HELP! columbs law/charges 2 charges on a line, looking for

1. Jan 12, 2010

1. The problem statement, all variables and given/known data
Two charges are placed on the x axis. One of the charges (q1 = +7.7 µC) is at x1 = +2.9 cm and the other (q2 = -22 µC) is at x2 = +9.2 cm.
(a) Find the net electric field (magnitude and direction) at x = 0 cm. (Use the sign of your answer to indicate the direction along the x-axis.)
(b) Find the net electric field (magnitude and direction) at x = +5.9 cm. (Use the sign of your answer to indicate the direction along the x-axis.)

2. Relevant equations

Columbs law, F=K*(|q1|*|q2|)/r2

3. The attempt at a solution

I'm fairly lost as to how to start this. I know that to figure out the force of the two charges acting on one another you would use the equation above with r being the distance between the two charges. What I don't know is how to figure out the charge on a spot where there is no charge previously. I think that it would be only the positive charge acting on the spot since (field wise) it is the only one that actually goes anywhere near the point. Or am I totally off? Help?

2. Jan 12, 2010

rl.bhat

The formula for electric field is
E = k*q/r^2.
The direction of field depends on the sign of the charge. It is away from the positive charge and towards the negative charge.