If a parallel-plate capacitor in a RC circuit has a 10 C charge on each plate when it is disconnected from the battery, and in 10 sec drops to 10% of its initial charge, what is its capacitance? (Assuming R = 10 ohms)

Question

If a parallel-plate capacitor in a RC circuit has a 10 C charge on each plate when it is disconnected from the battery, and in 10 sec drops to 10% of its initial charge, what is its capacitance?  (Assuming R = 10 ohms)

Not sure which formula to use/

Damon · Answer

V = Vi e^-(t/RC) .1 = e^-(10/10C) = e^-(1/C) ln .1 = -1/C