Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Electric Flux on one side

  1. Jun 13, 2008 #1
    1. The problem statement, all variables and given/known data

    A 1.3microC point charge is placed at the center of a cube with a volume of 9.8m^3. Calculate the electric flux through one side of the cube.

    2. Relevant equations

    E = [tex]\frac{k*q}{r^2}[/tex]

    [tex]\Phi[/tex] = E*A

    3. The attempt at a solution

    So what i was trying was this:

    I cube root the volume of the cube. This gives me the length of any side since its a cube. So then, i calculate E where k = 9.0E9 q = 1.3E-6 and r = 1.07 (this is half of one length of one side). I then put E back into the electric flux equation where A is (2.14^2)*6 (the total surface area of the cube) i then take my answer and divide by 6 to get the flux for one side which comes out to 4.68E4 N*m^2/C which is incorrect. Im thinking ive made a mistake calculating the Electric field (the r value specifically, since it isnt really constant. ie if we wanted to know the flux in the corner of the cube, we would need to use trig to get the r value. But i cant see what else the r value would be.) A little direction would be great :D

    my picture (not given):
    http://img214.imageshack.us/img214/1397/chargeyp3.png [Broken]
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jun 13, 2008 #2
    My advice is to think in terms of electric field lines.
    In order to avoid crazy integrals, simplify the problem to a sphere.
    Solve for the flux on a sphere of arbitrary radius, then divide by six.
    You see, the surface area of a sphere varies with r2, whereas the E-field varies as r-2, which cancel.
  4. Jun 13, 2008 #3
    Fantasic, i got it right. Thanks!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook