# First-order RL Circuit analysis

1. Jan 12, 2010

### Mohdoo

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

2. Relevant equations

I am not really sure which equations would be relevant since my question seems to be more so concepts instead of numbers, so forgive me please :(

I am familiar with how to do just about all of this problem, I am simply baffled as to how I handle the idea of a voltage source and a current source both being there at the same time. I have solved such problems only with one voltage source, or one current source, but never one of each or two of one of them.

3. The attempt at a solution

I see that there is a .5mA source, and a 30mA source. One is going to shoot current one direction into the capacitor, and one is going to shoot. I say a 30mA source because I assume that I can simply turn the 150V source into a 30mA source in parallel with the 5K resistor. Is it really as simple as subtracting .5mA from 30mA and then simply having i be negative? And then once the switch closes, the components to the left of the switch get cut off?

All my other problems on this homework assignment seemed so straight forward, this one is such a curve ball *sigh*

Many thanks!! :)

2. Jan 13, 2010

### vela

Staff Emeritus
You have a couple of things you can do. You can convert voltage sources into current sources and vice versa using Thevenin and Norton equivalence. If you have two voltage sources in series, you can combine them into one, and if you have two current sources in parallel, you can combine them into one. You could continue to combine elements to simplify the circuit, or you could use Kirchoff's and Ohm's laws to solve for the voltages and currents instead.

The techniques really don't change just because you have multiple sources. What specifically about multiple sources baffles you?

No, it doesn't work that way. You're going to have to work it out.

By the way, it's an inductor, not a capacitor.

3. Jan 13, 2010

### Mohdoo

Well, I figured that I could simply combine the two current sources. But in this case do they combine or do they subtract from one another? They both point upwards, but following each of them, would they not be going different directions when crossing through the inductor? It seems like the one on the left would be pushing current from left to right, and the one on the right pushing current from right to left. Which one is it? :(

4. Jan 13, 2010

### vela

Staff Emeritus
You can't combine them unless they're in parallel. I don't know if you've simplified he circuit further, but they're definitely not in parallel in the handwritten drawing.