Solving Discharging Circuit Homework

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SUMMARY

The discussion focuses on calculating the time it takes for the charge on capacitor C2 to drop to 1/e (approximately 36.8%) of its fully charged value after switch S2 is reopened. The key equation involved is the time constant, represented as RC, where R is the total resistance and C is the capacitance. Participants emphasize the importance of determining the equivalent resistance in the circuit configuration after S2 is reopened, noting that the resistors may be in parallel. Understanding the relationship between voltage and charge on the capacitor is crucial for solving the problem.

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Homework Statement


http://online.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys212/oldexams/exam2/fa06/fig17.gif
After a very long time with both switches in the closed position, switch S2 is reopened. How long does it take for the charge on capacitor C2 to drop to 1/e (36.8%) of its fully-charged value (i.e. of the value it had just before S2 was reopened)?


Homework Equations


Time constant = RC
capchg7.gif


The Attempt at a Solution


I try to find the total resistance, but am not sure how. I am not even sure what it means by "drop to 1/e (36.8%)". Does it mean 36.8% of the total charge?
 
Last edited by a moderator:
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driedupfish said:

Homework Statement


http://online.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys212/oldexams/exam2/fa06/fig17.gif
After a very long time with both switches in the closed position, switch S2 is reopened. How long does it take for the charge on capacitor C2 to drop to 1/e (36.8%) of its fully-charged value (i.e. of the value it had just before S2 was reopened)?


Homework Equations


Time constant = RC
capchg7.gif


The Attempt at a Solution


I try to find the total resistance, but am not sure how. I am not even sure what it means by "drop to 1/e (36.8%)". Does it mean 36.8% of the total charge?

yes, since charge is a function of voltage here. As the voltage drops, less charge is accumulated on the plates of the capacitor.
 
Last edited by a moderator:
Okay, to find the time constant, RC, I have to find the equivalent resistor at that point. But I don't know how? With only S2 reopened, are all the resistors now in parallel?
 

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