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Energy density

  1. Jul 23, 2016 #1
    1. The problem statement, all variables and given/known data
    Two large non-conducting plates of surface area A=.025m^2 carry equal but opposite charges Q = 75microC. What is the energy density of the electric field between the two plates.

    2. Relevant equations
    I wrote the equations on my attempt. This was a multiple choice problem and the correct answer he gave us was 1.7*10^7 but he's been wrong before.

    3. The attempt at a solution
    316fbdf3df98834d648106f0d95a8bd5.png
    Any help is appreciated!
     
  2. jcsd
  3. Jul 23, 2016 #2

    Charles Link

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    Try again. A=.025 The rest looks correct.
     
  4. Jul 23, 2016 #3
    The new answer I get is 508475 instead of 5085. I feel like I may be using a wrong equation or something? My teacher didn't really lecture on energy density at all so I'm a bit uncertain.
     
  5. Jul 23, 2016 #4

    Charles Link

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    I think you got the rest correct. I think your teacher's answer of 1.7E+7 may be incorrect.
     
    Last edited: Jul 23, 2016
  6. Jul 23, 2016 #5
    My answer isn't even one of the multiple choices?
     
  7. Jul 24, 2016 #6
    What is the distance between plates?
     
  8. Jul 24, 2016 #7
    Energy density is
    ω = W / V = E / (Ad)
    where W - energy, V - volume, d - distance between the plates.

    Capacity of the system (actually, we consider a capacitor)
    C = ε0A/d

    Energy
    W = Q2/2С

    Hope, it is enough to find the solution.
     
  9. Jul 24, 2016 #8
    The problem doesn't say the distance between the plates.
     
  10. Jul 24, 2016 #9
    But have it in your mind.
    Derive the final formula.
     
  11. Jul 24, 2016 #10

    Charles Link

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    @justin15501 Please let us know if your teacher agrees with your answer. I do think you computed it correctly.
     
  12. Jul 24, 2016 #11
    My answer is
    5.085⋅105 J/m3?

    Regards to your teacher )
     
  13. Jul 24, 2016 #12
    Well the way it works is my professor goes over this packet of problems (including this one) the day before the exam and leaves us to figure out the questions we got wrong on our own. And there's 165 questions in this packet. It's not a very ideal situation. If I get something wrong, it's not like I can go to him for help. I'm left to rely on the kind folks of this forum to help me however they can!
     
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