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Capacitors energy

  1. May 3, 2009 #1
    I am not sure if I have full understanding of capacitors energy, because I found a contradiction in solving one problem. The problem is: -
    If an uncharged capacitor connected to another fully charged capacitor in parallel, what will be the new charge distribution on both capacitors? “Assuming equal capacitance”
    I believe we have two rules of respect:-
    1- Conservation of charge
    2- Conservation of energy
    If we assume that C = capacitance, and Vi = initial voltage and Qi=the initial charge on the charged capacitance.
    Let the energy before connecting the two capacitance equal to the energy after linking them together:-

    0.5*C*Vi^2 = 0.5*(2C)*Vf^2
    then: - Vf=Vi/sqrt(2)

    Know consider the total charge after linking both capacitors:-

    Qt = Ct * Vf= (2C)*Vi/sqrt(2)=C*Vi*sqrt(2)

    Which is obviously not equal to C*Vi “the initial charge”.

    I am confused which one of these two fundamental concepts I have to base my calculation in, and why it doesn’t work with the other concept.
  2. jcsd
  3. May 3, 2009 #2


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  4. May 3, 2009 #3
    Charge is conserved as is energy, however,when the capacitors are joined together charge flows through the connecting wires and electrical energy is converted to heat energy this being dissipated to the surroundings.The result is that the energy stored in the field of the capacitor combination is less than the energy stored in the single capacitor that was originally charged,the amount of charge,however, is the same.
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