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!

Homework Help: Combination Please Help

  1. Mar 12, 2007 #1
    1. The problem statement, all variables and given/known data
    PROBLEM.jpg


    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

    User Avatar
    Science Advisor
    Homework Helper

    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
    PROBLEM2.jpg
    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

    User Avatar
    Science Advisor
    Homework Helper

    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

    User Avatar
    Science Advisor
    Homework Helper

    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

    User Avatar
    Science Advisor
    Homework Helper

    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:
    PROBLEM3.jpg
     
  13. Mar 12, 2007 #12

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    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

    User Avatar
    Science Advisor
    Homework Helper

    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

    User Avatar
    Science Advisor
    Homework Helper

    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

    User Avatar
    Science Advisor
    Homework Helper

    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.
     
  22. Mar 12, 2007 #21
    I'm pretty sure that V4 as 34.4V is right.. and V2 and V3 definitely aren't..

    In a parallel circuit, all !branches have the same voltage.. That means that the combined voltage (added) of R2 and R3 equal to the same voltage as R4's voltage..

    You have R4, V4, figure out I4..

    I(t)=I1=I(2,3,4).. In a combo circuit, total current I(t) is equal to/same in all series resistors (I1), or the sum of all current in a parallel (I2+I3+I4=It)..

    Now if you can figure I4..
    It-I4=I2+I3..
    and I2+I3 is the same thing as I(2,3) since in a series circuit, current is the same throughout...
     
  23. Mar 12, 2007 #22

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    Be careful, pugfug. He's new at this game and doesn't need to be confused. I2=I3!!! Not I2+I3=I(2,3)!!?
     
  24. Mar 12, 2007 #23
    so the voltage for V2 is 17.2 and V3 = 17.2--combined voltage of R2 and R3 = R4?
     
    Last edited: Mar 12, 2007
  25. Mar 12, 2007 #24

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    How can V2=V3 when they have the same current through them and different resistances??!!! Review Kirchoff, take a break and try the problem again.
     
  26. Mar 12, 2007 #25
    V4 = 34.4
    I4 = 17.2
    R4 = 2
    P4 = 591.68
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook