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Homework Help: Issue with circuit theory lab

  1. Sep 19, 2010 #1
    1. The problem statement, all variables and given/known data
    In last weeks lab, I was asked to find the resistance of two unknown resistors. I hooked them up in series with a 10V source and measured the current across each one. The ammeter read 5.77 mA at Rx and .014 mA at Ry. Shouldn't the ammeter have read the same value at both resistors?

    2. Relevant equations
    V = IR

    3. The attempt at a solution
    Once again, I'm puzzled as to why different current values are flowing through resistors in series with a voltage source. The same current should be flowing through everything, yes?
  2. jcsd
  3. Sep 19, 2010 #2


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    Science Advisor

    I think you're missing the point of how an ammeter works. To use an ammeter, you need to hook the ammeter up in series with the circuit you want to measure, so that you force all of the current to flow through the ammeter. Then the current you measure through the ammeter is the current flowing in your circuit. What it sounds like you did was to connect the ammeter across the resistor, in parallel with the resistor. Then what the ammeter is measuring is the current flowing through the ammeter, which will be different (probably very different) from the current flowing through the resistor, and will depend on the internal resistance of the ammeter.

    What you need to do is:

    (1) Hook up the ammeter in series with both resistors, so you know the current flowing in the circuit. The same current will be flowing through the ammeter and each of the two resistors.
    (2) Connect a voltmeter in parallel to each resistor to measure the voltage across each resistor. The resistance of each resistor is then the voltage drop across it, divided by the circuit current.
  4. Sep 19, 2010 #3
    also, you may want to use a different ammeter... the one you used is probably fried
  5. Sep 19, 2010 #4
    Thanks a lot guys.
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