Question

You have a RC circuit which contains a capacitor of 0.02 F and a resistor of 350 Ohm. Calculate how 

long it would take (in seconds) for the capacitor to be charged to 89% of its full voltage.

Answer 1

$$Q=Q_{\max}(1-e^{-t/RC})$$

$$1-e^{-t/RC}=0.89$$

$$e^{-t/RC}=0.11$$

$$t=-RC\ln 0.11\\ =-350*0.02*\ln 0.11=15\:\rm s$$