Charge on Capacitor plate

  1. Hi I have a question regarding the charge Q that would build up on capacitors in series when there is a potential difference between the plates.

    Consider the attached circuit.
    1. Knowing the potentials [tex]\phi_A, \phi_B[/tex] and the capacitances [tex]C_A, C_B[/tex], but not [tex]\phi_C[/tex] is there a way of calculating the charges on the plates?

    2. Is [tex]Q_A=Q_{Ca}[/tex] and [tex]Q_B=Q_{Cb}[/tex]?

    3. Is [tex]Q_{Ca}=Q_{Cb}[/tex]? If not, what would prevent charge flowing from one plate to the other?

    4. The series capacitors are equivalent to a single capacitor having capacitance
    [tex]C_T=(C_AC_B)/(C_A+C_B)[/tex] and hence charge [tex]Q_T=C_TV[/tex]. Is [tex]Q_A=Q_B=Q_T[/tex]?

    Sorry this is so long winded, and thanks in advance.

    Attached Files:

  2. jcsd
  3. berkeman

    Staff: Mentor

    Capacitors are DC open circuits, so the voltage in the middle is indeterminate, in a sense.

    But the way this problem is usually stated, you turn on the voltage source and ramp it up to its final voltage V. During this ramping process, a current flows through the capacitors according to the traditional equation:

    [tex]I(t) = C \frac{dV(t)}{dt}[/tex]

    That current results in charge displacement, which gives the final voltage values on the caps. You are correct that the series combination of the two caps results in a lower overall series equivalent capacitance. If the caps are unequal in capacitance value, the larger one will end up with a lower voltage across it.

    Now with that, are you able to answer your questions?
  4. berkeman

    Staff: Mentor

    Oh, and remember that [tex]I(t) = \frac{dQ(t)}{dt}[/tex]
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