1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Gold Member


    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


    User Avatar
    Homework Helper
    Gold Member

  6. Aug 12, 2017 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Just for info... Some specialist Voltmeters have very high input resistance, as much as 10*109 Ohms on some ranges.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted