I have two questions about insulators:

  1. Will the net charge on an insulator always be negative if it's rubbed?
  2. How can a negatively charged plastic rod transfer negative charges when it comes in contact with a metal sphere if plastic is an insulator and the electric charges are held in place?
  • $\begingroup$ i guess for second one may be there is potential difference btw rod and sphere? $\endgroup$ – user232991 Jun 2 '19 at 16:20

It is possible to generate both positive and negative static electric charges by rubbing (or even just contact). This is known as the Triboelectric effect and the list of materials involved is the Triboelectric series. Plastics tend to become negatively charged when rubbed and glass becomes positively charged.

Once the two materials are separated the remaining charge is not that strongly held, but since air is a poor conductor, this prevents the charge from quickly leaking away (although it will over time).

In the case of a plastic rod, excess electrons are on the surface and so not insulated by the bulk of the rod. If touched against a conductor the charge will escape (as the conductor acts like a sink), but you may have to run the conductor across the whole surface of of the rod to remove all of the charge as the insulation will prevent the charge from travelling from one part of the rod to another.

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