Question

How to find electric flux through a cube?

Answer 1

The flux through a simple homogeneous, non-absorptive (like vacuum) region is independent of the size and shape of the region. The amount of flux depends only of the amount of charge, Q that is contained in the region. Assuming the permittivity, e, is the same everywhere then the net flux is Q/e. If there is no charge in the region then the net flux is zero.