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Double-grounded circuit, find I across one of two resistors

  1. Apr 3, 2016 #1
    1. The problem statement, all variables and given/known data
    What is the current in the 2 ohms resistor?
    tmp_21127-20160403_000946-639066610.jpg

    2. Relevant equations
    Kirchhoff's loop law

    3. The attempt at a solution
    Grounding does not affect how the circuit behaves, so I ignored the ground in and applied the loop law (clockwise from the negative terminal of the 9V battery ):

    ΔV(loop) = 9V -(2 ohms)*I - 3V - (4 ohms)*I =0
    6V = (6 ohms)*I
    I = 1 A

    Correct?
     
  2. jcsd
  3. Apr 3, 2016 #2
    well i do not work with circuits much- but if you have grounded both terminals of a load -can you expect a current to flow in the lower 4 ohms resistance.
    grounding a circuit defines the potential to be zero (definitely)
    further i failed to understand -what you wish to convey through this expt.?
     
  4. Apr 3, 2016 #3

    Orodruin

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    Incorrect. The system is grounded at two different points, indicating that those points must have the same potential.
     
  5. Apr 3, 2016 #4

    epenguin

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    ^^...So you have the same simplification as in the last problem. After which...? :oldsmile:
     
  6. Apr 3, 2016 #5
    So I can ignore the 4 ohms resistor in my calculations? Then the current through the 2 ohms resistor would be given by:

    9V - (2 ohms)*I - 3V =0
    6V / 2 ohms = I = 3A
     
  7. Apr 3, 2016 #6
    From what I understand now, the 4 ohms resistor might as well not be there....it's between 2 points of zero potential and so no current flows through it. Right?
     
  8. Apr 3, 2016 #7

    Orodruin

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    Right. You need a potential difference between its ends to provide a current.

    In this respect, grounding the two points is like short-circuiting them.
     
  9. Apr 3, 2016 #8
    When connecting in Parallel you are doubling the capacity (amp hours) of the battery while maintaining the voltage of one of the individual batteries. Use a jumper wire between the positives of both batteries and another jumper wire between the negatives of both batteries. Connect your positive and negative wires to the same battery to run to your application.
     
  10. Apr 3, 2016 #9

    Orodruin

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    This does not add any insight to the current problem at hand. Please stay on topic.
     
  11. Apr 3, 2016 #10

    phyzguy

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    This is correct.
     
  12. Apr 3, 2016 #11
    This is confusing.....what's a jumper wire? Why connect the terminals?
     
  13. Apr 3, 2016 #12

    SammyS

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    Ignore the reply of @drvrm !
     
  14. Apr 3, 2016 #13

    CWatters

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    This is not relevant to this thread.

    In addition it is wrong and potentially dangerous. In this thread the batteries are different voltages and batteries with different voltages should never be connected in parallel.
     
  15. Apr 4, 2016 #14
    tmp_31440-20160403_224159-639066610.jpg I'm getting better at this circuit analysis thing!
     
  16. Apr 4, 2016 #15

    SammyS

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    upload_2016-4-4_12-23-45.png

    upload_2016-4-4_12-24-59.png
    etc.


    That seems correct.

    The image is hard to read. I had to zoom way in.

    In the future it's best to start a new tread for a new problem.
     

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  17. Apr 4, 2016 #16

    epenguin

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    And to use an app like DocScanHD to make the images more visible and less unpleasant to look at.
     
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