Homework Help: 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.

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