Net electric flux

Use Gauss's law,

1/ Since the charge is placed at the centre of one of the faces of the cube, there is no closed surface.

2/ To enclose the charge in a closed surface, assume that the another cube of same dimension has been stuck to the face, which carries the charge at it's centre.

3/ Now the first cube contributes 5 equal areas to the enclosing surface and second cube also contributes 5 equal areas. One equal area is the area of a face of the cube.

4/ The net electric flux is therefore q/ε₀, but it's through 10 equal cube face area.

5/ Hence the electric flux through one face is q/(10ε₀)

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