Capacitor charging/discharging for R=0

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Discussion Overview

The discussion revolves around the behavior of an ideal capacitor in a circuit with zero resistance (R=0) when a voltage is applied. Participants explore the implications for current and voltage over time, questioning the feasibility and physical interpretation of such a scenario.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that if R=0, the current would theoretically be infinitely large for an infinitely short time, leading to questions about the validity of such a graph.
  • Others argue that while the current may be very large, it cannot be infinite due to the finite speed of electrons and the limitations imposed by physical laws.
  • A participant describes the current versus time graph as resembling a delta function, indicating an instantaneous transfer of charge to the capacitor plates.
  • There is a discussion about whether the voltage across the capacitor can increase instantaneously, with some noting that this contradicts the principle that voltage cannot change instantaneously in a capacitor.
  • One participant mentions that in practical scenarios, resistance cannot be zero, and thus the concept of infinite current is not physically realizable.
  • Another participant introduces the idea of superconductors, which have zero resistance, suggesting a potential real-world application but not directly addressing the theoretical discussion.

Areas of Agreement / Disagreement

Participants generally disagree on the implications of R=0, particularly regarding the nature of current and voltage in this scenario. While some accept the theoretical notion of infinite current, others challenge its physical validity and implications.

Contextual Notes

The discussion highlights limitations in the theoretical framework, such as the assumptions about ideal conditions versus practical realities. The interplay between circuit theory and physical behavior of materials is also noted as a point of contention.

amith_elec
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Consider an ideal capacitor its given to a switch and at t=t0 switch closes and Voltage =V is applied ..Consider R=0 in the circuit..how will the graph of i(current) v/s time(t) and Voltage (V) v/s time look like ?in all waveforms i have seen R is never zero..:confused:
 
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amith_elec said:
Consider an ideal capacitor its given to a switch and at t=t0 switch closes and Voltage =V is applied ..Consider R=0 in the circuit..how will the graph of i(current) v/s time(t) and Voltage (V) v/s time look like ?in all waveforms i have seen R is never zero..:confused:

R cannot be zero.
In the theoretical case where it is, the current would be infinitely large for an infinitely short time.
I don't think the graph would tell you very much! (Even if it were possible to draw it)
 
Stonebridge said:
R cannot be zero.
In the theoretical case where it is, the current would be infinitely large for an infinitely short time.
I don't think the graph would tell you very much! (Even if it were possible to draw it)

Why would the current be infinitely large? There are a fixed number of electrons in the capacitor and they must move some distance to discharge. Their speed is bound by the speed of light, so some time must elapse. So the current should be REALLY BIG, but not infinite.

Now saying I'm right, just wondering...

-David
 
the graph of i vs t will be the delta function
(delta func;f(x); is zero at all values except at 0, where it is infinitely large such that the integral of f(x) from -infinity to +infinity is equal to some finite value[here, CV])
physically what happens in this ideal situation is that all the requisite charge is transferred to the capacitor plates in infinitesimal time. therefore, i~q/t;t tends to 0, so i tends to infinity.
 
DavidSullivan said:
Why would the current be infinitely large? There are a fixed number of electrons in the capacitor and they must move some distance to discharge. Their speed is bound by the speed of light, so some time must elapse. So the current should be REALLY BIG, but not infinite.

Now saying I'm right, just wondering...

-David

That's what I said!
The resistance cannot be zero in practice. There would be a very large current for a very short time. The fact that we can talk about a current and a time means that there are numbers that can be assigned to them.
I stated that in the theoretical case when R=0 (which is impossible in practice in this situation) the current would be (theoretically) infinitely large. In the real world things don't go to infinity. In this case because the resistance can't go to zero.
Hope that makes it clearer.
 
I think what the OP is asking is if the equation for current versus time (or time versus current) can be determined from the limit as R -> infinity.
 
Stonebridge said:
That's what I said!
The resistance cannot be zero in practice. There would be a very large current for a very short time. The fact that we can talk about a current and a time means that there are numbers that can be assigned to them.
I stated that in the theoretical case when R=0 (which is impossible in practice in this situation) the current would be (theoretically) infinitely large. In the real world things don't go to infinity. In this case because the resistance can't go to zero.
Hope that makes it clearer.

i got the point ...also will the voltage across capacitor increase to supplied voltage instantaneously which i presume should happen if i is infinite (theoretically)...however,we study that voltage cannot increase instantaneously in a capacitor
 
You are struggling with the difference between fields and circuits. There are no electrons in circuit theory just continuous current. A real capacitor has inductances that limit inrush current for example. Circuit answer is infinite current. Not physical answer.
 
Superconductors have zero resistance they are used in MRI's
 

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