Two easy circuit-analysis problems

[Molecular]

First problem:

Homework Statement

I'm supposed to find the voltage across the 8k resistor in the following circuit:
http://img79.imageshack.us/img79/4139/circuit1lk0.jpg

The Attempt at a Solution

This just seems so mindboggling easy. My first thought is to use kirckhoffs current laws on all three nodes, giving me three equations with three unknowns.

The problem is this always ends up giving me a 0 = 0 equation. I'm guessing this might be because the equation for the upper left node = 1, the down left node = 2 and the node on the right = 3, so you could say left node equations = right node equation. I don't know, at any rate I always get a 0 = 0 equation so there's no help there either.

I've also tried using the node-voltage method, but this also turns out wrong every time. Admittedly this may be calculation errors on my side, but to some extent since I can't see the paths back to whatever node I chose as the reference node I've got a feeling it won't be correct anyway.

The problem seems so easy, and I know for a fact it's supposed to be easy, but I'm somehow missing something essential here, any help would be greatly appreciated.

Problem two:
Nevermind this one, I figured it out =\

Homework Statement

I'm supposed to find the Thevénin equivalent voltage of the following circuit:
http://img156.imageshack.us/img156/9977/circuit3ab6.jpg

Again this is a somewhat easy problem.
I start by simplifying the voltage source to a power source:
230v / 20 = 11.5 A in parallel with a 20 ohm resistance and a 30 ohm resistor, which together becomes a 12 ohm resistor.

So now I've got this 11.5 A power source in parallel with a 12 ohm resistor in parallel with a 12 A power source.

First question: Considering I can now splice these two power sources together, how come I need to subtract them from each other when their current goes the same way? I know that's what I'm supposed to do, I'd just think it would be 12 + 11.5 instead of 12 - 11.5.

Second question: Aren't I correct in my observation that since the 12A current is stronger than the 11.5 one, the current after I put these together as one current source will be 0.5 but pointed downwards on the left side?

Anyway at the end I get a Tvh of -3.75, but it's not supposed to be negative. But I can't see how it's supposed to not become negative considering I'm almost positive that will be the direction of the current after adding the two sources up.

This is probably because I can't figure out the exact mechanics on adding two current sources together. All I know is I'm supposed to use kirckhoff's current law to do it, but I've really got no idea how, I just take it for granted as a rule that I can.

 

[Molecular]

Ok never mind problem two, I realize why now. 11.5 ampere goes down, 12 goes up, this means that the current across the 12 ohm resistor and the other 12 ohm resistor is a total of 0.5, this has to go upwards from the left current source because the 12 ampere current is headed upwards. Phew.

Still stumped at the first question though. I'm going to try the node voltage method one more time ;o.

Edit: And don't you know it, 5 minutes after I made this thread this one was right aswell. I get -12 V. I'd still like to ask though wether this is solvable in any easier way than using the node-voltage method? As I'm fairly sure it's supposed to be.

 

[piercas]

I would suggest approaching these problems by breaking them down into smaller, simpler parts. For the first problem, it may be helpful to redraw the circuit and label the nodes with the corresponding equations. Then, you can solve for the voltage at each node using Kirchhoff's laws and work your way towards finding the voltage across the 8k resistor. For the second problem, it may be helpful to first find the equivalent resistance of the circuit and then use Ohm's law to solve for the Thevenin equivalent voltage. As for the second question, when adding two current sources in parallel, you should use the current divider rule to determine the resulting current. It is also important to pay attention to the direction of the current and make sure it is consistent throughout the circuit. I would also recommend checking your calculations and equations to ensure they are correct. If you are still having trouble, it may be helpful to seek assistance from a classmate or instructor. Remember, even easy problems can be challenging if you are missing a key piece of information or making a small mistake in your calculations.