# Homework Help: Excess electrons and net charge

1. Jan 14, 2004

### eku_girl83

In my college physics class, we are working on electrostatics. Our homework problems relate to Coulomb's law, but I don't know how to manipulate it to solve them. Here are some examples:

1)Excess electrons are placed on a small lead sphere of mass 20 g so that its net charge is -2.60x10^-9 C. Find the number of excess electrons on the sphere.
2) How many excess electrons are there per lead atom? The atomic number of lead is 82, and the atomic mass is 207 g/mol.

Any help/explanations would be greatly appreciated!

2. Jan 15, 2004

### himanshu121

Apply Q=ne where Q is total charge and e electronics charge whereas n is the no of electrons

3. Jan 15, 2004

### HallsofIvy

Just to elaborate slightly on himanshu121's response:

(a) Do you know the charge on a single electron? If you don't, then you should look it up. I'll bet it's in your textbook. Once you know that, how many electrons would it take to make -2.60x10^-9 C?

(b) Once you know the answer to (a), you can divide it by the number of lead atoms to find the number of electrons per atom. Of course, to do that you need to know how many atoms of lead there are in 20 g of lead. How many atoms are there in 1 gram?

4. Jan 15, 2004

### eku_girl83

Thanks

Thanks you guys! I now have my problems worked correctly :)