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Homework Help: Net flux through a cylinder from a point charge

  1. Apr 8, 2015 #1
    1. The problem statement, all variables and given/known data
    My book demonstrates how a uniform electric field through a box generates a net flux of zero. I was wondering if the same would happen from a point charge outside of the cylinder on one end instead of a uniform electric field.

    2. Relevant equations
    Flux = E⋅A

    3. The attempt at a solution
    My book says that charges outside of the surface do not give a net electric flux through the surface. So does that mean if a positive point charge is at one end of the cylinder, all the flux cancels out?

    The flux through the nearest face would be negative because E and A are in opposite directions and the flux on the other end would be positive BUT smaller in magnitude because E is smaller (because the distance would increase) while A stays the same. And the flux around the surface would be positive.
    So is:
    Φnearest face = Φfurther face + Φaround the cylinder ?
  2. jcsd
  3. Apr 8, 2015 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Yes, you have the right idea. A consequence of Gauss' law is that the net flux through any closed surface is proportional to the charge enclosed. A charge outside the closed surface cannot create a net flux through the surface.
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