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Homework Help: Capacitors in parallel

  1. Jul 5, 2017 #1
    1. The problem statement, all variables and given/known data
    Two capacitors C1 and C2 are charged to 120V and 200V respectively.It is found that on connecting them together the potential on each one of them can be made zero.Then,
    B)3C1 + 5C2=0

    2. Relevant equations
    Q-charge on capacitor
    V-potential diff across plates
    Capacitors in parallel have same potential difference
    3. The attempt at a solution
    After charging the capacitors to their respective voltages,the charges on them are:
    On C1: 120C1 and -120C1
    On C2: 200C2 and -200C2
    Now,when we connect them in parallel,according to question,potential across them becomes zero.
    Through Q=CV,
    if V=0,Q=0 as well...
    So there's no charge on any capacitor

    So,the total charge on them ought to be zero..i.e..120C1+200C2=0
    Option B
    But,the answer given is A.
    I think..maybe my assumption of "connecting them together" in the question as a parallel configuration is wrong..maybe something else..
  2. jcsd
  3. Jul 5, 2017 #2


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    Staff: Mentor

    The problem with answer B is that the sign is wrong. If it were B)3C1 - 5C2=0 then it would be correct, and the same as answer A.

    The Q in Q=CV is the same polarity as CV. The reverse polarity comes because of the way the two capacitors are connected to cancel out the net charge. Does that make sense?
  4. Jul 6, 2017 #3


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    Homework Helper

    The capacitance is defined as non-negative. You can connect the capacitors positive side with positive side, and positive side with negative side as @berkeman suggested.
  5. Jul 6, 2017 #4
    Which way are the capacitors connected?that's my entire problem..

    Well,wouldn't the two cases give two different answers?
    Positive with positive would mean 120C1+200C2 is 0
    Positive with negative would mean 120C1 +(-200C2) is 0
  6. Jul 6, 2017 #5


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    Homework Helper

    C1 and C2 are capacitances, never negative. So connecting the plates of the capacitors positive to positive, negative to negative, the overall potential difference between the plates can not be zero.
  7. Jul 6, 2017 #6


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    Homework Helper
    Gold Member

    Arguing intuitively gives the correct answer a) and not b)
    But following through with your math, the two possible situations are
    Q1 = 120C1 and Q2= -200C2 or
    Q1=-120C1 and Q2=200C2
    neither of the other combinations can give zero when added

    so the toatal charge is Q1 + Q2 = 0, ie. 120C1 - 200C2 = 0 or -120C1 +200C2 = 0 , both giving a) but not b)
  8. Jul 6, 2017 #7
    Okk,I get it now.Thanks ppl !!
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