# Static Electricity: Charging by Friction: Understanding MIT demo

1. Feb 5, 2014

### poonintoon

MIT has a Physics Demo called "Rubber and Glass Rods with Tinsel and Balloon"
(Not sure if I can post links but easily found on youtube). I have watched the video several times but am a bit puzzled.

First the lab technician charges the rod by friction but then when he moves it close to the tinsel, the tinsel is repelled. Therefore the tinsel must already have the same charge as the rod. How did it get charged? The rod and tinsel never touch.

Also when he touches the balloon is this just charging by conduction but with an insulator/conductor instead of the more usual conductor/conductor pair?

Thank you for any help

2. Feb 5, 2014

### Staff: Mentor

See http://en.wikipedia.org/wiki/Electrostatic_induction, in particular this diagram:

The induced negative charge on the electroscope is closer to the rod than the induced positive charge, therefore it is attracted more strongly than the positive charged is repelled. There is a net attractive force.

You also get this effect with non-conducting objects in which the molecules act as electric dipoles. See the section Induction in dielectric objects at the end of the Wikipedia article.