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Capacitor equations

  1. Mar 5, 2013 #1
    hey guys, have a quick question

    just wanted to know what is the difference between these two equations, i couldn't find anything on google

    q=Q(1-e^-t/RC)

    and

    q=Qe^-t/RC


    why does one have 1 subtracting the rest of the equation, and the other doesn't

    thanks!
     
  2. jcsd
  3. Mar 5, 2013 #2
    I understand that in the first equation the capacitor is uncharged before the switch is closed, and when the switch is closed at t=0, charge (q) will increase.

    I just don't understand what the "1-" has to do with the problem
     
  4. Mar 5, 2013 #3
    wait i think i got it, in the second equation, the capacitor is charged, and it is discharging.

    in the first equation, it is uncharged. is this correct?
     
  5. Mar 5, 2013 #4
    The second equation is for a series connection of a voltage source, resistor, and a capacitor. Applying KVL around the loop and integrating, that is the result. The case is indeed for a capacitor discharging; hence the exponential decay.
     
  6. Mar 6, 2013 #5

    CWatters

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    Science Advisor
    Homework Helper

    Try calculating Q for t=0 and t=∞...

    1) q=Q(1-e^-t/RC)

    t=0, q=0
    t=∞, q=Q
    = charging

    2) q=Qe^-t/RC

    t=0, q=Q
    t=∞, q=0
    = discharging
     
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