Capacitors (charging after long time)

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Homework Help Overview

The discussion revolves around the behavior of capacitors in a circuit after a long time, specifically focusing on the energy stored in a capacitor and the voltage across it. The subject area includes concepts related to electrical circuits, capacitors, and resistors.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants explore the implications of current flow through resistors and capacitors after a long time, questioning the assumptions about voltage and current in the circuit. There are attempts to clarify the role of different resistors in determining the voltage across the capacitor.

Discussion Status

The discussion is active, with participants raising questions about the setup and reasoning behind the voltage across the capacitor. Some guidance has been offered regarding the removal of the capacitor from the circuit for analysis, and there is acknowledgment of the effect of other resistors in the circuit.

Contextual Notes

There are indications of confusion regarding which resistors are in the circuit and their impact on the voltage across the capacitor. The original poster expresses uncertainty about their calculations and assumptions.

jisbon
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Homework Statement
Find energy stored in capacitor after switch has been closed for long time.
Relevant Equations
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1568446283560.png

Hi there,
So as far as I know, after long time, current doesn't flow through the resistor That means the voltage drop across the capacitor is equal to the EMF of the cell = 9V
So will the charge be ##U = 0.5CV^2## = ##0.5(10)(81)##?
Answer appears to be wrong, not sure what I am doing wrong here.
 
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jisbon said:
Homework Statement: Find energy stored in capacitor after switch has been closed for long time.

Hi there,
So as far as I know, after long time, current doesn't flow through the resistor .
through what resistor? I think after a long time there won't be any current through the capacitor, and therefore no current through the 3 ohm resistor.
That means the voltage drop across the capacitor is equal to the EMF of the cell = 9V
I can really see no reason for that, no matter which resistor you meant.

If there's no current through the capacitor, you can remove it from the circuit, solve the remaining circuit to find the voltage across the capacitor.
 
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willem2 said:
If there's no current through the capacitor, you can remove it from the circuit, solve the remaining circuit to find the voltage across the capacitor.
Won't the voltage across the capacitor be 9v?
 
jisbon said:
Won't the voltage across the capacitor be 9v?
Isn't the 15Ω resistor connected too? Think about its effect.
 
Ah
sophiecentaur said:
Isn't the 15Ω resistor connected too? Think about its effect.
Ah..Solved it :) ##1.25x10^{-4}##
Thanks!
 
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