# Homework Help: Finding electric force between two particles

1. Sep 11, 2010

### theskyisgreen

1. The problem statement, all variables and given/known data
Two particles with electric charges Q and −3Q are separated by a distance of 2.5 m.
If Q = 4.9 C, what is the electric force between the two particles?

2. Relevant equations

Coloumb's equation: F=k*q1*q2/r^2
k=9.0*10^9

3. The attempt at a solution
I just tried to plug in the variables into the above equation.
so I got F= (9e9)*(4.9)*(3*4.9)/((2.5/2)^2)
F= 4.15e11
Apparently, that is not the correct answer, but I don't know what I did wrong!
I also tried to divide that number by 2 to get F= 2.07e11 which was also not right.

What am I doing wrong?

2. Sep 11, 2010

### kuruman

If the separation between the charges is r = 2.5 m, then r^2 = 2.5^2, not (2.5/2)^2. I am not sure what all those divisions by 2 are meant to accomplish.

3. Sep 11, 2010

### lewando

Why are you dividing the distance by 2? Consider your signs, too.

[edit: beaten by kuruman. not happy]