As far as I know, Gaussian surface's are surfaces that enclose a net charge and are shaped in such a way that the electric field vector's magnitude is the same over the entirety of the surface. My textbook states that the introduction of an external charge Q will not affect the net flux of the surface but it may affect the distribution of the field lines. Will the inclusion of Q affect the magnitude of E at points on the surface due to superposition and, if so, won't that affect the electric field vector used in the integral to calculate the net flux of the surface? Also, if that's the case then will the Gaussian surface no longer be Gaussian or is the magnitude of E at the surface due exclusively to the enclosed net charge the only constant?
Thanks