Time for a capacitor to discharge

AI Thread Summary
The discussion centers on a capacitor discharge problem where the user has derived the equation but received an incorrect mark from WebAssign. The user believes their calculations involving the natural logarithm are correct but is unsure due to the feedback from the platform. There is a suggestion to consider using 1/3 instead of 2/3 in the calculations, indicating a possible confusion in the problem setup. The clarity of the required format for answers is also questioned, with various notations mentioned. The conversation highlights the challenges faced when dealing with online homework systems and the importance of precise formatting.
joeyjoey
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Homework Statement
A capacitor with an initial charge q0 is discharged through a resistor. In terms of the time constant tau, how long is required for the capacitor to lose the first one-third of its charge?
Relevant Equations
Q(final) = Q(max)e^(-t/RC)
tau = RC
(I have no idea how to use Latex and I apologize)
I already how to properly set it up and execute the equation with natural log etc.

2/3q0 = q0e^(t/RC)
2/3 = e^(-t/tau)
ln(2/3) = -t/tau
t = -ln(2/3)tauThe problem should be trivial; however, -ln(2/3)tau is apparently incorrect. So is ln(3/2). Is there something I am missing?
 
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What you do here seems correct to me, what is the answer key again? Or you don't know it?
 
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Delta2 said:
What you do here seems correct to me, what is the answer key again? Or you don't know it?

I use WebAssign, which has marked it as incorrect. I do not know the answer, but I feel like what I did here is probably right.
 
joeyjoey said:
I use WebAssign, which has marked it as incorrect. I do not know the answer, but I feel like what I did here is probably right.
Is the required format perfectly clear? E.g. ln(3/2), ln(3/2)tau, ln(3/2)τ, -ln(2/3), 0.4τ, ...
 
haruspex said:
Is the required format perfectly clear? E.g. ln(3/2), ln(3/2)tau, ln(3/2)τ, -ln(2/3), 0.4τ, ...

1610254360164.png


this is how the question looks
 
joeyjoey said:
View attachment 275953

this is how the question looks
Hmm, ok.
Only other suggestion is to guess a confusion by the question setter and try 1/3 instead of 2/3.
 
haruspex said:
Hmm, ok.
Only other suggestion is to guess a confusion by the question setter and try 1/3 instead of 2/3.

ok. thank you for the help!
 
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