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Homework Help: Electric circuit

  1. Mar 20, 2010 #1
    1. The problem statement, all variables and given/known data

    A 500 ohm resistor and a 1000 ohm resistor are connected in series with a 12 V battery of negligible internal resistance . When a voltmeter is connected across the 500 ohm resistor , its reading is 3 V . What is the resistance of the voltmeter ?


    2. Relevant equations

    V =IR

    3. The attempt at a solution

    the effective resistance across the voltmeter and 500 ohm resistor (i am not sure across which) is Re=500Rm/(500+Rm) where Rm= resistance of meter and Re=effective resistance

    From V=IR , 12=I(1500) , so the current flowing through the circuit is 0.008 A .

    3=0.008(500Rm/(500+Rm))

    solving this , Rm=1500 ohm

    but the answer given is 1000 ohm , i wonder where have i gone wrong
     
  2. jcsd
  3. Mar 20, 2010 #2

    OmCheeto

    User Avatar
    Gold Member

    1000 ohms is the correct answer. Your current flowing through the circuit is incorrect as you did not include the resistance of the ohmmeter in that calculation. The simplest way to solve it would be to determine the voltage across and then the current through your 1000 ohm resistor.
     
  4. Mar 21, 2010 #3
    thank you
     
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