Consider 4 charges all of magnitude q located at the following positions: r1 = (+1; +1; 0) ; r2 = (-1; +1; 0) ; r3 = (-1;-1; 0) ; r4 = (+1;-1; 0) We now want to study the stability of such an equilibrium conguration against small displacements. Show that near the origin the potential energy felt by the test particle may be written as V (x; y; z) = V0 + Ax^2 + By^2 + Cz^2 + ... Where a test charge e is at the origin. I'm a bit confused about how to arrive at this derivation. In general we have E=-[itex]\nabla[/itex]V, so if we have the electric field at the origin, we can get the potential. However it does not turn out to be the above. Any help on getting me started?