Charging capacitor - no resistor

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Homework Help Overview

The discussion revolves around the behavior of a capacitor when charged without a resistor in the circuit. Participants explore the implications of Kirchhoff's Law and the assumptions regarding resistance in the circuit components.

Discussion Character

  • Conceptual clarification, Assumption checking, Mixed

Approaches and Questions Raised

  • Participants examine the reasoning that the potential difference (p.d) across the capacitor immediately equals the electromotive force (emf) of the source due to the absence of other components. Questions arise regarding the assumptions of zero resistance in wires and the implications of real-world conditions, such as internal resistance of the power source and inductance of wires.

Discussion Status

The discussion is active, with participants providing feedback on the original reasoning and suggesting additional considerations related to resistance and circuit theory. There is acknowledgment of the complexities involved when ideal conditions are not assumed.

Contextual Notes

Participants note the assumption of zero resistance in the wires and the ideal nature of the power source, while also recognizing that real components would introduce factors that affect charging time and current flow.

jsmith613
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Homework Statement



When charging a capacitor with NO resistor in the circuit, the p.d of the capacitor IMMIDIATELY reaches a maximum...

CapacitorQ.png


Homework Equations


The Attempt at a Solution



...is the following reasoning correct:

As stated by Kirchoff's Law, the sum of the potential differences across all components in a series circuit is equivalent to the the emf of the source...as there are no other compoenents in the circuit the p.d across the capacitor = emf of source immediatley.

(Note: when a resistor is present the p.d across the resistor decreases intil the p.d across capacitor = p.d source)

Is this correct?
 
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jsmith613 said:

Homework Statement



When charging a capacitor with NO resistor in the circuit, the p.d of the capacitor IMMIDIATELY reaches a maximum...

CapacitorQ.png


Homework Equations





The Attempt at a Solution



...is the following reasoning correct:

As stated by Kirchoff's Law, the sum of the potential differences across all components in a series circuit is equivalent to the the emf of the source...as there are no other compoenents in the circuit the p.d across the capacitor = emf of source immediatley.

(Note: when a resistor is present the p.d across the resistor decreases intil the p.d across capacitor = p.d source)

Is this correct?

I like your reasoning - but would like to see some reference to the idea that you have been instructed to assume the resistance of the wires is zero, and thus regardless of current flow, the p.d across them will be zero, and then an explanation that if real wires were used, their tiny resistance would mean a very short charge time, rather than an instantaneous charging.

Peter
 
PeterO said:
I like your reasoning - but would like to see some reference to the idea that you have been instructed to assume the resistance of the wires is zero, and thus regardless of current flow, the p.d across them will be zero, and then an explanation that if real wires were used, their tiny resistance would mean a very short charge time, rather than an instantaneous charging.

Peter

this was a question I made up myself based on a diagram in my book
thanks so much, through, for confirming my reasoning :)
 
You are also leaving out the internal resistance of the power source. There is no such thing as a totally ideal power source.
 
Also, any wire, no matter the resistance, has some inductance. Between that and the results of attempting to drive an infinite current (requiring infinite speed for finite charge carriers) you've left the realm of normal circuit theory and entered Maxwell's and Einstein's domain. Happy adventuring :smile:
 
The circuit as posed is not a physics problem but a circuit theory problem. The battery has zero internal resistance as do the wires. Assuming you switch the discharged capacitor onto the battery you will have an infinite current for an infinitesimal time as the capacitor charges. The math can be handled by the theory of distributions, aka delta functions.
 

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