- #1

fog37

- 1,568

- 108

I have been reflecting over this for the past few days. We can charge two insulators by rubbing them against each other. The two materials end up having an equal amount of opposite charge. For example, a glass rod rubbed with silk will become positively charged and the silk negatively charged.

Small bits of paper can be attracted and moved around when the positively charged glass rod is brought close (without touching) to the paper bits. The neutral paper bits gets polarized (separation between the centers of positive and negative charged) and Coulomb's attraction take place: the induced negative charge on the paper bit faces the positively charged rod. But we can also touch the paper bits with the rod and the bits will stick to the rod. Wouldn't the positive charge on the rod neutralize the negative induced charge on the paper bit leaving the paper bit with a positive charge? If that was true, the paper bit would immediately get repelled by the positive charged rod. But that does not happen. Why? Is it because, both materials are insulators and even when the rod touches the neutral paper the positive charge on the rod will remain on the rod and the induced negative charge on the paper bit will remain on the paper?

However, that seems to go against the fact that simple contact between two insulator can allow charge to transfer between them (rubbing simply increases the amount of contact). So the positively charged rod should neutralize the negative induced charge on the paper bit...

Thanks,

Fog37