Electric flux is not affected by charges that are not within the closed surface. But the net electric field, E, in the Gauss' Law equation, can be affected by charges that lie outside the closed surface. Gauss' Law is true in all situations, but people can only use it to calculate when high degrees of symmetry exist in the electric field. Examples include spherical and cylindrical symmetry. Otherwise, the calculations are too hard to do by hand and must be worked out using a computer.