How do I calculate the total stored energy when capacitors are connected?

[minidoggy]

Homework Statement

hey


A 2.21 uF capacitor is charged by a 20.0 V battery. It is disconnected from the battery and then connected to an uncharged 3.60 uF capacitor (Fig. 17-29).
http://www.webassign.net/giancoli5/17_28.gif

Determine the total stored energy at the following points in time.
(b) after they are connected

Homework Equations

C= Q/V
E=.5 QV

The Attempt at a Solution

I've tried to use the equations but I keep getting them wrong...
The answers I've gotten are
.00116
7.2e-4
.001279
.001162
4.42e-4
2.905e-4
884e-6

Thanks

 

[mjsd]

what points in time? also at which capacitor? or or both?

 

[minidoggy]

oh
after the two capacitors are connected
the first time was before they are connected.. which was 4.42e-4
I'm not sure .. but I assume it's both.. because that's all it says .. there's only one blank.. so I guess one answer.. >_<

 

[mjsd]

ok the question is not that clear to me. but a few things for you to think about: conservation of charge/energy, compare the voltage across C1 and C2think about what would happen upon connecting C2 to the system note C1 not eq to C2

 

[minidoggy]

I keep getting 4.42x10-6 ... but that's not the answer... TT________TT

 

[mjsd]

if it is not clear what the question is really asking, it is very hard for me to help. what 's the correct answer?

 

[minidoggy]

I don't know.. It only tells me if I'm wrong..

the whole problem looks like this, exactly

"A 2.21 µF capacitor is charged by a 20.0 V battery. It is disconnected from the battery and then connected to an uncharged 3.60 µF capacitor (Fig. 17-29).

http://www.webassign.net/giancoli5/17_28.gif
Figure 17-29

Determine the total stored energy at the following points in time.
(a) before the two capacitors are connected
_____J
(b) after they are connected
_____J
(c) What is the change in energy?
_____J"

I think the V will change...

 

[mjsd]

my guess is that since it is asking for "total" stored energy ... assuming no loss in the wires, there should be no change at all. NB: 3.6uF capacitor is not charged, ie. does n't store any energy initially

 

[minidoggy]

thanks for helping... it for some reason.. it's not the answer..

can anyone else please try and help! TT_TT

 

[minidoggy]

hey guys..
the answer is 0.000168
but i don't know how.. >_<
can someone explain? thanks