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

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Discussion Overview

The discussion revolves around the application of the series combination formula for capacitors in the context of an open circuit. Participants explore the implications of combining capacitors in series when the circuit is not closed and question the validity of calculating equivalent capacitance under these conditions.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions why the series combination formula for capacitors cannot be applied in an open circuit, seeking clarification on the concept.
  • Another participant requests a visual representation to better understand the scenario being discussed.
  • A reference to a previous problem is made, indicating that the discussion is based on a specific example from another thread.
  • A later reply asserts that while it is possible to combine capacitors in series, one must also determine the initial charge to ensure the circuit behaves correctly when closed. It emphasizes that simply adding charges is incorrect and provides a calculation that leads to a different voltage than expected.

Areas of Agreement / Disagreement

Participants do not reach a consensus; there are competing views on the applicability of the series combination formula in an open circuit and the correct approach to calculating equivalent charge and voltage.

Contextual Notes

The discussion highlights potential limitations in understanding how to apply the series combination formula in open circuits, particularly regarding initial charge and voltage calculations. There are unresolved mathematical steps related to these calculations.

K Dhiraj Bhak
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Capacitors , why can't series combination be used in an open circuit??

they say we cannot apply series combination formula for the capacitors in an open circuit,,,but why?? why can't we find the equivalent capacitance in an open circuit?
 
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what do you mean " in an open circuit "
show us a drawing of what you are referring to

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

NascentOxygen said:
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.
 
Last edited:

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