Goal Apply Coulomb's law in one dimension. Problem Three charges lie along the x-axis as in Figure 15.7. The positive charge 91 = 17 C is at x = 4 m, and the positive charge 92 = 6 C is at the origin. Where must a negative charge 93 be placed on the x-axis so that the resultant electric force on it is zero? -2.0 m- -2.0 m-x- 92 F23 93 F13 91 Figure 15.7 Three point charges are placed along the x-axis. The charge q3 is negative, whereas 91 and 92 are positive. If the resultant force on 93 is zero, then the force F13 exerted by q on 93 must be equal in magnitude and opposite the force F23 exerted by 92 on 93.