# LR Circuit Problem

1. Nov 18, 2007

### pronep

1. The problem statement, all variables and given/known dataIf You want to turn on the current through a coil of self inductance L in a controlled manner, so you place it in series with a resistor R= 2200 ohm, a switch, and a dc voltage source Vo=240 V. After closing the switch, you find that the current through the coil builds up to its steady=state value with a time constant T. You are pleased with the current's steady-state, but want T to be half as long. What new values should you use for Rand Vo.?

2. Relevant equations
T=L/R
I= Vo/R (1-e^(-t/T))

3. The attempt at a solution
I= 240/2200(1-e^(-t/T))
I'= Vo'/R'(1-e^(-2t/T))

Since, I=I'
240/2200(1-e^(-t/T))=Vo'/R'(1-e^(-2t/T)

Can somebody point out what I am missing here? Thanks

2. Nov 19, 2007

### learningphysics

You have the formula for the time constant T = L/R. You need the time constant to become half... so what do you need the R to become?

The steady state value of the current is Vo/R. considering the change in R above, what do you need to do to Vo to keep the same steady state current as before? We don't want I = I'... we only want the steady state I to be equal to the steady state I'.

3. Nov 19, 2007

### pronep

Thanks...Got it...I was making it more complex than it was.