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Homework Help: Nodal voltage?

  1. Feb 15, 2004 #1
    nodal voltage??

    im not real sure on how solve circuits using the nodal method. ive got an equation for node a but i dont know how to solve it. heres the equation
    (90-Va)/20 - (Vb-Va)/3 + Va/20 - Ia = 0

    thats at node a. how do i solve this?
    Ia = 6 A
     
  2. jcsd
  3. Feb 16, 2004 #2
    What kind of circuit u are dealing Will u pls provide the pic
     
  4. Feb 16, 2004 #3

    HallsofIvy

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    Science Advisor

    Taking Ia= 6, you still have two unknown quantities in your equation: (90-Va)/20 - (Vb-Va)/3 + Va/20 - 6 = 0 so you can't "solve" it for specific values. You could solve for either Va or Vb "in terms of" the other.

    (90-Va)/20= 9/2- Va/20 and (Vb- Va)/3= Vb/3- Va/3 so simply multiplying the terms in parentheses gives

    9/2- Va/20- Vb/3+ Va/3+ Va/20- 6= 0.

    The "-Va/20" and "+ Va/20" terms cancel so

    Va/3- Vb/3- 12/2+ 9/2= 0 or Va/3- Vb/3= 3/2.

    Then Va= 3(3/2+ Vb/3)= 9/2+ Vb or Vb= -3(3/2- Va/3)= Va/3- 9/2.

    You might just need Va- Vb= 9/2.
     
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