In a problem when you are placing a third charge so that E=0 at the origin, I know how you are suppose to sum the electric field of each point charge at the origin and set it to 0 in order to solve for the distance of the third charge. I have a slight confusion, however, on how to set the signs of each E with respect to the origin. To use a simple 1-D example to show my confusion: A point charge of +5 micro C is located on the x axis at x = -3 cm, and a second point charge of -8 micro C is located on the x axis at x = +4 cm. Where should a third charge of 6 micro C be placed so that the electric field at the origin is zero? The answer to the problem sets the direction of E on the -8 and +6 charges as -i, and +i on the +5 charge. I'm not sure why this is. Aren't we just pretending there's a positive test charge at the origin and looking at the direction each charge pushes it? If that is so, the 5 micro C chare would push it right so +i, the -8 micro C charge would pull it right so +i, and the +6 micro C charge, deduced to be places left of x = -3cm, would push it to the right so +i. Even if you reverse the direction sign for the negative charge it doesn't match. I'm just confused as to assigning these vector directions, not as to how to solve the rest of the problem.