# capacitorsWatch

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#1
Can some one help me with this question:
A 20 microfarrad capcitor is charged to 10V and isolated. A second uncharged 20 microfarrad capacitor is now connected to the plates of the other. The capacitors rapidly come to the same pd between their plates. What is the charge on each capacitor now? What is the total energy stored in the two capacitors? Comment on your answer. Now repeat with the second capacitor rated at 30 microfarrads.

While we're on the subject of capacitors does anyone know any good websites about capacitors?
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14 years ago
#2
(Original post by kat_r)
Can some one help me with this question:
A 20 microfarrad capcitor is charged to 10V and isolated. A second uncharged 20 microfarrad capacitor is now connected to the plates of the other. The capacitors rapidly come to the same pd between their plates. What is the charge on each capacitor now? What is the total energy stored in the two capacitors? Comment on your answer. Now repeat with the second capacitor rated at 30 microfarrads.
the first capacitor is charged to Q = CV = 10*20 = 200 microcoulombs

if voltage is the same accross both capacitors, and they have the same capacitance, they must have the same charge on them

charge is conserved.

therefore there is 100 microcoulombs each on the capacitors

the voltage accross them is (100/20) = 5 volts

energy = (QV/2)*2 = 100*5 = 500 microjoules

if the second capacitor has 30 microfarads capacitende, and they both have the same voltage, the charge must be shared in a 3:2 ratio. therefore the second one has 120 microcoulombs on it, and the first one has 80 microcoulombs.

the voltage accross them is (120/30) = (80/20) = 4 volts

energy = 120*4/2 + 80*4/2 = 400 microjoules
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