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Two capacitors connected in series

  • Thread starter songoku
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  • #1
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Homework Statement


Two capacitors, capacitance of 30 μF and 60 μF respectively, each has charge of 50 μF and 70 μF. The capacitors then connected in series. Find the potential difference on each capacitor now


Homework Equations


Q = CV


The Attempt at a Solution


I know that the charge should be same for series circuit but I don't know how to analyse this question. Each capacitors has its own charge and voltage and after connected in series, their charge should be the same. How to find it?
 

Answers and Replies

  • #2
phinds
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... each has charge of 50 μF and 70 μF?
uF is a measure of capacitance, not charge, so I don't understand what you are saying here.
 
  • #3
CWatters
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Think about what "The capacitors then connected in series" means. Presumably only one plate of each is connected together otherwise they would be in parallel. If only one side of each capacitor is connected together how much current can flow?
 
  • #4
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uF is a measure of capacitance, not charge, so I don't understand what you are saying here.
sorry, I meant 50 μC and 70 μC

Think about what "The capacitors then connected in series" means. Presumably only one plate of each is connected together otherwise they would be in parallel. If only one side of each capacitor is connected together how much current can flow?
Sorry I really don't know the concept here of how to determine the current when only one side of each capacitor is connected...I assume that one positive plate is connected to other negative plate. Will the case be the same if the series circuit connects positive plate to positive plate?

Thanks
 
  • #5
CWatters
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If the only one end of each capacitor is connected to anything there is no closed circuit so no current flow. Therefore the capacitors retain whatever charge and voltage they had before they were connected.
 
  • #6
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If the only one end of each capacitor is connected to anything there is no closed circuit so no current flow. Therefore the capacitors retain whatever charge and voltage they had before they were connected.
Sorry, actually there are choices for this question:
a. 4 and 8 Volt
b. 8 and 4 Volt
c. 5 and 5 Volt
d. 3 and 6 Volt
e. 6 and 3 Volt

If the capacitors retain their voltage, their voltage should be 5/3 and 7/6 Volt.

So there is no correct choice?

Thanks
 

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