Finding Electric Energy Density

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Homework Statement


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.


Homework Equations



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


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?
 

Answers and Replies

  • #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?
 
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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?
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] ?
 
  • #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.
 
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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.
E = sigma / e_o ?
 
  • #6
Dick
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E = sigma / e_o ?
Nope. E=??/r^2. What's the numerator?
 
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Nope. E=??/r^2. What's the numerator?
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 ?
 
  • #8
Dick
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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 ?
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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