Nodal Analysis

[petesmc]

Hi,

I'm having problems getting the right answer for the nodal voltages of the included circuit. They should be -4/3 V for Va and 4V for Vb, but i just can't get these. Can someone write a follow through of how to find these nodal voltages for me?

Thanks
-Peter

 

[evandude]

are you getting the following two simultaneous equations?

a/4+3+(a-b)/2=0

(b-a)/2+b/3-4=0

solving those gives -4/3 for a and 4 for b.

 

[petesmc]

I get -3 and +4 for the currents in each of the equations instead of 3 and -4. I thought current away is negative and current into is positive?

-Peter

 

[evandude]

you can do it either way, the signs will work themselves out. i did it with out of the node positive.

if you do it into the node positive, then you would have to negate ALL those terms, in which case all the negative signs would drop out anyway. sounds like you are getting the signs wrong for the current in the terms other than the current sources.

for instance, a/4 refers to the current going OUT of the node a. to get the current into the node you would have to use -a/4.