# Capacitors , why cant series combination be used in an open circuit?

 P: 13 they say we cannot apply series combination formula for the capacitors in an open circuit,,,but why?? why cant we find the equivalent capacitance in an open circuit?
 Sci Advisor PF Gold P: 2,651 what do you mean " in an open circuit " show us a drawing of what you are referring to cheers Dave
 P: 13 this was actually from a problem...http://www.physicsforums.com/showthread.php?t=730605
P: 3,151
Capacitors , why cant series combination be used in an open circuit?

I refer you to the last part of this reply on that thread..

 Quote by NascentOxygen Certainly you can combine them. Regardless of all else, if they are in series then it's always true that 1/C = 1/C1 + 1/C2 That gives you the capacitance that behaves equivalently, but you have yet to determine the initial charge it should have so as to lead to identical circuit current & voltage when the switch gets closed.
In short.. You can find the equivalent capacitance BUT you also need to find the correct equivalent charge.

Hint: You cannot just add the two charges together. If you do that the initial voltage will be wrong. The equivalent capacitance it 1.2uF. You calculated the charge to be 660μC so the initial voltage would be 660μC/1.2uF = 550V. That's wrong, it should be 100+180=280V according to the problem statement. Recalculate the correct charge.

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