Dielectric Effect in Capacitors

## Question:

How can a dielectric with high permittivity in a capacitor store
more charges than one with low permittivity dielectrics? How can
this by explained physically?
In the graphs of C (capacitance) against A (area) & C
(capacitance) against 1/d (1/ distance between plates of
capacitor), the experimental straight line obtained is higher
than the theoretical one & it does not pass through the
origin. Should the theoretical line have the same slope as the
expt one? Is there any prove?

## Answer:

Dielectrics are made up of insulating material in which the
charges do not move freely as in a conductor. However, the
charges consisting of the negative electrons and positive nuclei
of the atoms may be somewhat displaced by the application of an
electric field. This slight redistribution of charge will induce
a field which opposes the applied field. The net field between
the plates of a capacitor with a dielectric between the plates is
Et=Ea-Ei, where Et is the total field, Ea is the applied field
resulting from the charges on the plates and Ei is the induced
field due to charge redistribution in the dielectric.
If the dielectric is introduced while the voltage across the
capacitor is held constant, by a battery for example, the amount
of charge stored on the capacitor will increase until the Et
increases to the value that Ea had in the absence of the
dielectric. In other words, the reduction in field due to Ei
causes the voltage across the capacitor to momentarily decrease
until current flowing from the battery restores the original
voltage. This current piles up on the plates as additional
charge.

It is not clear to me from your question exactly what
experimental data was plotted. In general when you find a
difference between theoretical and experimental results you
should examine carefully any assumptions implicit in the
theoretical model. For example do you neglect the finite size of
your capacitor so that the effects occurring at the boundaries of
your real apparatus are not duplicated in the theory. Or perhaps
do you use the free space value of permittivity when your
apparatus has air between the plates. Or maybe your dielectric
material has some minute leakage current through it. Assuming
that your measurements are carefully made, the problem is always
in the theory, usually in some sort of over simplification.

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