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Voltage across voltmeter

  1. Nov 19, 2012 #1
    Now for this case when the voltmeter is not a ideal one, a small current is flowing. The voltmeter reading equal to the potential difference across itself (10V) and not emf of the battery(12V) . This happens because there is current flowing through the whole circuit, causing IR to exist across every component with resistance.

    Now if the resistance of the voltmeter approaches infinity meaning approaching an ideal voltmeter, it that the voltmeter reading approaches emf of the cell and theoretically at infinity it should show emf of the cell exactly right??

    and for the same instant, the potential difference across the resistor 10 kΩ should decrease until finally it reaches zero. Is this true??

    Thank you very much or your guiding.
     

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  3. Nov 19, 2012 #2

    phinds

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    Yep ... you've got it right
     
  4. Nov 20, 2012 #3
    I notice that your voltmeter resistance is 50kΩ.

    This is very low for a modern voltmeter. These usually have a fixed resistance of at least 1MΩ, maybe several megs on all ranges.

    Old fashioned needle pointer (analog) voltmeters offered a resistance of 50kΩ per volt.

    This was because the voltmeter contained a series resistor to provide full scale deflection at a particular voltage.
    This arrangement provided a constant resistance across that particular range of the voltmeter so for instance

    On the 12 volt range the resistance would be 12 * 50k = 600kΩ

    On the 3 volt range the resistance would be 3 * 5ok = 150kΩ etc
     
  5. Nov 20, 2012 #4

    Integral

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    Your drawing cannot be correct. That is not how a voltmeter is connected to measure voltage.
    If you want to measure voltage across the resistor then the meter should be connected parallel to the meter, if you want to measure source voltage then the meter needs to be parallel to the source.

    Currently you show it in series. that is simply NOT how a voltmeter should be connected.
     
  6. Nov 20, 2012 #5

    phinds

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    Actually, I think he contrived this "circuit" specifically to ask questions about how the voltmeter works, and he got it right.
     
  7. Nov 20, 2012 #6

    Integral

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    I can accept the drawing if we consider the 10KOhm resister as the internal resistance of the cell. So the meter is actually parallel to both the resistor and the cell emf. Now as the resistance of the meter goes up the current through the internal resistance drops. As the current drops the voltage across the internal resistance follows and the cell voltage approaches the ideal 12V.

    The OP should note that a voltmeter is always connected in parallel to the voltage being measured. He needs to be a bit more careful with his drawings if he really wants to understand what is happening.
     
  8. Nov 21, 2012 #7
    Thanks for your concern very much . I hav gt my answer. Thanks a lot.
     
  9. Nov 21, 2012 #8
    Thanks for you guiding too. actually I know this type of arrangement is odd,but I purposely arrange in tis way to study d the operation of voltmeter. Thanks by the way.
     
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