Determine the voltage across the 9.00-µF capacitor

[gamesandmore]

A 3.00-µF and a 5.00-µF capacitor are connected in series across a 30.0-V battery. A 9.00-µF capacitor is then connected in parallel across the 3.00-µF capacitor. Determine the voltage across the 9.00-µF capacitor.

I have no idea what to do.

 

[Astronuc]

Capacitors in series split the voltage, but differently than resistors.

Capacitors in parallel experience the same voltage drop.

So the 3 and 9-µF capacitors are put in parallel, what is the effective capacitance. Then what is the voltage drop across that capacitance? Hint - it splits the 30 V with the 5-µF capacitor.

What do you know about capacitance of parallel and series combinations of capacitors?

 

[gamesandmore]

Here is what I did so far:

Cp = 12 uF
Cs = 5 uF
Ct = 1/12 + 1/5 = 3.529 uF
q = CtV = 1.0588e-4 V
Vt = 30V = V1 + V2

 

[Astronuc]

Seems right - but make sure charge is in Coulombs (C) and voltage is V (volts = J/C).

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capac.html