Flux through a cube corner

A point charge of charge q = 96 ϵ 0 q = 96\epsilon_0 is placed at one corner of a cube as shown. What is the electric flux through the shaded face?


The answer is 4.

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3 solutions

Imagine 8 cubes around a point charge. If you focus only 1 cube, the flux in that area would be 1 8 \frac{1}{8} of total flux. Since the cube has only 3 sides that electric fields cross, then the flux of 1 side would be 1 24 \frac{1}{24} 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 ( 1 24 ) ( q ϵ 0 ) = ( 1 24 ) ( 96 ϵ 0 ϵ 0 ) = 4 (\frac{1}{24})(\frac{q}{\epsilon_0}) = (\frac{1}{24})(\frac{96\epsilon_0}{\epsilon_0}) = 4 .

Lawrence Chiou
Mar 3, 2016

By Gauss' law , a surface containing the entire charge has total electric flux q / ϵ 0 = 96 q/ \epsilon_0 = 96 . 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 96 / 24 = 4 96 / 24 = 4 .

The symmetry argument would be much more obvious if you considered a 2 × 2 × 2 2 \times 2 \times 2 cube centered at the origin.

Calvin Lin Staff - 5 years, 3 months ago
Jacopo Vi
Apr 12, 2016

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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