For a thin shell, you can use two of these, a closed spherical one inside and a spherical one outside of the shell.
For a thin shell, you can use two of these, a closed spherical one inside and a spherical one outside of the shell. For an infinite sheet, a closed cylinder perpendicular to the sheet is convenient. Choosing them is arbitrary, but ought to be selected with consideration given to symmetry so that at least part of it has a constant electric field that can be factored out of an equation. FTP, name these imaginary closed surfaces named after one of Maxwell's equations dealing with a system's total enclosed electric flux.
For a thin shell, you can use two of these, a closed spherical one inside and a spherical one outside of the shell.
{Gaussian} surfaces