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Calculating the missing resistance when only given voltage

  1. Dec 13, 2016 #1
    1. The problem statement, all variables and given/known data
    I am asked to calculate the missing resistance(R5) when only given the voltage drop of of 14v across it

    2. Relevant equations
    current and divider rule
    ohms law
    3. The attempt at a solution

    i tried finding the current of the 6+10 ohm series resistor that is parallel to R5, which would mean they have the same voltage drop. Not sure where i would go from here since i don't know the total resistance, i can't find the total current that is being split between R5 and 6+10 resistor.
     

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    Last edited: Dec 13, 2016
  2. jcsd
  3. Dec 13, 2016 #2

    nrqed

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    Hello,

    We would need the actual question with all the information provided to help you. Can you provide more details?
     
  4. Dec 13, 2016 #3
    whoops sorry, reuploaded the circuit
     
  5. Dec 14, 2016 #4

    nrqed

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    Ok, thank you.

    As a first step, think of the resistors R3, R4 and R5 as a single equivalent resistance, let's call it Req. The voltage across this Req will be 14 volts too. Now, just from the information given, can you determine what must be the value of this Req?
     
  6. Dec 14, 2016 #5
    3 and 4 are in series, which is in parallel to R5? would i not require current to find the value of Req, since i am missing the value of R5
     
  7. Dec 14, 2016 #6

    nrqed

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    What I suggest is to do the problem in two steps. As a first step, it is better to forget completely about R3, R4 and R5. Just call the combination of these three Req. Can you figure out the value of Req? Once you will know that, we will figure out R5.
     
  8. Dec 14, 2016 #7

    nrqed

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    Hint: you know the potential of the battery (28 volts), you know the other resistors in the circuit an you know that there is 14 volts across Req. This is enough to find Req.
     
  9. Dec 14, 2016 #8
    voltage divider rule?
     
  10. Dec 14, 2016 #9

    nrqed

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    Yes, that's one way to do it. Here the numbers given make the calculation very easy.
     
  11. Dec 14, 2016 #10
    so i got 14 = 28 x Req/ R(1+2+2)
    = 2.5?
     
  12. Dec 14, 2016 #11

    nrqed

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    I am not sure what R(1+2+2) means.

    The correct equation is ##14 = \frac{28 R_{eq}}{1+2+2+R_{eq}}##
     
  13. Dec 14, 2016 #12
    forgot to add Req back, got a vlaue of 10 for R5
     
  14. Dec 14, 2016 #13

    nrqed

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    Good job!
     
  15. Dec 14, 2016 #14
    thank you for the help
     
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