# Nodal analysis with current source

Discussion in 'Homework Help' started by Monique Nogueira, Oct 16, 2015.

1. ### Monique Nogueira Thread Starter New Member

Oct 15, 2015
2
0
I need to solve this question using nodal analysis, but I'm not sure what to do with that current source right there. I could only find I1 = 1A, which I think it's the right answer. Using the EveryCircuit app I found I2 = 2A, I3 = 500mA, I4 = 3.5A and I5 = 2.5A, but I just can't figure out how to find the answers Can you guys help me find the right equations?

2. ### RBR1317 Active Member

Nov 13, 2010
270
54
If you need to solve this question using nodal analysis, then you would need to write the node equations to solve for the node voltages. Why are you solving for the currents instead? Which of the nodes have unknown voltages to be solved? What is the method for handling a current source in a node equation?

Feb 19, 2010
3,518
515
Too funny.

4. ### Monique Nogueira Thread Starter New Member

Oct 15, 2015
2
0
RBR1317: So, I just ignore the current source?
shteii01: Why is that funny? ._.

5. ### bwack Active Member

Nov 15, 2011
113
10
You can ignore the current source for R1 because of the voltage sources (Use kirchhoff's voltage law), and you got it right.
Write the node equations using kirchhoffs current law. I suppose you can do two nodes, two equation and solve them algebraically.

Monique Nogueira likes this.
6. ### RBR1317 Active Member

Nov 13, 2010
270
54
Nodal analysis applies the current law to each node; however, each current is expressed in the node equation as the difference between adjacent node voltages divided by the resistance between the adjacent nodes - except when there is a current source, in which case the current is equal to the value of the source. Note that a node equation is not written for the ground node, or for any node that forms a supernode with the ground node (such as you have in this problem).

7. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,518
515
My textbook has explanation on how to apply Node-Voltage Method. What assumptions are made. What procedure to follow. Which brings up a question. Why aren't you using your textbook?

See? I told you that you are being funny.

8. ### WBahn Moderator

Mar 31, 2012
18,093
4,920
Are we supposed to be mind readers who can divine what currents you are talking about when you throw around I1, I2, I3, I4, and I5? Even if we assume that they correspond to the resistor designators, you give no indication of what direction each current is flowing, which is critical.

Don't you think it might, just possibly, be useful to indicate what those current designations refer to?

How can we possibly tell you whether your equations are right or wrong when you don't show us any of your equations at all? Back to that mind reading assumption?

How are you dealing with the voltage sources in your nodal equations? Those are the tricky part -- current sources in nodal analysis are very easy to deal with. But, without seeing some work, how can we possibly help you find what you are doing right and what you are doing wrong? More mind reading, I guess.

9. ### RBR1317 Active Member

Nov 13, 2010
270
54
This is your circuit slightly modified to eliminate distractions and facilitate writing the node equations. Write the node equations to solve for the node voltages that are unknown. With enough experience you should be able to redraw the circuit in your mind as you write the node equations directly.