A point charge of charge q = 9 6 ϵ 0 is placed at one corner of a cube as shown. What is the electric flux through the shaded face?
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Imagine 8 cubes around a point charge. If you focus only 1 cube, the flux in that area would be
8
1
of total flux. Since the cube has only 3 sides that electric fields cross, then the flux of 1 side would be
2
4
1
of total flux. (The other 3 sides of the cube are parallel to the field, so there is no component that is perpendicular to those sides left.)
The answer would be
(
2
4
1
)
(
ϵ
0
q
)
=
(
2
4
1
)
(
ϵ
0
9
6
ϵ
0
)
=
4
.
By Gauss' law , a surface containing the entire charge has total electric flux q / ϵ 0 = 9 6 . However, the cube as shown only contains one-eighth of the total flux. Furthermore, each of the three sides that contain non-zero flux has exactly one-third of the total flux of the cube. Therefore, the flux that goes through just one of the three sides is 9 6 / 2 4 = 4 .
You can consider a cube C centered at the origin and with length-side two. The flux is the equal to 96/6 over each of the six faces of such a cube. The area considered is one fourth of of the area of the face C and by simmetry we obtain flux=96/6/4=4
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