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How to calculate resistance and voltage using a multimeter

  1. Dec 1, 2009 #1
    1. The problem statement, all variables and given/known data
    This is just something I want to know, so it doesn't really have a problem statement.

    But I'm doing a lab tomorrow that uses voltage dividers. So it's two resistors R1 and R2 in series with a power supply. The power supplies Vin (the input voltage). A voltmeter is connected across R2 to measure Vout.

    The lab also asks me to measure the values of R1 and R2, as well as the current going through R2. I want to make sure I know how to measure each of these properly.

    2. Relevant equations



    3. The attempt at a solution
    R1 and R2: connect the multimeter with the power supply disconnected, across the resistors and measure?

    Vin would just be the amount of voltage supplied by the power supply, correct? I guess I just hook up the multimeter across the terminals of the power supply...

    Current across R2: Can't I just connect an ammeter in series with the circuit anywhere and take the reading? Can you measure current with a multimeter (I've never used one). Thanks!
     
  2. jcsd
  3. Dec 1, 2009 #2

    ideasrule

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    Homework Helper

    All of the above is correct, but to get a more accurate reading on Vin you should measure the voltage while the circuit is closed. The power supply's internal resistance is going to lower the voltage by a non-negligible amount.

    I have a suspicion that the teacher wanted you to calculate R1 and R2 by measuring voltages and currents instead of measuring resistance directly. In that case, would you know how to get R1 and R2?
     
  4. Dec 1, 2009 #3
    In case this helps, the voltage division and current division rules are:

    When a voltage V is divided across two series resistors, the voltage divides in the same proportions as each resistor.
    voltage across R1 = V [R1/(R1+R2)]
    voltage across R2 = V [R2/(R1+R2)]

    However, when a current I divides between two parallel resistors, the current in each resistor divides in proportion with the OTHER resistor:
    current through R1 = I [R2/(R1+R2)]
    current through R2 = I [R1/(R1+R2)]
     
  5. Dec 1, 2009 #4
    What does "measure the voltage while the circuit is closed" mean? (Can you tell I'm new at this? haha) And yep, I could find R1 and R2 theoretically if I needed to.

    And thank you mike, that does help =)
     
  6. Dec 2, 2009 #5
    Measure the voltage when you have R1 and R2 connected to the power supply.
     
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