A square surface of side L metre is in the plane of the paper. A uniform electric field E⃗ (V/m), also in the plane of the paper, is limited only to the lower half of the square surface (see figure). The electric flux in SI units associated with the surface is

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(3) zero. The area vector of a surface points along its normal. The square lies in the plane of the paper, so its normal points straight out of the page. The field also lies in the plane of the paper, so it is at right angles to that normal: φ=E⃗·A⃗=EA cos 90°=0 That the field is confined to the lower half changes nothing. Over the upper half there is no field and hence no flux; over the lower half the field runs parallel to the surface, so no line crosses it there either. The total flux is zero.

A square surface of side L metre is in the plane of the paper. A uniform electric field E⃗ (V/m), also in the plane of the paper, is limited only to the lower half of the square surface (see figure). The electric flux in SI units associated with the surface is

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