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Homework Help: Finding Electric Energy Density

  1. Mar 8, 2007 #1
    1. The problem statement, all variables and given/known data
    Find the electric-field energy density at a point which is a distance of 10.0 cm from an isolated point charge of magnitude 7.30 nC.


    2. Relevant equations

    [tex] u = \frac {1} {2} * e_{o} * E^{2} [/tex]


    3. The attempt at a solution

    [tex] u = \frac {1} {2} * 8.85*10^{-12} * 7.30^{2} [/tex]

    This was the equation we learned in class, and the online program say's its wrong. From the book it says that factors of A and d cancel out because of geometry, though I'm second guessing now, since the book says the equation is only in a vacuum, but this problem doesn't state either or?
     
  2. jcsd
  3. Mar 8, 2007 #2

    Dick

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    First you want to know what the value for E is in that situation. What formula might you use for that?
     
  4. Mar 8, 2007 #3
    The only equation from this chapter that I could find that would be relevant and give you E would be:

    [tex] V_{ab} = E*d [/tex]

    [tex] E = \frac {V_{ab}} {d} [/tex] ?
     
  5. Mar 8, 2007 #4

    Dick

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    It's the electric field around a POINT charge. It HAS to be one of the first formulas you encounter. Hint: it's an inverse square law. Keep looking.
     
  6. Mar 8, 2007 #5
    E = sigma / e_o ?
     
  7. Mar 8, 2007 #6

    Dick

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    Nope. E=??/r^2. What's the numerator?
     
  8. Mar 8, 2007 #7
    ooh [tex] E = \frac {KQ} {R^{2}} [/tex]

    I was thinking about this one but I thought you just meant in this chapter.

    Q = 7.30 * 10^-19 ?
     
  9. Mar 8, 2007 #8

    Dick

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    You don't have units on Q. I can't tell you if it's right. It's given as 7.30 nC. How many coulombs is that?
     
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