1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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




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

  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


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted