In a region of space the electric field is in the x-direction and proportional to x, i.e., E→=E0xi^. Consider an imaginary cubical volume of edge a with its edges parallel to the axes of coordinates. The charge inside this volume will be

Field at face ABCD = E0x0i^Flux over the face ABCD =-E0x0a2The negative sign arises as the field is directed into the cube.Field at face EFGH =E0x0+ai^Flux over the face EFGH =E0x0+aa2Flux over the other four faces is zero as the field is parallel to the surfaces.Total flux over the cube =E0a3=1ε0qWhere q is the total charge under the cube∴q=ε0E0a3