Capacitors and energy

  • Thread starter minidoggy
  • Start date
  • #1
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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
 

Answers and Replies

  • #2
mjsd
Homework Helper
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what points in time? also at which capacitor? or or both?
 
  • #3
6
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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.. cuz that's all it says .. there's only one blank.. so I guess one answer.. >_<
 
Last edited:
  • #4
mjsd
Homework Helper
726
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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
 
  • #5
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I keep getting 4.42x10-6 ... but that's not the answer... TT________TT
 
  • #6
mjsd
Homework Helper
726
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if it is not clear what the question is really asking, it is very hard for me to help. what 's the correct answer?
 
  • #7
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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...
 
Last edited:
  • #8
mjsd
Homework Helper
726
3
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
 
  • #9
6
0
thanks for helping... it for some reason.. it's not the answer..

can anyone else please try and help!! TT_TT
 
Last edited:
  • #10
6
0
hey guys..
the answer is 0.000168
but i dunno how.. >_<
can someone explain? thanks
 

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