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Combined resistance in this circuit

  1. Dec 31, 2017 #1
    1. The problem statement, all variables and given/known data
    upload_2017-12-31_21-5-20.png
    What is the combined resistance when the switch is closed?
    2. Relevant equations
    IR=V

    3. The attempt at a solution
    I found that I1=5.2A and that I2=9.1A. The answers follow suit.
    Now I thought that calculating the combined resistance with 60/(I1+I2) would be correct, however the answer is 4Ω.
    Can anybody help me out in this? Thanks.
     
  2. jcsd
  3. Dec 31, 2017 #2

    phinds

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    Gold Member

    Show your work. Show how you combine various resistors to get the total resistance
     
  4. Dec 31, 2017 #3
    I didn't think I should do this. Why is it wrong to just divide the voltage by the total current? This gives 4.2 instead of 4. We didn't study how to really combine the resistors when they are connected like this.
     
  5. Dec 31, 2017 #4

    phinds

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    You have computed something with the switch open and you are applying it when the switch is closed. That just flat does not work. You HAVE to figure out the combined resistance with the switch closed.
     
  6. Dec 31, 2017 #5
    What do you mean? These currents are for when the switch is CLOSED, not open.
     
  7. Dec 31, 2017 #6

    phinds

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    They how could you possibly have gotten it without combining the resistors?
     
  8. Dec 31, 2017 #7

    gneill

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    How do you know that the answer is 4Ω ? Was this a multiple choice question? Or perhaps they want the result rounded to an appropriate number of significant digits?
     
  9. Dec 31, 2017 #8
    I thought it doesn't matter how I reached those currents. I did combine the resistors in order to reach those current, and they are correct.
     
  10. Dec 31, 2017 #9
    Maybe they did but it would be very unusual. This is the answer written in the answers as correct.
     
  11. Dec 31, 2017 #10

    phinds

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    So you don't think you should combine the resistors but you did combine the resistors?
     
  12. Dec 31, 2017 #11

    phinds

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    Using a delta-wye transform I get an equivalent resistance that is compatible with the sum of the currents you stated, and of course, it's not 4 ohms. Since you combined the resistances, you MUST have gotten the same thing I did, else you could not have gotten the right total current.
     
  13. Dec 31, 2017 #12

    gneill

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    This method is correct provided that your current values are correct. If, for correct current values, your result doesn't match the supplied answer for the total resistance with the appropriate number of significant figures for the given data, then that supplied "correct" value is not accurate.
     
  14. Dec 31, 2017 #13
    Alright, thanks for the help!
     
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