In a cuboid of dimension 2L × 2L × L, a charge q is placed at the centre of the surface S having area 4L². The flux through the opposite surface to S is given by:
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In a cuboid of dimension 2L × 2L × L, a charge q is placed at the centre of the surface S having area 4L². The flux through the opposite surface to S is given by:

G1 Physics · Accepted Answer

Reflect an identical cuboid across the face containing the charge so that the charge becomes the centre of a 2L × 2L × 2L cube. By symmetry, the total flux q/ε₀ is equally shared by the six faces. Hence the flux through the opposite face is q/(6ε₀).