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Homework Help: Electric Flux and Gauss' Law of point charges

  1. Oct 2, 2007 #1
    1. The problem statement, all variables and given/known data

    A point charge [tex]q_{1} = 4.15 \times 10^-6 [/tex] is located on the x-axis at x = 1.80 m, and a second point charge [tex]q_{2} = -5.80 \times 10^-6[/tex] C is on the y-axis at y = 1.10 m. What is the total electric flux due to these two point charges through a spherical surface centered at the origin and with radius r = 1.45 m?
    Take the permittivity of free space to be [tex] 8.85 \times 10^{-12}\:{\rm C}^{2}/{\rm N \cdot m}^{2}.[/tex]

    2. Relevant equations

    [tex] \phi_{E} = \frac{Q_{enclosed}}{\epsilon_{0}} [/tex]

    3. The attempt at a solution

    I just divided the one charge inside the sphere by epsilon naught.

    [tex] \frac{q_{2}}{\epsilon_{0}} [/tex]

    So I get, [tex] -6.55 \times 10^{5} [/tex]

    I'm sure I screwed something obvious up, any suggestions?
  2. jcsd
  3. Oct 2, 2007 #2


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    Homework Helper

    Why are you sure you messed up? Show the calculation you made and also be sure to show your units.
  4. Oct 2, 2007 #3
    I know I'm wrong because this hw's online and I got it wrong, and I loose points for every wrong answer suckily. Well the units for q were nanocoulombs which I changed to coulombs and then epsilon naught is [tex] {\rm C}^{2}/{\rm N \cdot {m}}^{2} [/tex]. So they cancel out do give [tex] {\rm N \cdot m}^{2}/ \rm C} [/tex]

    Calculation was [tex] \frac{-5.8 \cdot 10^{-12}}{8.85 \cdot 10^{-12}} [/tex]

    crap... I used the wrong changing of units it [tex] 1 \cdot 10^{-9} [/tex] coulombs per nanocoulombs... not -6.

    Thanks solo.
  5. Oct 2, 2007 #4


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    Homework Helper

    Well, that was easy on me... As a suggestion, when you present a problem in the forum, type it in exactly as it appeared originally. That would have made the SI prefix error easy to spot. Your method was correct!
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