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RL circuits - finding resistance and inductance

  • #1
295
4

Homework Statement


upload_2017-2-12_12-8-34.png

I am practicing questions from the book in preparation for an exam, and I am having trouble with this one

Homework Equations




The Attempt at a Solution


According to my professor, the answers are: R1=12ohm, R2=9ohm, L=34.28 H
So far this is what I have done:
Voc = 6V
τ = 20/7
Vo = 3V
Rt=7L/20
Rt=6/(-24+Isc(R2+3)
This is just from analyzing the equation that was given in the question. I don't know if my logic is correct, and I am not sure what the next steps will be.
 

Answers and Replies

  • #2
165
45
Start with your equation for v(t). What is its value when t equals 0?

Since the current through L can't change instantaneously, prior to closing the switch, what is the current flowing in the circuit? That is the same current that flows through the 3 ohm resistor, so the v(0) value should help.

See if this helps any.
 
  • #3
cnh1995
Homework Helper
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Heed @magoo's clues. The important thing here is to note that the current through the inductor doesn't (or can not) change instantaneously, but voltage across it can.
 
  • #4
cnh1995
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Voc = 6V
τ = 20/7
Vo = 3V
Rt=7L/20
Rt=6/(-24+Isc(R2+3)
What do these symbols (Rt, τ, Voc) stand for? Which method are you using?

Given the voltage equation, what will be the equation of current for t>0?
 
  • #5
295
4
What do these symbols (Rt, τ, Voc) stand for? Which method are you using?

Given the voltage equation, what will be the equation of current for t>0?
τ is the time constant
Voc is open circuit voltage
and Rt is thevenin equivalent resistance
(I think)
 
  • #6
cnh1995
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τ is the time constant
Voc is open circuit voltage
and Rt is thevenin equivalent resistance
(I think)
You need not use Thevenin for this circuit.
What is the current through the circuit at t=0+?
 

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