Question

why insulator cannot conduct electricity?

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Answer 1

free electrons are necessary for the conduction of electricity .

Insulators do have have free electrons therefore they do not conduct electricity

Answer 2

Insulators are materials that have just the opposite effect on the flow of electrons. They do not let electrons flow very easily from one atom to another. Insulators are materials whose atoms have tightly bound electrons. These electrons are not free to roam around and be shared by neighboring atoms.