How long after the switch is closed does the voltage across

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Homework Help Overview

The discussion revolves around a circuit problem involving the voltage across a resistor after a switch is closed. Participants are tasked with determining the time it takes for the voltage to drop to a specified value, Vf = 20V, using relevant equations related to RC circuits.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss the application of the equation Q=Qm(1-e-t/RC) and question where the value of 20V is incorporated into the calculations. There is an exploration of different attempts to solve for time, with some participants noting previous incorrect answers.

Discussion Status

Some guidance has been offered regarding the correct application of formulas, with one participant indicating a change in approach that led to a correct answer. Multiple interpretations of the problem setup are being explored, particularly concerning the use of voltage values in the calculations.

Contextual Notes

Participants are working under the constraints of homework rules, which may limit the information shared. There is an ongoing examination of assumptions related to the circuit parameters and the equations being used.

beeteep
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Homework Statement


Consider the circuit
2s5yyd0.jpg


How long after the switch is closed does the voltage across the resistor drop to Vf = 20V. Answer in units of s.

Homework Equations


Tc = RC
Q=Qm(1-e-t/RC)

The Attempt at a Solution


[/B]
RC = .0003026

11 = 31(1-e-t/RC)

.3548 = 1-e-t/RC

1.3548 = e-t/RC

ln(1.3548) = -t/RC

.3037*.0003026 = -t

91.886x10-6 s
Answers I've also come to that have been incorrect:

Previous Tries

Try#1: .945

Try#2: .000313554 (using 20 instead of 11 for Q)

Try#3: 91.886e-6
 

Attachments

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Where in your calculations does the 20V appear?
 
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rude man said:
Where in your calculations does the 20V appear?
20 = 31(1-e-t/RC)
 
beeteep said:
20 = 31(1-e-t/RC)

Or what I think you may be asking

(31V-20V) = 31(1-e-t/RC)
 
Looks like I was using the wrong formula.

I changed to I = (V0/R)*e-t/RC and was able to get the correct answer.
 

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