Question

could you find v1, v2, and v3 in this case..

Answer 1

I'm a little lost here. Usually I find the voltages in resistors or between two points of the circuits. What does mean those v1, v2 and v3?

Answer 2

it's voltage at a node when it compared to ground..

Answer 3

Have you tried Kirchoff equations?

Answer 4

yea, i've tried it KCL and KVL, but i can't do when there are current source and voltage source present at the same time..

Answer 5

Yes, what you need is nodal analysis.
http://en.wikipedia.org/wiki/Nodal_analysis

Answer 6

oh, i get it, just by combine the voltage source.. thanks

Answer 7

eh, could you helpme with my second question

Answer 8

to find Io and Vab

Answer 9

Ok, well. I would do a simplification first. The three resistive elements that form a triangle can be rearranged to have 2 resisntance in serie and this two in parallel with the third one.

Answer 10

Then you can select to go around that way or apply directly Kirchoff equations. In this case, I think, it is possible do it better than the first case.

Answer 11

delta to wye transformations? But after i transform it, i still can't find Io and Vab

Answer 12

However, can you solve the circuit that gives you after transformation?

Answer 13

i'm still stuck after transformations, there aren't current anywhere..

Answer 14

i can't find the current value..

Answer 15

You can do this without applying the transformation.
There are three loops in the given picture (picture004.jpg). I will assume the current flowing in these loops to be I1 , I0 and I2 in clockwise direction starting from the top. Now you can apply Kirchoff's law in each loop separately. You should get the following 3 equations:
80V = 20I2 + 30(I2-I0) +20(I2-I1) .........(i)
80V = 20(I1-I2) +30(I1-I0) +20I1 ...........(ii)
0 V = 30(I0-I2) + 30I0 + 30(I0-I1) .........(iii)
You have got 3 equations and three unknowns. You can solve for I0
The find Vab = I0*R