In this equation, Q 1 is the charge of one of the point charges, and Q 2 is the charge on the other one. This
equation is known as Coulomb’s Law.
To get comfortable with these three equations, we’ll provide you with a rather comprehensive
problem.
Two point charges, labeled “A” and “B”, are located on the x -axis. “A” has a charge of —3 μC, and
“B” has a charge of +3 μC. Initially, there is no charge at point P , which is located on the y -axis as
shown in the diagram.(a) What is the electric field at point P due to charges “A” and “B”?
(b) If an electron were placed at point P , what would be the magnitude and direction of the force
exerted on the electron?
(c) What is the electric potential at point P due to charges “A” and “B”?Yikes! This is a monster problem. But if we take it one part at a time, you’ll see that it’s really not too
bad.
Part 1—Electric Field
Electric field is a vector quantity. So we’ll first find the electric field at point P due to charge “A,” then
we’ll find the electric field due to charge “B,” and then we’ll add these two vector quantities. One note
before we get started: to find r , the distance between points P and “A” or between P and “B,” we’ll have
to use the Pythagorean theorem. We won’t show you our work for that calculation, but you should if you
were solving this on the AP exam.