Compute the charge on the capacitor

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    Capacitor Charge
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Homework Help Overview

The discussion revolves around calculating the charge on a capacitor after a switch is toggled between two positions in an RC circuit. The original poster mentions specific voltages before and after the switch is thrown, indicating a transition from charging to discharging states.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the use of equations related to RC charging and discharging to find the charge on the capacitor. There are inquiries about the governing equations for capacitors and the importance of time in the calculations. Some participants express confusion about the original poster's calculations and the need for more detailed work to identify errors.

Discussion Status

The conversation is ongoing, with participants attempting to clarify the problem and share relevant equations. There is a focus on understanding the relationships between voltage, charge, and time in the context of the capacitor's behavior. No consensus has been reached, but there is a collaborative effort to explore different aspects of the problem.

Contextual Notes

Participants note the values of resistance, capacitance, and emf provided in the problem statement, as well as the initial conditions of the capacitor being uncharged. There is an emphasis on the specific time frame of 10.0 ms after the switch is moved back to position 1.

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Homework Statement


http://img259.imageshack.us/img259/1535/question2az5.th.jpg

Compute the charge on the capacitor 10.0 ms after the switch is thrown from position 2 back to position 1.I know from previous parts of this problem that the Voltage across the resistor before the switch is thrown is 8.28 V. Afterwards it is 9.72V.

I figured that the charge q before switch is thrown is 133 micro coulombs.

Homework Equations


The Attempt at a Solution



I tried using the new voltage after the switch is thrown to find q but it didn't work. Can someone please help me out?
 
Last edited by a moderator:
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C = Q/V;

and I found the voltages using the equations relating to RC charging and discharging.

but I am still not getting the right answer.
 
I can't tell you what you did wrong, because you didn't show your work. Didn't the problem
give R, C and E?
 
What kind of equation governs a capacitor?
 
You're looking for an equation with time t in it. You're going to need that to use your given value of 10.0 ms.
 
A capacitor with C = 1.30×10−5 F is connected as shown in the figure View Figure with a resistor with R = 990 Omega and an emf source with E = 18.0 V and negligible internal resistance. Initially the capacitor is uncharged and the switch S is in position 1. The switch is then moved to position 2, so that the capacitor begins to charge. After the switch has been in position 2 for 10.0 ms, the switch is moved back to position 1 so that the capacitor begins to discharge.I used the equations for V and I for discharging and charging capacitors.

V = E(1-e^(-t/RC)) charging
V = V_oe^(-t/RC) discharging

I = (E/R)e^(-t/RC) charging
I = (V_o/R)e^(-t/RC) dischargingI then used C = Q/V to find Q for the first part where before switch is thrown...hope this helped clarify some things, sorry for the lack of info
 

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