Dielectric Effect in Capacitors


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?


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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