In a region of space, the electric field is in the x-direction, proportional to x and given by E→=αxi^, where α is a positive constant. Consider an imaginary cubical volume of edge a, with its edges parallel to the axes of coordinates. The charge inside this cubical volume is

The field at the face ABCD =αx0i^So, flux over the face ABCD =−αx0a2 The negative sign arises as the field is directed into the cube The field at the face EFGH=αx0+ai^So, flux over the face EFGH =αx0+aa2The flux over the other four faces is zero as the field is parallel to the surfacesSo, total flux over the cube =αa3=qε0 here q is the total charge inside the cube⇒ q=ε0αa3