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Current question

  1. Aug 22, 2015 #1
    • members are reminded that your thread titles must be accurate and descriptive of the topic under discussion
    1. The problem statement, all variables and given/known data
    When direct current is measured with ammeter of resistance RA1=100mΩ, current is I1=12 A. When direct current is measured with ammeter of resistance RA1=50mΩ, current is I2=15 A. What is the value of current if there is no ammeter (RA3=0)?

    2. Relevant equations
    Ohm's law

    3. The attempt at a solution
    I am getting that I3=18 A. The result should be 20 A.

    Could someone check this?
  2. jcsd
  3. Aug 22, 2015 #2


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    Staff: Mentor

    How did you get 18 A?
    It is hard to find a mistake if you don't show your work, but here is a guess: using your approach, what would you expect for an amperemeter with a resistance of 1 Ohm? Is that result realistic? If not, something is wrong with the approach.
  4. Aug 22, 2015 #3


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    Staff: Mentor

    What electrical circuit have you sketched to use as a model for the unknown circuit where you are trying to measure current?
  5. Aug 23, 2015 #4
    In the question, circuit is irrelevant.
  6. Aug 23, 2015 #5


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    Staff: Mentor

    Show how you worked out your answer, then.
  7. Aug 23, 2015 #6
    whenever you have a physical problem, unless it is trivial, it is always a good idea to make a sketch.
  8. Aug 24, 2015 #7


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    Gold Member

    +1 to that. Not sure I could do it without making a sketch diagram but then I'm old.
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