1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electric flux through a surface

  1. Sep 29, 2013 #1
    1. The problem statement, all variables and given/known data
    A point charge q is situated a distance d/2 above a square surface of side d as shown (see attached).
    a) It cannot be determined
    b) 2q/ε0d
    c) q/4[itex]\pi[/itex]ε0
    d) q/[itex]\pi[/itex]ε0d2
    e) q/6ε0

    2. Relevant equations


    3. The attempt at a solution
    Shouldn't the answer be zero because whatever enters the area also leave the area? If not I know I should use the integral of E*dA but I don't know what E would be.

    Attached Files:

    Last edited: Sep 29, 2013
  2. jcsd
  3. Sep 29, 2013 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    If the area were a thin volume you'd be correct, the net flux into the volume = 0, but flux thru a surface A = the integral over the surface of the dot-product of the electric field E and an element of surface area dA: flux = ∫∫ E*dA.

    So yes, you need to integrate E*dA over the surface. dA is normal to the surface at every point on the surface.
  4. Sep 29, 2013 #3
    You can manage without integration.
    Imagine the charge is in the middle of a cube with side d.
    Due to symmetry, the flux through each face of the cube is the same.
    And the total flux through the cubic, closed surface can be found from Gauss' law.
  5. Sep 29, 2013 #4

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Nasu has come up with the right and simple way to solve this.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted