Capacitor Voltages at t = 0: Zero or Split?

  • Context: Undergrad 
  • Thread starter Thread starter danadak
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Discussion Overview

The discussion revolves around the behavior of capacitor voltages at time t = 0 in a circuit with two capacitors in series, particularly in response to a step function voltage input. Participants explore whether the voltages across the capacitors are zero or if they share the total input voltage equally at that instant, considering various assumptions and circuit conditions.

Discussion Character

  • Debate/contested
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • Some participants argue that the voltages across the capacitors (Vc1 and Vc2) are zero at t = 0 due to the rule that a capacitor's voltage cannot change instantaneously.
  • Others propose that Vc1 and Vc2 are equal and sum to the input voltage (vi) at t = 0, suggesting that the Heaviside step function creates a Dirac impulse of current that charges the capacitors equally.
  • A participant notes that an ideal circuit with only capacitors and voltage sources leads to an impossible situation, requiring additional components like resistance or inductance to resolve the analysis.
  • Some participants emphasize the need for a mathematical analysis using Kirchhoff's laws, pointing out that the derivative of the Heaviside function does not exist at t = 0, complicating the solution.
  • Concerns are raised about the realism of simulations that assume ideal conditions, with participants discussing the implications of physical parameters like resistance and inductance in practical circuits.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether Vc1 equals Vc2 at t = 0 or if both voltages are zero. Multiple competing views remain regarding the implications of ideal versus non-ideal circuit conditions and the mathematical treatment of the problem.

Contextual Notes

Limitations include the assumptions about ideal components, the instantaneous nature of the voltage change, and the mathematical challenges posed by the Heaviside function at t = 0. The discussion highlights the complexities of circuit analysis when dealing with transient responses.

  • #31
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So we can say my conjecture that R at output node would rob current at t = 0 from
the current coming from Dirac impulse is not true, hence forces Vc1 = Vc2, until the
momentum of charge occurs, which due to mass does not occur until t > 0 ?

Would that be a correct interpretation ?
 
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  • #32
With ideal circuits the mass and momentum of the electrons is always assumed to be zero. The problem in the OP is about the ideal circuit (which has no solution).

There are many ways that you can “fix” the problem. Adding the resistance is one. Looking at the mass and momentum of the charges is another. But all of them are a change from the initial problem (which has no solution). So any conclusion you make will be rejected by anyone who doesn’t agree with it because it is a different problem from the original problem (which has no solution).
 

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