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Combination Please Help

  1. Mar 12, 2007 #1
    1. The problem statement, all variables and given/known data
    [​IMG]


    2. Relevant equations
    P = IV
    V = IR


    3. The attempt at a solution
    I know how to solve the problem once i figure out which parts are parallel and which parts are series. So would it be R2 and R3 are in parallel and R4 is series, but what about R1? i have done a problem similar to this but without the R1 on the side (shown in the picture above)...if anyone can explain to me what is different about this, i would really appreciate it.:smile:
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Mar 12, 2007 #2

    Dick

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    I think you misspoke. R2 and R3 are in series. R4 is in parallel with them. R1 is in turn in series with the combination of the other 3. Yes?
     
  4. Mar 12, 2007 #3
    yeah, I got that a little mixed up there! thank you for the help!
     
  5. Mar 12, 2007 #4
    [​IMG]
    I think i've got it now, if anyone can double check my work, it would be greatly appreciated (still not sure if im doing it correctly, but i think ive got the idea)
     
  6. Mar 12, 2007 #5

    Dick

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    Well, Rt looks good. Lots of problems elsewhere. Why on earth would you say V1=120?? Since the sum of the voltages around a loop is zero? And how could I1 not equal It??? Where is the rest of the current going?
     
  7. Mar 12, 2007 #6
    I dont really know, im gonna go back and try again: we just learned this today, so im really new at this.
     
  8. Mar 12, 2007 #7
    so V1= 0 and I1 = 0??
     
  9. Mar 12, 2007 #8

    Dick

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    V1 is not 0V and it's not 120V. Are you just guessing? What is the total current running through the circuit? Hint: You got that right. How much of it goes through R1? THINK about that!
     
  10. Mar 12, 2007 #9
    all of it. 21.4
     
  11. Mar 12, 2007 #10

    Dick

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    That's better. Now you know I and R for R1. Find V1. Now continue stepping through the circuit...
     
  12. Mar 12, 2007 #11
    What I've got so far:
    [​IMG]
     
  13. Mar 12, 2007 #12

    Dick

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    Ok. Now do V4. Sum of voltages around a closed loop is zero. You are keeping track of signs in your head, right?
     
  14. Mar 12, 2007 #13
    so V4 = 85.6 because it is in parallel
     
  15. Mar 12, 2007 #14

    Dick

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    It's NOT in parallel with R1! It's in series! Draw a loop through the battery including R1 and R4. Sum of all those voltages better be zero.
     
  16. Mar 12, 2007 #15
    well, since it is in series, wouldnt that make I4 = 21.4? so that would make V4 = 42.8
     
  17. Mar 12, 2007 #16

    Dick

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    No. It's not really in 'series'. Part of the 21.4A flows through R4 and part of it flows thru the R2 R3 combo. Concentrate on V4. Concentrate on the loop I suggested. Vbattery+V1+V4=??????
     
  18. Mar 12, 2007 #17
    R(2,3,4) are in series with R1.. That means that V1+V(2,3,4)=Vt.. Therefore, V(2,3,4) equals 34.4V (120-85.6)..And in a parallel circuit, voltage is the same across each !branch.. That means that 34.4V=V(2,3,4)=V4=V(2,3)

    And the parentheses is used to indicate how they are when combined..

    Now that I've showed you V4, you can get I4..
     
    Last edited: Mar 12, 2007
  19. Mar 12, 2007 #18
    V4 = 34.4, am i understanding that correctly?
    34.4 +85.6 = 120
    and then V2 = 34.4 and V3 = 34.4
     
  20. Mar 12, 2007 #19

    Dick

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    Nooooo. V2+V3=34.4V. Please review Kirchoff laws, ok?
     
  21. Mar 12, 2007 #20
    Sorry for all of the questions, i am not very familiar with this material, we only touched on it a little today for the first time... i will go look up Kirchoff laws and see if that will help some.
     
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