Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Capacitor charged and disconnected then connected to capacitors

  1. Jul 21, 2012 #1
    Initially, a 15uF capacitor is charged by a 100V battery. Once charged, it is disconnected from the battery and connected to two uncharged capacitors(C_2 = 20uF, C_3 = 30uF) in parallel. What is the charge and potential difference on each capacitor.

    (The two capacitors mentioned above are in series to each other but parallel to the charged capacitor.)

    So, I found out the Q on C_1 as being 1.5mC when fully charged.

    Now since they are in parallel the voltage across the two uncharged capacitors is 100V.
    I find the equivalent capacitor of those two as 12uF and the charge being 1.2mC.

    Now I am confused about how there is 1.5mC on C_1 but just 1.2mC on the two capacitors in series.
    Shouldn't the charge across the whole circuit be the same? Or am I mistaken?
     
  2. jcsd
  3. Jul 21, 2012 #2

    vk6kro

    User Avatar
    Science Advisor

    When you add the extra capacitors, the voltage on the combination will drop.

    I make the series combination 12 μF and the parallel combination 27 μF when you add the 15 μF.

    The charge must be constant,

    Initial charge = Q = C * V = 0.000015 Farads * 100 volts = 0.0015 Coulombs, as you said.

    So, the new voltage will be Q = C * V

    or 0.0015 Coulombs = 0.000027 Farads * V volts

    V= 55.56 volts
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Capacitor charged and disconnected then connected to capacitors
  1. Charging the capacitor (Replies: 4)

  2. Capacitor Charge Time (Replies: 5)

Loading...