# Help with series combinations

1. Feb 25, 2009

### xswtxoj

1. The problem statement, all variables and given/known data

a series combinations is 11.6k ohms resistor and an unknown capacitor is connected to the a 180 V battery. One second after the circuit is completed, the voltage across the capacitor is 16.8v. determine the capacitance of the capacitor?

2. Relevant equations

V=18
C?
1 sec later
V=16.8v
3. The attempt at a solution
c= q/v
i=v/r

2. Feb 25, 2009

### swuster

Re: 16

Derive the differential equation for the current/voltage across the capacitor when it's charging, using Kirchoff's Voltage Law, and solve.

Or if you're allowed to, just look up the solution of this equation, which should be in the form $$V_c (t) = V_o (1 - e^{-t/\tau})$$ where $$\tau$$ is the time constant $$RC$$. You can then plug in your known values of $$V_c$$ to solve for the capacitance.