Need help finding current on RC circuit

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Homework Help Overview

The discussion revolves around an RC circuit problem involving capacitors and a resistor. The original poster seeks to determine the current in the circuit after the capacitors have lost 80% of their initial stored energy, given specific values for capacitance and resistance.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to calculate the necessary charge to achieve a specific energy loss and derives equations related to energy and current in the circuit. Some participants question the visibility of the circuit diagram, which may be crucial for understanding the problem.

Discussion Status

Participants are actively engaging with the original poster's calculations, with some expressing the need for visual context to provide better feedback. The original poster has indicated a potential mistake in their approach but is seeking confirmation on their solution.

Contextual Notes

There are constraints regarding the visibility of the circuit diagram, which has led to difficulties in providing feedback. The original poster has attempted to share the diagram through various means.

Ruskointhehizzy
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Homework Statement


In the circuit shown in the figure each capacitor
initially has a charge of
magnitude 3.60nC on its plates. After the switch S is closed, what will be the
current in the circuit at the instant that the capacitors
have lost 80.0% of their initial stored energy?
the shown 3 capacitors are 10, 15, and 20 pF and the one resistor has 25ohms - all in series with one switch S.

Homework Equations


E = 1/2Q^2/c = QV/2
V_c(t)=Q/c * (1-e^(-t/RC))
i = Q/RC * e^(-t/RC)

I have since solved this but I would really appreciate it if I could get this checked for correctness.
Thank you.
IMG_2196.JPG


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I think I have figured out my mistake but please can I still get a check to see if my solution is correct?
first I find what Q needs to be to get .2E
to do this I say:
E0 = ½Q2/C, E = ½ ( √.2 Q )2/C = .2(½ Q2/C) = .2E0
I then use this to find -t
-t = RCln(Q/Q0) = RCln(√.2Q0/Q0) = RCln(√.2)
then finally
i = - Q/RC * e-t/RC = Q/RC * eRCln(√.2)/RC = Q/RC * eln√.2 = = -(3.6nC/(25Ω*4.62pF)) * √.2
i = = -3.6e-9C * √.2 / ( 25Ω * 4.62e-12F) = 13.9A
 
We can't see your images, which is probably why nobody has responded.
 
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phyzguy said:
We can't see your images, which is probably why nobody has responded.
Yeah I tried to post a image with a link to my google drive but it didn't let me - then when I tried to remove it wouldn't let me.
I finally resorted to posting it in the comments lmao.
Did you have a chance to check over my solution? I hope I got this correct I've been stuck on it for awhile
 
I don't see the figure anywhere. Have you tried just dragging the figure into the comment entry window?
 
FactChecker said:
I don't see the figure anywhere. Have you tried just dragging the figure into the comment entry window?
oh that worked thank you

that was just my first wrong attempt - I will change the op
 

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