# Homework Help: Node voltage method

1. Sep 7, 2014

### orangeincup

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

Use the node-voltage method to find v0 in the circuit below.

2. Relevant equations

i1+i2+...in=0
i=v/R

3. The attempt at a solution
So I set my reference node to the top of the middle line(above 25Ω)
Equation...
24V/(20+80)Ω+v0/25+.004A=0

Now I can solve for v0=(-.004-24/100 )* 25, v0=-6.1V

I think I did something wrong, I'm confused about a few things. How do I know where to take my reference node? Also, I'm solving for v0, yet I'm adding .004 amps to the answer. Since I'm solving for voltage, how is this possible?

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Last edited: Sep 7, 2014
2. Sep 7, 2014

### ehild

I do not see any figure.

ehild

3. Sep 7, 2014

### ehild

that is not correct. The current through the 20+80Ω resistors flows through the potential difference v0-24V.

ehild

4. Sep 7, 2014

### orangeincup

Oh I see. I updated my solution:

(v0-24)/100+v0/25+0.004A=0

v0/100Ω+v0/25Ω=24V/100Ω-0.004A

0.05v0=.24A-.004A

v0=4.72V

5. Sep 7, 2014

### ehild

It is much better, but 40 mA=0.04 A.

ehild

6. Sep 7, 2014

### orangeincup

Okay, I fixed it. v0=4.00V. Thank you.