A surface is so constructed that, at all points on the surface, the vector points inward. Therefore, it can be said that
A) the surface encloses a net positive charge.
B) the surface encloses a net negative charge.
C) the surface encloses no net charge.
D) the surface vector $\Delta$S at all points on the surface is necessarily parallel to the electric field vector .
E) the surface vector $\Delta$S at all points on the surface is necessarily perpendicular to the electric field vector .

Question

A surface is so constructed that, at all points on the surface, the  vector points inward. Therefore, it can be said that
A) the surface encloses a net positive charge. 
B) the surface encloses a net negative charge. 
C) the surface encloses no net charge. 
D) the surface vector $\Delta$S at all points on the surface is necessarily parallel to the electric field vector . 
E) the surface vector $\Delta$S at all points on the surface is necessarily perpendicular to the electric field vector .

Quizplus · Accepted Answer

The fact that the surface normal vector points inward implies that the enclosed charge is negative. This is because the electric field lines start from positive charges and terminate at negative charges. Since the surface normal vector points inward, the surface is enclosing the termination points of electric field lines, which are negative charges. Therefore, the answer is option B.

A Surface Is So Constructed That, at All Points on the Surface