Gaussian surface

A Gaussian surface is a closed surface used with the integral form of Gauss's law to relate electric flux to enclosed electric charge. It need not coincide with the surface of any charged body. Choosing it to match a symmetry can make the normal component of the electric field constant on a portion of it, allowing that component to be taken outside a surface integral. A concentric sphere works for an isolated spherically symmetric charge distribution; an arbitrarily placed sphere generally does not.

Zero enclosed electric charge implies zero total electric flux. Inferring zero electric field additionally needs information such as the spherical symmetry used in the electric field of a uniformly charged spherical shell. The imagined boundary does not itself block or redirect the electric field.