madchiller said:
thanks a lot. But then who is right? if i get a negative voltage or current do i take the absolute value because the polarity that i chose was wrong?
tx for the help
No, you most certainly don't do that. What is means is that the actual current is going in the opposite direction.
The easiest way to do node voltage analysis is as follows:
1) label all of the node voltages in your circuit
2) pick a node (note, do not pick a nose)
3) write expressions for currents
leaving the node in terms of node voltages (I=V/R)
4) write a node equation setting the sum of all currents leaving the node = 0
5) repeat 2)-4) for all other nodes
6) solve the system of equations for the node voltages
That's it. Just do that consistently and don't worry about the fact that some of the currents in 3) are actually coming into the node instead of leaving the node.
In 3), since the convention is to write expressions for current leaving the node then, if your circuit element is a resistor and if you are working at node i then you will always have an expression of the form (Vi-Vj)/R. If you solve the system and get Vj>Vi, all that means is that the current is actually entering the node instead of leaving it. Do not try to guess if Vj>Vi or not, always use the convention of currents leaving the node. This applies also when you get around to node j, when you are at node j you assume again that the current is leaving the node, i.e. at node j you write (Vj-Vi)/R. Your assumption of currents leaving will therefore always be wrong exactly 50% of the time, but as long as you are consistent you will get the right voltages.