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Old Mar 22nd 2009, 04:40 PM   #1
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Join Date: Nov 2008
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RC circuit

How long would it take a 430mF capacitor to charge to 75% of its maximum?

I think im supposed to use this equation

V = Vo (1-e^ -t/T)

but im not sure what to do next? any help is appreciated
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Old Mar 22nd 2009, 08:18 PM   #2
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The charge Q during charging of the capecitor is given by
Q = Qo ( 1- e^-t/T). In your question Q = 75% 0f Qo, or Q= (3/4)Qo,hence
3Qo/4 = Qo (1-e^-t/T)
3/4 = 1 - e^-t/T
e^-t/T = 1- 3/4 = 1/4
e^t/T = 4 ( taking reciprocal both the sides )
Now take natural log (ln)
t/T = ln4
t = T*2ln2.
Now T, the time constant should have been given in the question.
Or as T = RC, R should have been given.
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