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B Discharging capasitors and it's general electrical propertie

  1. Jun 3, 2016 #1
    It's just that i've never properly sorted this confusion of mind, so i would like to sort it out

    so it 2 different capacitors are in series with a resistor: (let's say 0.15F, 0.45F, 150M ohm)
    1. both current and voltage drops exponentially
    2. capacitance on capacitors remain same over time?
    2. the charge on capacitors is same over time?
    because Current decreases, and Time increases, does that mean Charge (Q=I*T) is unchanged
    3. at t=0 charge on cap 1 = charge on cap 2?
    Since they're in series, current should be same, therefore charge is same? But Q=CV, and capacitance is different


    thankyou.
     
  2. jcsd
  3. Jun 3, 2016 #2
    You mean the two capacitors and the resistor are all in series?
     
  4. Jun 3, 2016 #3

    jbriggs444

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    If all three components are in series then the two capacitors are equivalent to a single capacitor. You are needlessly complicating the situation by having two capacitors instead of one. Let's take a 1 F capacitor in series with a 1 ohm resistor.
    Yes.
    Yes. The capacitance of an ideal capacitor is a constant.
    More basic than that. If you have a pool of charge and drain some charge, the pool shrinks. Charge is a conserved quantity.

    If we have the two terminals of a 1F capacitor connected to the two terminals of a 1 ohm resistor then current decays by a factor of 1/e every second. If it starts charged at 1 Volt then the capacitor also starts with 1 Coulomb of charge.

    After one second, there is only about 0.37 Volts and 0.37 Coulombs. 0.63 Coulombs having drained through the resistor at an average of 0.63 amps during that first second.

    After two seconds there is only about 0.14 Volts and 0.14 Coulombs remaining. 0.23 Coulombs having drained through the resistor at an average of 0.23 amps during that second second.

    After three seconds there is only about 0.05 Volts and 0.05 Coulombs remaining. 0.09 Coulombs having drained through the resistor at an average of 0.09 amps during that third second.
     
  5. Jun 3, 2016 #4

    sophiecentaur

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    In the general case of two charged capacitors discharging in series with a resistor, the result can be that the two Cs can end up with (opposite) charges on them when the current has stopped flowing. It all depends on the values and the initial charges.
     
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