Node analysis help

Discussion in 'Homework Help' started by Learnphysics, Nov 5, 2010.

  1. Learnphysics

    Thread Starter New Member

    Nov 5, 2010
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    [​IMG]


    I'v set up the equation for node v1 node, but v2 node confuses me.

    would it be correct to assume that the 1A current flows into node 2? Or does only part of it flow into node 2, (with the other part going down the 10 ohm resistor into ground)

    How do i find out how much current that 1A source is contributing into node 2.

    Also when setting up the KVL for node 1, i have (v1/5ohms) + (v1 - v2)/5ohms... but i can't figure out the current comming through the 10v Voltage source to V1. I coudl do V1-V2, and then divide it by the resistance of the voltage source. but i don't know the resistance of the voltage source.

    I'v been at this question for a very long time, and managed to get a whole range of answers from -4 to ~6V. The back of the book says the answer is -3.33V.

    How do i do this? I have a feeling i'm missing some fundemental rule with this.

    the two equations i got using nodal analysis are:
    at V1 I have

    (V1/5) + Is -2 = 0

    and at V2 i have

    Is + 2 + (v2/10) +1 = 0

    Please help
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    HINT: For the purpose of calculating the voltage at node v2, the 5 ohm resistor in parallel with the 10V dc source can be ignored. This is the case because an ideal voltage source can be considered a zero resistance.

    Once you have calculated the voltage v2, you will then be able to calculate the current contribution to Is by the 5 ohm resistor in series with the 10V dc source.

    Don't forget to consider the current contribution to Is of the 5 ohm resistor in parallel with the 10V dc source.

    hgmjr
     
  3. Learnphysics

    Thread Starter New Member

    Nov 5, 2010
    2
    0
    Hmm, Maybe i'm not setting up my equations right. Is there any particular directions for the currents? (the textbook said to use arbitrary directions)

    My equations were:

    V2 : Is + 1 - (v2/10) = 0

    V1= V2 + 10

    V1: -(v1/5) -Is =0

    Do i consider the 5ohm resistance when calculating the equation for V1?

    If so V1's equation would be

    V1: -(v1/5) - Is -(v1-v2)/5 = 0

    Are these equations set up right?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Initially let's focus on calculating V2. In this case, your equation above needs a rethink.

    Start by redrawing the circuit with the parallel 5 ohm resistor absent from the circuit. This means that you will be calculating the current in the left loop but this current is only part of Is.

    With this simplification and using KCL as you are trying to do, write your KCL equation to find V2.

    Once you have that part of the calculation in hand we will move to the next step in the calculation of Is.

    hgmjr
     
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