# Coulombs Law Problem - Find the Force

1. Jul 6, 2009

### KatieD

Coulombs Law Problem -- Find the Force

1. The problem statement, all variables and given/known data
Two small objects, A and B, are fixed in place and separated by 3 cm in vacuum. Object A has a charge of 2 uC and object B has a change of -2 uC. How many electrons must be removed from A and put onto B to make the electrostatic force that acts on each object an attractive force whose magnitude is 68 N?

We know an electrons charge is 1.60*10^-19 C

2. Relevant equations
F=kq1q2/r^2 (coulombs law)

3. The attempt at a solution

I tried to solve for q (the charge). Since they are both the same I took the square root. Then I divided by the charge of e-. I am out of ideas :(

I can do simple coulombs law problems in this chapter, but the harder ones like this stump me. :((

Thanks,
Katie

2. Jul 6, 2009

### queenofbabes

Re: Coulombs Law Problem -- Find the Force

So after doing this, what is the quantity that you have arrived at? What is q/e?

3. Jul 6, 2009

### KatieD

Re: Coulombs Law Problem -- Find the Force

F = K*Q^2/r^2
68 = (8.99 * 10^9 N * m^2/C^2)(q^2)/(3*10^-2)^2
q=2.6*10^-6 C
e=1.6*10^-19
(2.6*10^-6C)/(1.6*10^-19)=1.63*10^13

the answer in the back is 3.8*10^12

4. Jul 6, 2009

### queenofbabes

Re: Coulombs Law Problem -- Find the Force

I'm not asking for the number.

I'm saying: if q is the charge, and e is the charge per electron, what does the quantity q/e mean? How does that relate back to what the question wants?

5. Jul 6, 2009

### KatieD

Re: Coulombs Law Problem -- Find the Force

q/e is going to be the number of e. But I guess that would be the number of e- for that charge. So they ask for the e- to change the charge. Would I then find the difference by subtracting from 2 and then dividing by e-?

6. Jul 6, 2009

### queenofbabes

Re: Coulombs Law Problem -- Find the Force

The question wants x electrons per object.
You just calculated the no. of electrons you need
From the question you can find out how many electrons you had at the start.

Hopefully its obvious enough now?

7. Jul 6, 2009

### KatieD

Re: Coulombs Law Problem -- Find the Force

okay got ya, I just need to take it into consideration with the initial formula
instead of solving for just q I am solving for q+ex

8. Jul 6, 2009

### KatieD

Re: Coulombs Law Problem -- Find the Force

Ok, I can't seem to get the right answer and I don't know where I'm going wrong.

I set the equation up as

F = k(q1 + ex)(q2 - ex) / r^2

where q1 is the positive charge and q2 is the negative charge.

So this works out to

68 = (8.99*10^9)(2*10^-6 + x*1.6*10^-19)(-2*10^-6 - x*1.6*10^-19) / (3*10^-2)^2

but this doesn't lead to the right answer. Where is my set up going wrong?

9. Jul 6, 2009

### queenofbabes

Re: Coulombs Law Problem -- Find the Force

It does, I've checked. Calculator error somewhere?