The Principle of Superposition for Electric Potential
The formula Φ = kQ/r tells us how to find the potential due to a single point source
charge, Q. To find the potential in an electric field that’s created by more than one
charge, we used the principle of superposition. In fact, applying this principle is
even easier here than for electric forces and fields because potential is scalar (we
will not be concerned with orientation, just the sign of the charge). When we add up
individual potentials, we’re simply adding numbers; we’re not adding vectors.
Let’s illustrate with an example. In the figure below, the source charges Q 1 = +10
nC and Q 2 = −5 nC are fixed in the position shown; the charges and the two points,
A and B, form the vertices of a rectangle. What is the potential at point A? At point
B?
The potential at Point A due to Q 1 alone (ignoring the presence of Q 2 ) is
ΦA1 = k Q 1 /rA1 = (9 × 10^9 )(+10 × 10−9)/(3 × 10−2) = 3000 VSince Point A is 5 cm from Q 2 (it is the hypotenuse of a 3-4-5 right triangle), the