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Voltage across resistors - Series & Parallel

  1. Nov 30, 2011 #1
    Hey, I have a circuit which you can see in the images attached.

    I had 3 questions and I *THINK* I've answered one of them. But I can't seem to get to the next part. Here's the questions below

    Questions:
    a) The total Resistance (Rt) of the circuit.
    b) The voltage across each resistor, V1, V2, V3
    c) The currents I1, I2, I3, I4

    Attempts:

    a) What I have done so far is RT = 1/(1/330 + 1/470 + 1/650) = 144Ω
    Series + Parallel = Rt = 220 + 680 = 900Ω (Series)​
    900Ω + 144Ω = 1.044kΩ​

    b) *Haven't really gotten that far* all that I've gathered is V= I * R I only have the variable R so I'm not sure what to do :(

    Cloud2go%2030%20Nov%202011%2013:35.jpg
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 30, 2011 #2
    You have the total resistance and the battery voltage so you can calculate the current from the battery. V1 and V3 should then be straightforward....
     
  4. Nov 30, 2011 #3
    Thank you! Can't believe I didn't think of that :( What about V3 ? Do I calculate each resistor & then add them together?
     
  5. Nov 30, 2011 #4
    Alright I'll try that.. The current through R1 is 9.578mA and I suppose R5 should be the same ?
     
  6. Nov 30, 2011 #5
    Can you now find V2? The resistors R2, R3 and R4 are in parallel
     
  7. Nov 30, 2011 #6
    Well because they're in parallel wouldn't the voltage remain the same throughout each resistor? so If I fidn the voltage for one would they all be the same? My teacher is really bad sorry if these seems stupid !
     
  8. Nov 30, 2011 #7
    You are correct, the voltage across R2, R3 and R4 is the same (do you know what it is)
    So now you can calculate the current through R2, R3 and R4.....
     
  9. Nov 30, 2011 #8
    Sorry but I can't think of a formula to calculate the voltages for the parallel circuit :(
     
  10. Nov 30, 2011 #9

    gneill

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    Staff: Mentor

    What's the equivalent resistance of the parallel resistors?
     
  11. Nov 30, 2011 #10
    144Ω I believe...
     
  12. Nov 30, 2011 #11
    You should be able to calculate V1 and V3....which should then give you V2 !!!!
     
  13. Nov 30, 2011 #12

    gneill

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    Staff: Mentor

    If that's the case, what's the voltage drop across that resistance?
     
  14. Nov 30, 2011 #13
    I don't know how to work the v.d out :( I have it drawn on pcbwizard so I can check my answers but I need to be able to do manual way :/
     
  15. Nov 30, 2011 #14

    gneill

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    Staff: Mentor

    Ohm's law! You've already calculated the current, right?
     
  16. Nov 30, 2011 #15
    I have the current for R1 and R5, not the parallel circuit though ?
     
  17. Nov 30, 2011 #16

    gneill

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    Staff: Mentor

    Surely it's the same current? R1 and R5 are in series with the equivalent resistance of R2||R3||R4. Note: The individual currents through the parallel resistors must sum to that.
     
  18. Nov 30, 2011 #17
    Yes R1 and R5 are the same current, but I don't get the formula which links the equivalent current from the parallel to series ? If that makes sense.. :/
     
  19. Nov 30, 2011 #18

    gneill

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    Staff: Mentor

    R1, R5, AND the equivalent resistance for the parallel resistors all share the same current.
     
  20. Nov 30, 2011 #19
    I realise that R1 and R5 have equal current. I also realise that if you add all of the parallel resistors up you receive the equivalent current of R1 and R5, but what I don't get is how you work the current out for the parallel circuit? I only realise that they are the same because I'm using a circuit simulator to try and help me...
     
  21. Nov 30, 2011 #20

    gneill

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    Staff: Mentor

    Let's call the equivalent resistance of the three parallel resistors R234. Then R1 and R234 and R5 are in series. They all MUST have the same current because they are in series.
     
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