ELECTRIC POTENTIAL
Consider a point charge q 1 at a distance r 1 from a fixed charge q 2 moving by some
means to a position r 2.
How much work did the electric force perform during this displacement? The
answer is given by the equation.
Therefore, since ∆UE = −WE, we get U 2 − U 1 =.
Let’s choose our U=U reference at infinity. Then this equation becomes
.
Notice that if q 1 and q 2 have the same sign, UE is positive. This means it took
positive work by an external force to bring these charges together (since they repel
each other). If q 1 and q 2 have opposite signs, UE is negative. This means the
electric force does positive work to bring these changes together (since they
attract).