You have already seen that if an electric field is applied to a polar material, the dipoles will rotate to align with the electric field. However, this picture is in fact an oversimplification. To see why, we must first consider how dipoles behave in the absence of an electric field.

The molecules in a structure always possess some energy and this causes random motion. For a system at equilibrium, there is as much random motion in any one direction as in the opposite direction, therefore the average positions of the molecules remain constant.

The electrostatic forces created by the field influence the molecules to rotate and align with the field, as we saw before.

However, the molecules are also still undergoing thermal motion. This means that at any given instant, not all the molecules are perfectly aligned with the field. It is only the average orientation of the molecules, viewed over a long period of time, that displays this alignment.

Try adjusting the temperature and observe the effect on the rotation of the molecules. Thus deduce how temperature affects the dielectric constant.

COLD HOT

As the temperature is increased, the dielectric constant will.

         That's correct!

As the temperature increases, the molecules have more thermal energy and therefore the amplitude of random thermal motion is greater. This means that the range of deviation from a perfect alignment with the field is greater, therefore the molecules are less closely aligned with each other, therefore the orientational polarisation of the material - and hence the dielectric constant - is less.

Unfortunately that's not right. Take another look at how increasing the temperature affects the motion of the molecules, and remember that the dielectric constant expresses the ability of the material to polarise.

No - we have already stated that the dielectric constant will vary with temperature. Look more closely at what happens to the molecules and see if you can work out how.