# Columb's law

1. Jan 8, 2008

### rcmango

1. The problem statement, all variables and given/known data

In a vacuum, two particles have charges of q1 and q2, where q1 = +3.5 µC. They are separated by a distance of 0.25 m, and particle 1 experiences an attractive force of 2.3 N. What is q2 (magnitude and sign)?

2. Relevant equations

3. The attempt at a solution

not sure why this wouldn't work 2.3 N = (8.99 x 10^9 * Q2)/ .25^2

should that work? and would the sign be + or -?

thanks.

2. Jan 8, 2008

### olgranpappy

you want the force to be attractive, as stated in the problem. do like charges attract or repel?

3. Jan 8, 2008

### rcmango

like charges repel. however, is my formula equation correct?

4. Jan 8, 2008

### Mindscrape

You are missing a charge, remember

$$F = k \frac{qQ}{r^2}$$

5. Jan 8, 2008

### rcmango

Okay, I see, so then I solve for Q2, correct?

which shows now 2.3 N = ((8.99 x 10^9) * (2.3) * (Q2) )/ .25^2

i get something like: -4.57 x 10^-12 C

6. Jan 8, 2008

### Mindscrape

You should solve for what you want algebraically before you put in numbers. It is a lot easier that way. So you are right, you want the value of the other charge.

$$q_2 = \frac{r^2 F}{kq_1}$$

in terms of the numbers that would be... (you do it)

*note: you made a mistake for the value of the first charge. Also note that I just gave you the magnitude of the charge, you will have to figure out the sign.