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Confusion on Farad calculation

  1. Jun 1, 2012 #1
    Hi,

    Please apologize the ignorant question, but I am confused on applying the discharge capacity of a capacitor.

    If Farads = Coulombs/Volt, then a hypothetical 1 Farad capacitor could generate 1 volt at 1 amp for one second. But by the same formula, a .00005 Farad capacitor could generate 20,000 volts at 1 amp for one second.

    What am I missing?

    Thanks for your patience!
    Jerry
     
  2. jcsd
  3. Jun 1, 2012 #2
    jerbeast,
    You have it correct, but your wording is a little unusual. See if this version makes more sense to you...

    Starting with uncharged capacitor:

    If I pump 1A of current for 1 sec into a 1F capacitor, its voltage will be pumped up to 1V.

    If I pump 1A of current for 1 sec into a .00005F capacitor, its voltage will be pumped up to 20,000v.
     
  4. Jun 1, 2012 #3

    NascentOxygen

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    Staff: Mentor

    Hi jerbeast! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    Q = C·V

    There is no mention of current or time in that equation.
     
    Last edited by a moderator: May 6, 2017
  5. Jun 1, 2012 #4

    NascentOxygen

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    Staff: Mentor

    That's a good way of looking at it. Perhaps OP could instead consider two capacitors that start off already charged. To nominate a figure, let's imagine each is pre-charged to 40kV.

    Now, if you draw 1A for 1 sec from the 1F capacitor, its plate voltage will fall by just 1V, and the capacitor will still be almost fully charged, at 39,999V.
    If you draw 1A for 1 sec from the .00005F capacitor, its voltage will fall by 20kV. So that capacitor will as a result become half discharged.
     
  6. Jun 1, 2012 #5
    Thanks for the quick responses. That helped considerably.
     
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