Question

Hi, I don't understand the experiment (45th minutes, lecture 3) to charge the sphere. The plate is in contact with the glass or is there some insulator between the metallic plate and the glass.

If there is no insulator on the metal plate, plate would be + and so, if we touch, charges go away.
Thanks for answer.

Answer 1

There is no insulator on the metal plate. What happens is next:
We charge the glass by rubbing it, it gets charge, it doesn't matter which sign is the charge.
Then when we put the plate over the glass, the glass transfers only a really tiny amount of charge to the plate, this amount of charge is negligible, this is because the glass is a really good insulator.
The really important thing is that the electric field produced by the charge on the glass makes that the charges on the plate get redistributed and so one type of charge will be atracted by the glass and the other will be repeled (the repeled charge is of the same type of that in the glass), so when you touch it the excess charge that is repelled by the glass will go through you and the result is that you get an excess of charge in the plate, the plate is no longer neutral because you took away the excess of charge.
I hope this will be clear for you.

Answer 2

I thought it was the explanation but i wasn't sure that charges from the glass was'nt able to go on the plate. Yes very clear (but read following).
Nethertheless, when you rub the glass with cat skin, charges goes from one to the other so charges travel from one insulator to the other.
And when we approach the metal, we say they don't travel. It's the key point....no ?
Is that phenomenon possible only when we give energy to the glass by rubbing it that charges travel. Does that mean that if we put into contact glass and the rubber (after each has been charged), charge will not flow from one to the other ?
gratefully for answer

Answer 3

I got the answer to the first question... I think that we could see in an experiment the answer to the second... I did the second at the university, the only bad thing about my experiment is that I'm too bad manipaluting things and my glass rod didn't get any charge.
Well, the explanation of the phenomena in the first question is the follows: We already know that some materials have more or less tendency to get charge of one type or the other. When we put two materials in contact they interchange charge. When you put glass in contact with cat skin the surface of contact is small, what you do by rubbing the glass is that you change the contact points between the surfaces and you get more change transference getting an appreciable change of charge. So when you put the glass and the plate in contact there is no appreciable transference of charge between the surfaces and that's because (in a microscopic scale) the materials touch each other just in a few points as the same Walter Lewis said.

Answer 4

Thank you very much for your answers wich helped me a lot.