Coulomb’s law allows us to find the force between two point charges. Imagine two
point charges are held fixed in place on the xaxis. One has positive charge +Q and is
fixed to the origin. The other has negative charge –Q and is fixed at x=6. Another
positive point charge +q is released from rest halfway between them (at x=3). Consider
the following student comment about this situation: “There will be zero net force on the
charge in the middle. Using Coulomb’s law, the force due to the +Q charge is positive,
and the force due to the –Q charge is negative. The forces cancel.”
(a) Do you agree with the statement?
(b) How does Coulomb’s law apply to situations in which there are more than two
point charges?
1 Answer

a) No. They don’t cancel, they add. Both forces are in the +ve x direction.
b) The forces obey the principle of superpositon. The total force due to any number of charges is calculated by calculating the force for each charge individually, as if it were the only one, and adding them all together (vector sum).