# Homework Help: How many electrons

1. Jan 21, 2010

### shimizua

1. The problem statement, all variables and given/known data
Part A (which I already got right) A person scuffing her feet on a wool rug on a dry day accumulates a net charge of -72.1 uC. How many excess electrons does this person get?
Part B (Which is what I need help on) Given the static charge buildup in the previous question, what is the magnitude of the force on an electron at a distance 6.03 m from the person?

2. Relevant equations
So i thought that what you would do is simply find how many coulombs are in an electron, which is 1.6E-19 C. Then convert the uC to C and you would get .000072 C or 7.2E-5 C. So i divided the 7.2E-5 by the 1.6E-19 and got 4.5E14 which was the correct answer. But the part I need help with is finding the magnitude of the force. I would think I could just use coulombs law but I am not really sure what my q1 and q2 would be. Would I use 7.2E-5 and the 4.5E14? or would i use something else?

3. The attempt at a solution
I did
F=((8.9E9)(4.5E14)(7.2E-5))/6.03^2 and got an answer of 7.9E18 N. which was wrong. not sure where i went wrong

2. Jan 21, 2010

### Senjai

Yes colombs law, its asking you what force a SINGLE electron would experience, 6.03m away from the person..

Try treating the person as a single charge. Find out what the charge of an electron is, and try colombs law.

3. Jan 21, 2010

### shimizua

Well the charge of a single electron is 1.6E-19 C. so would it be then
F=(8.9E9)(1.6E-19)/6.03^2? cause i tried that and it was still wrong. Would the equation just be then kQ/r^2 or would it be kQ^2/r^2
just not sure

4. Jan 21, 2010

### Senjai

Colombs law is $$F = \frac{kQ_1Q_2}{r^2}$$

It measures the force two charges exerts on eachother at a given distance r. So if one charge is the charge of an electron, the other charge is?

5. Jan 21, 2010

### shimizua

thank you so much. got it. so it was using the charge of the net charge from what i was given. the 7.2uC

6. Jan 21, 2010

### NewtonMDC

I'm stuck on this problem too. Did you end up using
(k)(charge of girl)(charge of electron)/r^2?

7. Jan 21, 2010

### shimizua

you do k(charge of how many she accumulated)(charge of electron)/r^2