Resistor - Capacitor - Resistor

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    Capacitor Resistor
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Discussion Overview

The discussion revolves around the behavior of a series circuit consisting of an electromotive force (emf), two resistors, and a capacitor during the charging interval. Participants explore whether the two resistors can be considered in series and how this affects voltage measurements across the capacitor and the reference points used for those measurements.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether both resistors can be considered in series during the capacitor's charging interval, given that they have the same current flowing through them.
  • Another participant suggests that the resistors can be combined when considering the voltage across the capacitor, but not when measuring voltage relative to Earth, indicating a need for different approaches in those scenarios.
  • A participant seeks clarification on the implications of measuring voltage with respect to Earth, questioning how this affects the situation and whether it relates to charge loss.
  • Another participant reflects on the relationship between current flow, resistance, and voltage across the capacitor, noting that the voltage at the top of the capacitor changes over time and is influenced by the resistance configuration.
  • It is mentioned that there is no "charge loss," but rather a difference in reference points for measuring potential difference (PD).
  • Concerns are raised about the phase relationship of the potential difference across the capacitor compared to the input voltage, particularly when considering the capacitor's position in the circuit.

Areas of Agreement / Disagreement

Participants express differing views on how to approach the analysis of the circuit, particularly regarding the treatment of the resistors and the implications of reference points for voltage measurements. The discussion remains unresolved, with multiple competing perspectives presented.

Contextual Notes

Participants highlight the importance of reference points in voltage measurements and the potential impact of different configurations on the analysis. There is an acknowledgment of the transient behavior of the circuit but no consensus on the implications of these factors.

Sefrez
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This should be a simple question.

If you have a series circuit in the following order: emf, resistor1, capacitor, resistor2

Are both resistors in series during the charging interval considering that they both have the same current through them? That is, would a resistance of resistor1 + resistor2 either before or after the capacitor be equivalent?
 
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If you are only considering the voltage across the capacitor then you can combine the two resistors. BUT NOT if you are considering the voltage between the 'top end' of the capacitor and Earth, for instance. For that, you need to do things in a different order.
 
I believe I am speaking of the first you mention. For the second, could you explain in a little more detail what that would be? The way I am picturing it now is that your saying the wire at which terminates at the negative terminal of the emf is earthed. Is that correct? If so, how does this change the situation? Charge loss or something?

Thanks!
 
I had to stop and think there!
The problem was that you didn't actually ask the question which answered! (HAHA - very common)
I was initially talking in terms of measuring volts in the AC situation but the same must apply to the transient waveform at switch on. The current flow with time will be the same (defined by R(total) and C) but, with the resistance 'split' across the C, the voltage at the top of the C will start at Vin, modified by the potentiometer ratio to a value above zero and end up at Vin. Think of it this way: if the top resistor were nearly zero, the value of PD to Earth would be nearly Vin all the time.

There is no "charge loss". The difference is the reference point from which you would be measuring the 'output' PD.

If the capacitor is 'in the middle' then the phase of the PD between positive end of the capacitor and Earth will not be the same as the phase of the PD between the ends of the capacitor. (wrt the phase of Vin)
 

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