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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.
COLDHOT
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.