Find Charges on Capacitor w/ Floating Plates

[Hydrous Caperilla]

Homework Statement

Ignore those darkened black lines as they are just consequences of my editing and there's a SWITCH" between A and C2 which is opened initially

So here's the question I was working with.

The switch is closed at t=O,find the charges on the capacitor.The batteries are both 2OV and the C1=2uF,C2=4uF and C3=2uF.

Homework Equations

3. The Attempt at a Solution
After the switch is closed at t=O ,the charges on the capacitors will change so I used Khirchoff rule on the 2 loops but it didn't help me because there are 3 unknown charges and my loops are 2 so I got stuck.When I asked my teacher ,He said that the circled plates are floating plates because these plates are isolated from the circuit so charges can't come on these plates from outside and so sum of charges on these plates would be zero but I couldn't catch that...Isn't C2 connected with the battery so charges can come from the battery?Anyway why are the circled plates floating plates how can I identify them
[/B]

 

[PumpkinCougar95]

Would the charges change?

 

[cnh1995]

Use superposition theorem.
Consider one battery at a time and short the other one. You can find the voltages across the capacitors in both the cases and then add them to get the resultant.

 

[Hydrous Caperilla]

I believe so because the capacitors would cease to remain in series as seen when the path through capacitor 3 was open

 

[Hydrous Caperilla]

My doubt is not about solving the question but in identifying the floating plates...Why should I consider those circled plates to be floating plates

 

[gneill]

My doubt is not about solving the question but in identifying the floating plates...Why should I consider those circled plates to be floating plates

They have no wired connection to anything but other capacitor plates. Can you trace a path (via wire) from any of those plates to one of the battery terminals?

 

[Hydrous Caperilla]

Nope...they are only connected to other capacitors

 

[gneill]

Nope...they are only connected to other capacitors

So the net charge on that circuit node cannot change, right? There's no path for new charge to enter or to leave.

 

[Hydrous Caperilla]

Okay I get this