By symmetry,
the electric field will not depend on y and z coordinates and its direction
at every point must be parallel to the x-direction We can take the Gaussian surface to be a
rectangular parallelepiped of cross-sectional area
A, as shown (A cylindrical surface will also do ) As
seen from the figure, only the two faces 1 and 2 will
contribute to the flux; electric field lines are parallel
to the other faces and they, therefore, do not
contribute to the total flux