For physicists, regarding electricity.

[Firetaffer]

Hey, I have a test tomorrow and just need an answer to a simple question.

There have been several exams in the mock-papers I've done where a piece of cloth is rubbed onto a metal/plastic tube. The piece of cloth always ends up negative, is there a reason for this? Does cloth always end up getting negative charges? Or is it just pure luck, thanks in advance!

 

[GBlair88]

Anything to do with the fact cloth is an insulator and therefore cannot hold a charge?

 

[Spadge]

As far as I know, it is dependant on the material - I doubt 100% of all weavable substances would become, through friction, negativally charged.

http://en.wikipedia.org/wiki/Triboelectric_effect

"Nylon tend to give up electrons and become positively charged" - http://www.electrostatics.com/page2.html

EDIT: The best you can do, for an exam like this, is look at the materials used. They will either hint that the material of the bar always gains a certain charge, or the material of the cloth always gains a certain charge.

 

[spartan231490]

Anything to do with the fact cloth is an insulator and therefore cannot hold a charge?

Actually, if i remember, it's the opposite. cloth is an insulator which means that it's electrons are hard to move, metal is a conductor, and the electrons move freely, onto the cloth. cloth becomes negative, metal becomes posative. At least, that's the way I remember it from physics class, but it's been years. Try google.

 

[Firetaffer]

As far as I know, it is dependant on the material - I doubt 100% of all weavable substances would become, through friction, negativally charged.

http://en.wikipedia.org/wiki/Triboelectric_effect

"Nylon tend to give up electrons and become positively charged" - http://www.electrostatics.com/page2.html

EDIT: The best you can do, for an exam like this, is look at the materials used. They will either hint that the material of the bar always gains a certain charge, or the material of the cloth always gains a certain charge.

Thanks for that! I didn't know about the Triboelectric effect, I've always wondered why cloth seems to get negatively charged, but for some reason haven't asked... thanks!