1. Not finding help here? Sign up for a free 30min 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!

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

    User Avatar
    Science Advisor
    Homework Helper

    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

    User Avatar
    Science Advisor
    Homework Helper

    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

    User Avatar
    Science Advisor
    Homework Helper

    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

    User Avatar
    Science Advisor
    Homework Helper

    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?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?