Need help with RC circuits...Please!?

Question

A capacitor in an RC circuit is initially uncharged. What is the charge on the capacitor being charged up by a 25 V battery after a period of 2 time constants of the RC circuit. ( C=15uF or 1.5 x10-5, R=unknown)? Thank you so much in advance I have tried to work out this problem for over an hour! :)

Answer 1

An RC circuit charges the capacitor at a rate of

v(t)=v(supply)*(1-1/e^(t/T))

where T is the time constant
v(t) at t=2*T
25*((1-1/e^(2))
=25*0.864
=21.6 v
knowing the voltage, compute the charge

Q=C*V
Q=21.6*15 uCoulombs
=324 uCoulombs


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