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What is the resistance between two resistors?

  1. Dec 1, 2016 #1
    1. The problem statement, all variables and given/known data
    upload_2016-12-2_11-24-19.png

    2. Relevant equations
    R=V/I

    3. The attempt at a solution
    Would it be 10ohms? Since PQ is between two 10ohms resistor.
     
    Last edited by a moderator: Dec 1, 2016
  2. jcsd
  3. Dec 1, 2016 #2

    berkeman

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    Are the two resistors in parallel or series or some other configuration?

    Hint -- what do parallel resistors share? :smile:
     
  4. Dec 1, 2016 #3
    They have the same voltage across both resistors? But the voltage across the two resistors is after the voltage has been reduced by the first resistor right? I still am not seeing how I can solve the equation with just these information. :<
     
  5. Dec 1, 2016 #4

    berkeman

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    They are on purpose trying to make the question confusing. one terminal goes to one side of each resistor, and the other terminal goes to the other side of each resistor. Are the resistors in parallel?
     
  6. Dec 2, 2016 #5

    CWatters

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    Homework Helper

    Try colouring all wires connected to "P" one colour and all wires connected to "Q" another colour.
     
  7. Dec 2, 2016 #6
    But how would I know the resistance just from the resistance of the other two resistors?
     
  8. Dec 2, 2016 #7

    gneill

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    What does your textbook or course notes say about how resistors combine to create a net resistance?
     
  9. Dec 2, 2016 #8
    In parallel, the resistors combine by the rule 1/r1 +1/r2 = 1/rTotal, in series the resistors combine by r1+r2=rTotal. So the net resistance in this circuit would be rTotal= r1+ 1/(1/r1 +1/r2)
     
  10. Dec 2, 2016 #9

    gneill

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    How do you justify that? Can you explain your reasoning?
     
  11. Dec 2, 2016 #10
    As in a series, the total resistances are simply added up, and in parallel, the resistances are added up by 1/r1 +1/r2 = 1/rTotal. So if I added up the parallel resistors and pretend it is just 1 resistor with a resistance of 1/(1/r1 +1/r2)=rX, then the total resistance in the circuit would be rX+r1(Where r1 is the 10ohms resistor in the series)=rTotal.
     
  12. Dec 2, 2016 #11

    berkeman

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    Very good. So in the end, are these two resistors in series or parallel? What is your final answer? :smile:
     
  13. Dec 3, 2016 #12
    The unknown resistor and 1 of the known resistor are in parallel, but on the whole both are in series with another known resistor. Yet I still don't understand how to convert all of these into the value of the unknown resistor. :(
     
  14. Dec 3, 2016 #13

    NascentOxygen

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    No.

    Once you have accounted for a resistor, you can't then include it in a second accounting.

    Has anyone told you to redraw the circuit, using as many trials as is necessary, until you can draw them unequivocally as a pair of resistors in either (a) a series arrangement, or (b) a parallel arangement. It must be one or the other.
     
  15. Dec 3, 2016 #14
    Actually, I used v1 both times as the two resistors provided are of the same ohms, and I was too lazy to change its symbol. So you can treat one of the v1 as vb or vy. Sorry for the confusing equations xD. Also, I did convert the circuit into a series, by adding the parallel resistors together and treat it as a series along with the 10ohms resistor.
     
  16. Dec 3, 2016 #15

    NascentOxygen

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    I have no idea what all that is about.

    ►► So, is your answer to the question that started this thread now A, B, C or D?
     
  17. Dec 3, 2016 #16
    I still have no idea, I tried totaling the resistance on the parallel resistors (Rtotal) and treat it as a series with the 10ohms resistor. I then assumed that the current that passes out of the 10ohms resistor is equivalent to the current into the parallel resistors:

    Current out of 10ohms resistor: I=V/10

    Current into parallel resistors: V/10 = I1+I2
    V/10 = V1/10 + V1/X (As the voltage is the same between parallel resistors where V1 is the voltage)

    but I still cannot solve for x.
     
  18. Dec 3, 2016 #17

    NascentOxygen

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    Why do you keep referring to 3 resistors when there are only 2?

    Once you have worked out the equivalent resistance of the pair of resistors, then STOP! That is your answer! There is nothing more to be done!

    The question being addressed is: with what single resistor could you replace this two resistor arrangement so that the current between P and Q would be unchanged (obviously when some voltage is applied between P and Q).
     
  19. Dec 3, 2016 #18
    So since the known resistors are in a series, do I just add them together to get the unknown resistance? I'm really confused ._.
     
  20. Dec 3, 2016 #19

    NascentOxygen

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    You have not yet determined whether the two resistors are in a parallel arrangement, or are in series?

    How would you recognize a parallel arrangement? How would you recognize a series arrangement?
     
  21. Dec 3, 2016 #20
    A series arrangement would be where the resistors are connected next to each other. Parallel would be where the path of the resistors are divided. So the unknown resistor and 1 of the known resistor are in parallel. There is also one known resistor of 10ohms that are in series to the parallel resistors. Right?
     
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