# Homework Help: Helping setting up Nodal Equations

1. Sep 24, 2007

### bengaltiger14

Helping setting up Nodal Equations....

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

I need to find the node voltages at N1 and N2. See attachment for circuit.

3. The attempt at a solution

I think I need to setup KCL at each node. At Node 1, I got this:

(v1-v2)/12 - (v3-v2)/20 - (v2-0)/40 =0

v1 is the source voltage, and v3 is the voltage at node 2. Is this correct thus far?

#### Attached Files:

• ###### ECE221Circuit.doc
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2. Sep 24, 2007

### Staff: Mentor

Yes, you are correct so far. However, I personally like to write the KCL equations from the perspective of the node in question, showing that the sum of the currents out of the node is zero. So I would have written your first equation like this instead.

(v2-v1)/12 + (v3-v2)/20 + (v2-0)/40 =0

It's a matter of personal preference, I guess, but thinking about the KCL equations in this way helps me to keep them straight in my head.

3. Sep 24, 2007

### bengaltiger14

Cool.. thanks. And the equation at Node 2 should look something like:

(v2-v3)/20 - (0-v3)/60 = 0

Plug in v2 from previous equation, solve for v3, and then plug v3 back into first equation?

4. Sep 24, 2007

### Staff: Mentor

I still have trouble with your sign conventions, but I think that is correct.

5. Sep 24, 2007

### bengaltiger14

Any suggestions on solving these equations?

6. Sep 24, 2007

### Staff: Mentor

You have two unknowns and two equations. You can either set up the equations so that you can add them to eliminate one of the unknowns, or you can use the determinant math trick to solve them.