# Homework Help: Find postition on third charge question

1. Jan 17, 2008

### flaming

Find postition of third charge question

1. The problem statement, all variables and given/known data
A charge of 1.57E-9 C is placed at the origin, and a charge of 4.23E-9 C is placed at x = 1.51 m. Find the position at which a third charge of 3.11E-9 C can be placed so that the net electrostatic force on it is zero.

2. Relevant equations

3. The attempt at a solution

Last edited: Jan 17, 2008
2. Jan 17, 2008

### Tom Mattson

Staff Emeritus
First question: Given that the charges are all positive, and that positive charges repel, do you think the 3rd charge should be placed to the left of both charges, to the right of both charges, or inbetween the two charges? Why?

3. Jan 17, 2008

### flaming

i guess it would be at the left of the two charges to counter them..i guess

4. Jan 17, 2008

### Tom Mattson

Staff Emeritus
Don't guess. Think.

If the 3rd charge were to the left of the other two, and the other two were both repelling it, then is it even possible that the net force on the 3rd charge is zero? Think about the directions of the forces on the 3rd charge.