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Homework Help: Spherical surface surrounding charges find flux

  1. Jan 22, 2009 #1
    A spherical surface completely surrounds a collection of charges. Find the electric flux (with its sign) through the surface if the collection consists of (a) a single +5.5 x 10-6 C charge, (b) a single -4.8 x 10-6 C charge, and (c) both of the charges in (a) and (b).

    Ok, so this is what I've been able to figure out:
    Ea=kq1/r^2=49445/r^2

    Eb=kq2/r^2=-43152/r^2

    and for part c: Ea + Eb = 6293/r^2

    I'm just so confused as to how to get rid of the r^2.. Have I used the wrong equation entirely? Thanks so much to anyone who can help!
     
  2. jcsd
  3. Jan 22, 2009 #2

    LowlyPion

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  4. Jan 22, 2009 #3
    The total of the electric flux out of a closed surface is equal to the charge enclosed divided by the permittivity.

    so then for example on part a, would I not just take:

    q/k = (5.5x10^-6)/(8.99x10^9) = 6.117x10^-16

    ?? I know that's not right but I don't understand why. Do I have the sign backwards?
     
  5. Jan 22, 2009 #4

    LowlyPion

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    Φ = Q/εo

    But you used the right value.

    If the charge is - then the flux is -, meaning that it points towards the charge. If + it would be pointing outward through the closed surface.
     
  6. Jan 22, 2009 #5
    so you're saying my answer is right then, right? I submitted it on my online homework thing, and it says I'm incorrect :/
     
  7. Jan 22, 2009 #6

    LowlyPion

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    Do they want 6.1 instead of 6.117, since they only gave you 5.5 for the charge in the first place?

    I ask in case this is something your instructor likes to do.
     
  8. Jan 22, 2009 #7
    No, our answers can be to whatever decimal point we want, just as long as it's within +/- 2% of the correct answer
     
  9. Jan 22, 2009 #8

    LowlyPion

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  10. Jan 22, 2009 #9
    Ohhh. lol. okay, now if only I knew what eo was..
     
  11. Jan 22, 2009 #10
    oh wait... ok so the equation is Φ = Q/εo

    do I just need to solve for Eo?
     
  12. Jan 22, 2009 #11
    I see! lol thanks so much for your help! :]
     
  13. Jan 22, 2009 #12

    LowlyPion

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