Question

A glass rod was rubbed with silk and was found to have +5micro coulomb of charge. How many electrons got transferred and from which material?

Answer 1

When a glass rod is rubbed with silk, electrons move from glass to the silk leading to the glass getting positively charged. So there is a very slight loss in its mass. New mass = Original mass - Mass of lost electrons. But we need not consider this loss as it is very small.

eg : Assume that the glass rod is found to have a charge of +3 x 10-7 C after rubbing.

We have q = ne . So n (no of electrons lost ) = q/e.

So mass of electrons lost = q me/ e = 3 x 10-7 Cx 9.1 x 10-31 kg / 1.6 x 10-19 C

= 2 x 10-18 kg (approximately). See how small the number is!!

Hope that helps..