As capacitance is the ability to store charge therefore we say conductors can show self-capacitance but why can't insulators? we can store charge on insulators as well so why can't they show capacitance?
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2 Answers
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The self-capacitance of a conductor is defined by the ratio of charge and electric potential. The operation of a capacitor relies on the fact that we can change charge by applying voltage. Otherwise, how would you charge your capacitor? In dielectric, the total charge can not be changed by applying voltage.
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Of course insulators can store charge as is shown in this video with charge being stored on the glass beaker.
The problem with defining the capacitance of an insulator, $Q/V$, is that the potential at each position on an insulator will be different and so a unique value for the capacitance of an insulator cannot be defined/determined.
