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Simple circuit analysis - can someone check if it's right?

  1. Oct 29, 2008 #1
    simple circuit analysis -- can someone check if it's right?

    1. The problem statement, all variables and given/known data

    (circuit can be found on http://www.MailFreeOnline.com/uploader/A723E91C.jpg)

    i. write down equations relating i1 and v1, i2 and v2 and i3 and v3
    ii. solve for the voltages v1, v2 and v3
    iii. determine the currents i1, i2 and i3


    2. Relevant equations
    KVL
    KCL


    3. The attempt at a solution
    i.

    i1 = 3 - V2
    i2 = 2 - 2V2
    i3 = V3/10


    ii.
    i1 + i2 = i3
    3 - 2V1 + 2 - 2V2 = V3/10
    therefore 5 - 2(V1 + V2) = V3/10

    iii.
    (this is the hard part for me. have i used KVL right?)
    first window:
    -1.5 + 1 + V1 + V2 = 0
    therefore v1 + v2 = 0.5

    second window:
    -1 + V2 + v3 = 0
    therefore V2 + v3 = 1

    overall window:
    V1 + V2 + V3 = 1.5
     

    Attached Files:

    Last edited: Oct 29, 2008
  2. jcsd
  3. Oct 29, 2008 #2
    Re: simple circuit analysis -- can someone check if it's right?

    hey if you host your image on imageshack.us and past the link into the forum, the image can be viewed in the post without the approval of the admin. not saying that the admin will block it, but it will allow the image to be viewed sooner

    if i see the image i may be able to help
     
  4. Oct 29, 2008 #3
    Re: simple circuit analysis -- can someone check if it's right?

    hey. sorry about that. here's a link to the circuit: http://www.MailFreeOnline.com/uploader/A723E91C.jpg
     
  5. Oct 30, 2008 #4
    Re: simple circuit analysis -- can someone check if it's right?

    Think about what you did for I1 and I2, the left side of the equation is in the units of Ampere, and the right side of the equation is in the units of Voltage.

    Stick with Ohm's Law to find I1 and I2, similar to what you did for I3
     
  6. Oct 30, 2008 #5
    Re: simple circuit analysis -- can someone check if it's right?

    huh? isn't i1 = (1.5 - V1)/0.5, for example?
     
  7. Oct 30, 2008 #6
    Re: simple circuit analysis -- can someone check if it's right?

    You're right, sorry about that.

    So how did you get I1 = 3 - V2? Solving it your way, wouldn't it be 3 - 2V1?
     
  8. Oct 30, 2008 #7
    Re: simple circuit analysis -- can someone check if it's right?

    whoops sorry... it must have been a typo. how about part 3?
     
  9. Oct 30, 2008 #8
    Re: simple circuit analysis -- can someone check if it's right?

    Looking at your steps, it looks like you're doing okay. But somehow I can't agree with you that V1 + V2 + V3 = 1.5. I don't think you can combine it like that. I want to say that V1 = V2 = V3 because they're the same node.

    (but then your equation V1 + V2 = 0.5 and V2 + V3 = 1 no longer makes sense...)
     
  10. Oct 30, 2008 #9
    Re: simple circuit analysis -- can someone check if it's right?

    hmm... what does that mean?
     
  11. Oct 30, 2008 #10
    Re: simple circuit analysis -- can someone check if it's right?

    Perhaps redrawing the circuit will help you.
     

    Attached Files:

    • ckt.pdf
      ckt.pdf
      File size:
      10.5 KB
      Views:
      31
  12. Oct 30, 2008 #11
    Re: simple circuit analysis -- can someone check if it's right?

    ah i see what you mean now. so where did i go wrong? i thought my KVL logic was correct...
     
  13. Oct 30, 2008 #12
    Re: simple circuit analysis -- can someone check if it's right?

    also, isn't it true that i1 + i2 = i3? if you plug in my values, you get something else.
     
  14. Oct 30, 2008 #13
    Re: simple circuit analysis -- can someone check if it's right?

    I vaguely remember the windows separation method, like what you did. But I think you're supposed to follow the path of the current, so you can't do something like

    -1.5 + V1 + V2 + 1 = 0

    Instead, you're supposed to go to R3,

    -1.5 + V1 + V3 = 0

    It's been a while since I've done it, so maybe? heheh.. but if you look at the circuit I came up with, it should be more clear.

    And yes, I1 + I2 = I3. What do you mean by that?
     
  15. Oct 30, 2008 #14
    Re: simple circuit analysis -- can someone check if it's right?

    if you substitute in my (supposedly correct) equations for i1, i2 and i3, you don't reach the conclusion that V1 = V2 = V3...

    also, using the window method, i'm sure you can inspect every window individuall. i just don't know how to do it lol.
     
  16. Oct 30, 2008 #15
    Re: simple circuit analysis -- can someone check if it's right?

    I do believe the right way to solve this is to assume that V1 = V2 = V3. Let's look at the windows of the final circuit I drew then.

    I1 = (Vs1 - V3) / R1
    I2 = (Vs2 - V3) / R2

    But we also know that V3 = I3 * R3 by Ohm's Law.

    By KCL, we know that I3 = I1 + I2.

    V3 = (I1 + I2) * R3 and plug in I1 and I2 from above, and you should have V3, which is equals to V1 and V2. From there you should be able to solve for I1 and I2.
     
  17. Oct 30, 2008 #16
    Re: simple circuit analysis -- can someone check if it's right?

    ah thansk for that. i'm just waiting to see your circuit (it's still being approved).
     
  18. Oct 30, 2008 #17
    Re: simple circuit analysis -- can someone check if it's right?

    d'oh! didn't read the rules =]

    http://rapidshare.com/files/159049612/ckt.pdf.html

    for your convenience..
     
  19. Nov 8, 2008 #18
    Re: simple circuit analysis -- can someone check if it's right?

    Could anyone solve this problem
     

    Attached Files:

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