Electric Forces - Force from Multiple Charges

Question 1

forces from multiple charges

Answer 1

• Electric charges create electric forces on each other. As you hopefully recall, forces like Fe are vectors: they have a direction as well as a magnitude. The magnitude of Fe is given by Coulomb’s Law; the direction is either toward the other charge (if they attract) or away from the other charge (if they repel).
• Coulomb’s Law is easy enough to figure out if there are two charges. But what if there are three, or more? In that case, each charge will feel a force from every other charge. You can find the amount of force between each pair of charges using Coulomb’s Law. The TOTAL force on a charge will be the vector sum of the forces from each of the other charges. To figure out the total net force on a charge, you find the force that each of the other charges makes separately on the charge; then you add those forces as vectors to get the total force.
• In this lesson, we’ll only consider problems where the charges are placed along a single line. That essentially makes the problem 1-Dimensional, so the math isn’t too hard. You just need to know if the forces on a given charge point in the + or – direction. The best way to do this is to DRAW A DIAGRAM. Draw the forces acting on the charge. If another charge is of the same sign, draw the force repelling; if yet another charge is of the opposite sign, draw the force attracting. Forces pointing right (or up) are positive; forces pointing left (or down) are negative. Add the positive forces, subtract the negative forces, and you’ll get the total force.
• REMEMBER: The magnitude of the electric force from Coulomb’s Law is a positive number. The direction, + or -, comes from the diagram.