fix charges

belleamie

assume the charges 1-4 are fized ( q1=+3uC, q2=+4uC, q3=-3, q4=-4)
Find the net force felt by charge 5 (q5=-2uC) both magnitude and direction. I'm not sure wut equation to use?

Chen

I assume the charges are fixed in a square structure, and the fifth one is in the center of the square?

It doesn't really matter, what you need to do is find the force that each of the four charges exerts on the center charge. This force is defined as:
[tex]F_E = K\frac{q_1q_2}{r^2}[/tex]
To find the net force, simply add the four vectors you got. You will notice that you have two pairs of forces that work along the same action line, add them first since it's easiest. Then you will be left with two force vectors, perpendicular to each other. Use pythagoras to find the magnitude of the net force and the trigonometric function tan to find its direction.