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Determine the resistance of an ammeter.

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  1. Apr 23, 2017 #1
    1. The problem statement, all variables and given/known data
    An inductor is connected in series with an AC source, the rms output voltage of it is 260V and a hot-wire ammeter reads 2A. If the ratio of the voltage across the ammeter to the voltage across the inductor is 5/12, determine the resistance of the ammeter. (Neglect the inductor resistance).
    2. Relevant equations
    1nxNk.jpg

    i=V/R (where i is the current, V is the voltage and R is the resistance).

    3. The attempt at a solution

    (Va is the ammeter voltage and Vl is the inductor voltage)
    My result is not what in my text book, could you explain why?
     
  2. jcsd
  3. Apr 23, 2017 #2

    ehild

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    The voltage across the ammeter (UA) is not in phase with the voltage across the inductor (UL)
    upload_2017-4-23_13-4-14.png
     
  4. Apr 23, 2017 #3
    So, what should I do?
     
  5. Apr 23, 2017 #4

    ehild

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    Look at the picture, it is the phasor diagram of the voltages. The resistance of the ammeter is in series with the inductor. The voltage across the inductor is 90°ahead of the voltage of the resistor. You have to add the voltages as vectors, and the resultant voltage (the red vector) is equal to the voltage of the AC generator.
     
  6. Apr 23, 2017 #5
    You mean it will be like this?
    exY0o.jpg
     
  7. Apr 23, 2017 #6

    ehild

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  8. Apr 23, 2017 #7
    I checked my book and found it was the right answer. Thank you, ehild!
     
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