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Different equations for Electric Energy, please help me understand!

  1. Feb 17, 2013 #1
    I've been reading recently about Electric Potential Energy and when introduced with the following situation of two charged plates (one (-) and the other (+)) and a charged particle between them I've been taught the following equation.

    a) U=E/Q ⇔ E=QU

    However now when I'm reading about a capacitor the equation for the stored electric energy in the capacitor is

    b) E= (QU)/2

    But how is this possible? A capacitor is to my understanding exactly what I've described above, two charged plates. So how come there is a different equation for the same thing?

    For references I've read equation a) in Heureka A page 215 and b) in Heureka B page 180. The books are written in Swedish. I am very grateful for any help I can get to help me understand.

    P.S I've been searching for the same question without luck so I believe it was appropriate to post this thread
     
  2. jcsd
  3. Feb 17, 2013 #2
    Re: Different equations for Electric Energy, please help me understand

    These two equations do not describe the same thing. The first one describes the potential energy of some particle (of charge Q) in some external electric field. The second describes the energy stored in a capacitor; Q is the charge stored in that same capacitor, not of some unrelated particle.
     
  4. Feb 17, 2013 #3
    Re: Different equations for Electric Energy, please help me understand

    Thanks voko, I think i got it!
     
  5. Feb 17, 2013 #4
    Re: Different equations for Electric Energy, please help me understand

    Very well. Keep in mind, though, that the second equation is obtained from the first one. It is done by considering some small charge dq that has to be moved from one plate to another. Assuming that the capacitor already holds some charge q, the potential difference is u = q/C; the energy required to move the small charge dq from one plate to another is then dE = u dq = q dq/C; this is the first equation. Integrating this we obtain E = Q^2/(2C) = QU/2.
     
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