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Homework Help: Reading of a voltmeter and an ammeter

  1. Aug 11, 2017 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    Part a)

    Voltmeter can be replaced by an open circuit and ammeter can be replaced by a short circuit .By doing so the current flows in the outer loop consisting of both the batteries and both the resisters .The current in the circuit is given by i = (45-15)/(20+10) = 1A .

    Potential drop across Voltmeter is U2-iR2 = 45-(1)(20) = 25 V

    So reading of ammeter is 1A and that of voltmeter is 25V .

    Is that the correct answer for part a) ?

    I am having some trouble with the use of "ideal voltmeter" .An ideal voltmeter has infinite resistance .It can be replaced by an open circuit . But if voltmeter has to read a potential difference it must draw some current .If no current flows through the voltmeter ,then it's reading would be simply zero .

    Sorry for being a bit confused .

    Attached Files:

  2. jcsd
  3. Aug 11, 2017 #2


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    Your analysis is correct, BUT ... ideal volt meters are assumed to draw an infinitesimally small current that has zero effect on the circuit. It's a problem-simplification technique.
  4. Aug 11, 2017 #3

    And in part b) when the two meters are interchanged

    No current flows through U1 and R1 , so entire terminal voltage of U1 appears across the voltmeter .It reads 15V .

    Current flows in the loop consisting ammeter and U2 and R2 .Current i = 45/20 = 2.25A .

    Ammeter reads 2.25A and voltmeter reads 15V .

    I hope this part is also correct .
  5. Aug 11, 2017 #4


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  6. Aug 12, 2017 #5


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    Just for info... Some specialist Voltmeters have very high input resistance, as much as 10*109 Ohms on some ranges.
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