1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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 Problem - please help

  1. Aug 20, 2014 #1
    Hi, V1 for this problem is 400mA and no matter how I work this problem I cannot get the correct answer.
    V2=V1+6
    For the second equation, I don't know what to do with 14V. I've tried subtracting it from V1 divided by the 4 ohm resister, most logical, and that doesn't work. I've tried multiple things and I'm having no luck. Please point me in the right direction?
     

    Attached Files:

  2. jcsd
  3. Aug 20, 2014 #2

    donpacino

    User Avatar
    Gold Member

    1. V1 is a voltage and 400 mA is a current. I'm not sure whether you fundamentally do not understand the difference between the two, or you communicated it to us wrong.

    2. you should solve this problem using a super-node as described in your previous post. Try writing a kcl using the current going out of the V1-V2 supernode.
     
  4. Aug 20, 2014 #3
    Yes I meant mV and I do know the difference thanks. I have tried so far:
    I1=i2+i3+i4
    I1+i2+i3+i4=0
     
  5. Aug 20, 2014 #4

    donpacino

    User Avatar
    Gold Member

    so your first equation was incorrect.
    your second equation is correct.

    I1+i2+i3+i4=0

    using ohms law, we know that I=V/R
    using that equation replace the current with the voltage difference across the resistor over the resistance. Instead of using V2, use V1+6.

    I will do i4 for you
    I1+I2+I3+ ((V1+6)-0)/6 =0
     
  6. Aug 20, 2014 #5
    Alright thanks! But now I have a more baffling aspect to consider: why is i3 equal to v2/r and i4 equal to (v1+6)/r?????
     
  7. Aug 21, 2014 #6

    NascentOxygen

    User Avatar

    Staff: Mentor

    Why have you introduced "r" when there is no such element shown on your schematic?

    What equation should you write to relate i3 and v2?

    There's that mysterious "r" again? :confused:

    Can you write the equation relating i4 to R4?
     
  8. Aug 21, 2014 #7
    Yeah, R is resistance. Is this because i4 is not included as part of the supernode? i3 is equal to v2/R3, so why isn't i4 equal to v2/R4?
     
  9. Aug 21, 2014 #8

    NascentOxygen

    User Avatar

    Staff: Mentor

    There is no R3 element on you schematic!

    If there were a resistor R4 and it had V2 volts across it, then its current certainly would be V2/R4. But I can see no R4 on your schematic. I presume you are referring to the 6Ω resistor?

    Components should be labelled so you can refer to them by that label.
     
  10. Aug 21, 2014 #9

    donpacino

    User Avatar
    Gold Member

    V2=V1+6...

    When you have two nodes directly linked by a voltage source do not label them V1 and V2.
    Assuming the voltage increase is Vs, label them V1 and V1+Vs.

    does that make sense?

    also olny refer to resistor values as r or r3 when then are labeled that way in the schematic. referring to elements in a way other than how they are labeled creates confusion.
     
  11. Aug 21, 2014 #10
    Yes, why isn't i4 equal to V2/6 if i3 is equal to V2/2? Is it because i4 is not part of the supernode?
     
  12. Aug 21, 2014 #11

    donpacino

    User Avatar
    Gold Member

    replacing the v2 with v1+6 is a way of reducing variables simplifying the equations. if it is confusing to you feel free to keep the v2 in the equation.
    With more complicated circuits this will add another level of confusion
     
  13. Aug 21, 2014 #12

    donpacino

    User Avatar
    Gold Member

    You say yes it makes sense, but then you use the same faulty logic...

    you already said that V1+6=V2

    so if i4=V2/6, then i4=(V1+6)/6

    does that make sense?

    If it does not i suggest you open up an algebra textbook
     
    Last edited: Aug 21, 2014
  14. Aug 21, 2014 #13

    NascentOxygen

    User Avatar

    Staff: Mentor

    It is! And V2 = V1 + 6, so you can make that substitution at the same time.

    It seems you are attempting a problem for which you lack the necessary background. I suggest that you forget about the concept of a supernode, you don't need it, and first solve this exercise by any means you are familiar with to find all currents and voltages. It is clear that you need lots of practice, and the only way you will gain practice is by attempting exercises.

    Can I conclude that you have obtained the solution from someone else and are trying to understand his working? You will learn a lot more by attempting the problem without another person's solution.

    Try it yourself, any way you know.
     
  15. Aug 21, 2014 #14
    Thank you all very much. I have an AS in electronics and have been working as a tech for years, but I am forced to use alternative methods to solve circuits now while pursuing my EE degree. If you want to degrade me and ask if I know anything about this subject that's unfair. If I was as stupid as some of you suggest I wouldn't be asking questions. But thank you all for your help, but please try to be respectful.
     
  16. Aug 22, 2014 #15

    donpacino

    User Avatar
    Gold Member

    Did not mean to be degrading mate. best of luck to you!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Nodal Analysis Problem - please help
  1. Nodal analysis help (Replies: 2)

Loading...