Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Nodal Analysis

  1. Sep 13, 2006 #1

    I'm pretty good with all of this stuff... I'm just a bit confused on how to start this one. I mean, usually you begin by figuring out where the reference node is... But I'm not sure what to use as my reference node/ground node...

    In all the other problems I've done, it's been pretty obvious. I usually get circuits that are organized top to bottom, where I've got a bunch of resistors on the top of a ladder-like chain of components. So usually the bottom portion of the circuit is the ground node.

    However, that's obviously not the case with this one. The voltage source is in upside-downish. And the current sources are going opposite of the way they seem like they should.

    Would that mean that the top area of the circuit would be grounded?

    I'm awfully confused. Any ideas on this one?

    http://www.synthdriven.com/images/deletable/EEN201-04.jpg [Broken]
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Sep 13, 2006 #2
    Don't worry about which node is grounded or which direction the sources are pointing. Just write out your node equations. This problem is really easy if you pick the right node to start with. Since your node equations say all the current entering and leaving the node must sum to zero, the best node to start with is a node with current sources, because then you already know some of the currents. The more sources on the node the better. If you pick the right node to start you should be able to solve this problem by inspection. You're after Vo and know the resistor values, so you just need to find the current flowing through the 6k resistor.
  4. Sep 13, 2006 #3
    ...I do need to know which node is grounded in order to write out my equations... if the right node is v2 and the center node is v1, i can write out KCL for v2 just fine in terms of v1 and v2. but I need v1 in order to solve for v2, and when writing out v1 I get confused as to where the ground is so I can properly write out the equation.
  5. Sep 13, 2006 #4
    You don't need to know v1 or v2, you only need to know (v1-v2). And in this case (v1-v2) is the current through the resistor multiplied by the resistor value. Use KCL to find the current through that resistor (easy if you use the right node).

    When you only care about voltage differences, the absolute value doesn't matter. You could define v2 to be ground, or you could define it to be 37. In either case, you will always get (v1-v2)=v0. But as you said, since there is no reference voltage, you can't find the absolute value of any of the nodes individually.
  6. Jul 19, 2010 #5
    According to KCL and KVL, any node should result the same. Only difference is some nodes are appearing easier than other depending on how complicated they breach out from the node.Usually, I would select the node which will involve the least equations which will provide more convenient way to solve the simultaneous equations.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook