# Homework Help: LR Circuit.

1. Apr 21, 2006

### Gott_ist_tot

I have a circuit problem. There is a 30 V power source conected in series with a switch and a ten ohm resistor. The circuit then spling to an inductor of 10 mH and a 20 ohm resistor. I am having difficulty finding the current across the 20 ohm resistor. I got the current immediately after the switch closes as 1 A. Then a long time after the switch closed the current across the 20 ohm resistor is 0 A. What is the current when the switch is reopened. The circuit is now an LR circuit L = 10mH and R = 20 ohms. by V=IR I thought that the current across the resistor would be 1.5 A. However, it is not. Is V of the inductor different than the power supply? If so, I con't find a rule governing that. Thanks for any help.

2. Apr 23, 2006

### lightgrav

Just before the switch closes, the current through the inductor is 0 Amp.
The current after the switch closes is 1 Amp, as you calculated.
So the current through R20 = 1 Amp (at t=0, Current through L is zero).
What is the Voltage across R20 at t=0? _____
So, what is the Voltage across the inductor at t=0? _____
What is the rate of change in Current through L at t=0? _____

After a long time, the rate of change in Current through L becomes zero.
This does NOT imply that I through L is zero, only that it is constant.
So the Voltage across L is zero ... What Current goes through R10? ____
What current was going through the Inductor just before switch opens?

As the switch opens, there's no longer any power supply in the circuit.
(How would the Inductor Voltage be equal to some dis-connected thing?)
But the Inductor Current has not changed from the I just before opening.
What is the current through R20 now? ____
So, what is the Voltage across the inductor? _____
So, what is the rate of change in Current thru inductor? ___