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A point charge q is located at the centre of the open (uncovered) mouth of a cylindrical container. What is the net electric flux passing through the entire surface of the cylinder?

1. zero
2. q/(2*epsilon0)
3. q/epsilon0
4. q/(4*epsilon0)

Correct answer: q/(2*epsilon0)

Solution

The charge sits exactly in the plane of the open end of the cylinder. Imagine completing the surface with an identical cylinder on the other side to enclose q fully; that full closed surface would have flux q/epsilon0 by Gauss's law. By symmetry the actual cylinder captures exactly half of the total flux, namely q/(2*epsilon0).

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