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Capacitor Discharge Report Urgent Help

  1. Dec 9, 2008 #1
    Hi,

    I am studying an engineering program in university. I have a piece of lab coursework to do and I am struggling to find information on it.
    The lab report was based on 'The discharge of a capacitor with time using an oscilloscope'

    It sounds silly but what is the theory of a capacitor discharging? I also need to find the principle of why is discharges relevant to the experiment at hand.

    I know I'm asking a lot but I think I'm in the right place to ask :)

    A huge thanks to anyone that can help me :)
     
  2. jcsd
  3. Dec 9, 2008 #2

    tiny-tim

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    Hi Tobus! :smile:

    From capacitor in the PF Library

    Inverse exponential rate of charging:

    A capacitor does not charge or discharge instantly.

    When a steady voltage [itex]V_1[/itex] is first applied, through a circuit of resistance [itex]R[/itex], to a capacitor across which there is already a voltage [itex]V_0[/itex], both the charging current [itex]I[/itex] in the circuit and the voltage difference [itex]V_1\,-\,V[/itex] change exponentially, with a parameter [itex]-1/CR[/itex]:

    [tex]I(t) = \frac{V_1\,-\,V_0}{R}\,e^{-\frac{1}{CR}\,t}[/tex]

    [tex]V_1\ -\ V(t) = (V_1\,-\,V_0)\,e^{-\frac{1}{CR}\,t}[/tex]

    So the current becomes effectively zero, and the voltage across the capacitor becomes effectively [itex]V_1[/itex], after a time proportional to [itex]CR[/itex].
     
  4. Dec 9, 2008 #3
    Hey thanks :)

    Thank you for the help,

    For my introduction I have written this,

    "We are doing this experiment to understand if the capacitor discharges at a particular rate to show that the theory of a capacitor discharges at an exponential rate when a direct current is removed. Capacitors are used in many circuits and electronic storage devices. As the capacitor is charged from the direct current supplied there is a force of attraction between the two plates as one becomes positively charged and the other becomes negatively charged. Once the charge/voltage? has been removed the capacitor starts to discharge and the flow of electrons moves across the plates. The rate of discharge decays exponentially to time."

    What do you think, do I get my point across properly? More importantly is the physics correct! lol
     
  5. Dec 9, 2008 #4

    tiny-tim

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    Hi Tobus!

    hmm …

    i] the electrons do not move across the plates … they go the long way round!

    ii] you haven't actually explained anything … why do the electrons go the long way round (especially when, as you point out, there's an attractive force the short way round) … and why is it exponential, or at least, what is the governing equation? :smile:
     
  6. Dec 9, 2008 #5
    What do you mean by long way around :S. Sorry I'm not so sure lol.
    I know its exponential decay as the current decreases at the same ratio in successive equal intervals of time.

    How about that? Do you think I should put it in somewhere.

    For my introduction I simply need to explain the theory of what a capacitor does. :)
     
  7. Dec 9, 2008 #6

    tiny-tim

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    Again from PF Library:

    Displacement current:

    No current ever flows through a functioning capacitor.

    But while a capacitor is charging or discharging (that is, neither at zero nor maximum charge), current is flowing round the circuit joining the plates externally …​
    but why? (how does it know? :biggrin:)
     
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