2 Description of field lines

2.4 Arbitrary systems of charges -- conclusions

2.4.1 Four equal charges as a square

• there are five equilibrium points (the green circles); only the one in the center of the square is stable. A test charge placed near this point would invariably end up in the equilibrium position, which is a local minimum for the potential. The other four points are local maxima of the potentials.
• this example shows that there doesn't have to be a charge inside every equipotential (look in the center)