# RL circuits - finding resistance and inductance

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1. Feb 12, 2017

### Cocoleia

1. The problem statement, all variables and given/known data

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

2. Relevant equations

3. 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.

2. Feb 12, 2017

### magoo

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. Feb 12, 2017

### cnh1995

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. Feb 12, 2017

### cnh1995

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. Feb 12, 2017

### Cocoleia

τ is the time constant
Voc is open circuit voltage
and Rt is thevenin equivalent resistance
(I think)

6. Feb 12, 2017

### cnh1995

You need not use Thevenin for this circuit.
What is the current through the circuit at t=0+?