation field) that is weaker and opposite to $\vec{E}$.¹

This results in the total field decreased by a factor of ${ϵ}_r$, the relative dielectric permittivity.

The polarisation field $\vec{P}$ is proportional to $\vec{E}$:

$$\vec{P}={ϵ}_0{\chi }_e\vec{E}$$

where ${\chi }_e$ is the electric susceptibility.

See also

• Magnetisation, the magnetostatics analogue

Footnotes

1. Prof Sarris used this analogy. A dielectric is like a packed subway train, where people “end up out the windows”. The charges protrude out of the capacitor (i.e., the train is the dielectric between a capacitor’s plates). So the net charge reduces and thus the net field also reduces. ↩