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Homework Help: Parallel Circuit Problem. Please help

  1. Aug 5, 2016 #1
    1. The problem statement, all variables and given/known data
    Resistor R1, R2, R3 are connected in parallel, have an equivalent resistance of 720(ohms). The currents are labeled I1, I2,and I3. I2 is 3x the value of I1, and I3 is 6x the value of I1. Calculate R1, R2, R3.

    2. Relevant equations
    I used the parallel circuit equation. 1/RT=1/R1+1/R2.....

    3. The attempt at a solution

    I tried to solve it like this....

    1/720= 1/R1+1/(1/3R1)+1/(1/6R1)---> 720=R1+3R1)+6R1-----> 720= 10R1-----> R1=72 ohms.
    I know I made a mistake or I have been following the wrong path. Please help I really don't understand this question. Thank You!
  2. jcsd
  3. Aug 5, 2016 #2


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    Staff: Mentor

    Can you upload a copy of the problem/schematic? :smile:
  4. Aug 5, 2016 #3
    I'm so sorry, this was a word problem that came from a classwork that the teacher gave me. There was no diagram that came with it.
    I have seen one person who tried to solve it in this website ---> https://answers.yahoo.com/question/index?qid=20140911191648AArMp36
    but he didn't explain it, he just showed his equation so I still didn't understand how he arrived on his answers.

    If you could help, then thank you!
  5. Aug 5, 2016 #4


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    Science Advisor

    With the same voltage across the resistors (parallel circuit), when the current is higher, is the resistance higher or lower?
    Then look at the equation you generated. Does it indicate higher or lower resistances for the associated currents?
  6. Aug 6, 2016 #5
    When the current is higher the resistance is lower because current and resistance are inversely proportional.
    Based on what you said, my equation indicates that it has higher resistance when I simplify it.
  7. Aug 6, 2016 #6


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    Homework Helper
    Gold Member

    Equivalent resistance of the parallel combination is less than the least of R1, R2 and R3.
    Voltage across all the resistors is same since they are in parallel. Use Ohm's law to find the resistances.
  8. Aug 6, 2016 #7
    Yea but the amount of voltage is not given so how do I even use Ohm's Law?
  9. Aug 6, 2016 #8


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    Homework Helper
    Gold Member

    So, I1R1=I2R2=I3R3..
    You know the relation amongst I1, I2 and I3.
    Last edited: Aug 6, 2016
  10. Aug 6, 2016 #9
    Thank you so much! I got it!
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