How long after the switch is closed does the voltage across

[beeteep]

Homework Statement

Consider the circuit

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

Homework Equations

Tc = RC
Q=Q_m(1-e^-t/RC)

The Attempt at a Solution

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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

 

[rude man]

Where in your calculations does the 20V appear?

 

[beeteep]

Where in your calculations does the 20V appear?

20 = 31(1-e^-t/RC)

 

[beeteep]

20 = 31(1-e^-t/RC)

Or what I think you may be asking

(31V-20V) = 31(1-e^-t/RC)

 

[beeteep]

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.