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Calculating voltage which voltmeter is showing

  1. Jan 15, 2017 #1
    1. The problem statement, all variables and given/known data
    Schematic of circuit is not given, only the text which I translated into English.

    Using a voltmeter with internal resistance of 6k ohm, voltage between two points, 1 and 2 of a circuit of constant current, is measured to be 8v.
    Then, using a voltmeter with internal resistance of 10k ohm, between the same points, voltage is measured to be 12v.
    What voltage will the voltmeter with internal resistance of 15k ohm measure?

    2. Relevant equations
    I=U/R

    3. The attempt at a solution
    I tried to solve it by calculating currents getting 1.3mA and 1.2mA in first and second case, but wasn't sure what to do next.
    These voltmeters should be represented as resistors and the answer is 16V.
     
  2. jcsd
  3. Jan 15, 2017 #2

    gneill

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    Staff: Mentor

    You're told that the current in the circuit is constant, which I take to mean that the current leaving node 2 is the same as the current entering node 1:

    upload_2017-1-15_15-51-48.png

    So ##I## is an unknown constant and ##R## is some unknown resistance lying between nodes 1 and 2. That's two unknowns. Fortunately you were given two cases where the meter resistance and the measured voltage are given, so you can construct two equations in two unknowns.
     
  4. Jan 15, 2017 #3
    I would like to thank you for looking into this but I am unsure on how to do that. I mean, how would system of equations look like?
     
  5. Jan 15, 2017 #4

    gneill

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    Staff: Mentor

    Start symbolically: don't plug in any numbers, just use variables. See if you can write an expression for the voltage ##V## in terms of ##I##, ##R## and ##R_m##. Or, write an expression for ##I## in terms of ##V##, ##R## and ##R_m##. Either way is fine (although the latter may be more straight forward).
     
  6. Jan 15, 2017 #5
    So like voltage/current divider?
    V = (Rm || R) * I
     
  7. Jan 15, 2017 #6

    gneill

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    Staff: Mentor

    Sure, that would work. Or since the voltage is the same across both resistors it's easy to write the sum of the currents:

    ##I = \frac{V}{R} + \frac{V}{R_m}##

    Whatever you are more comfortable with.
     
  8. Jan 15, 2017 #7
    So it's like this.
    upload_2017-1-16_0-55-9.png
    upload_2017-1-16_0-55-14.png

    R=30k ohm
    I = 0,0016A

    Replacing those values gets me the voltage of 16V.
    Thanks, was struggling with this trivial problem.
     
  9. Jan 15, 2017 #8

    gneill

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    Staff: Mentor

    Glad I could help.
     
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