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Circuit Voltage

  1. Apr 9, 2009 #1
    A voltmeter with internal resistance of 100k ohm reads 40V across the 200k ohm resistor of the circuit. Determine the actual voltage across the resistor( without the voltmeter)

    my anawer:
    across 100k ohm = 13V
    across 200k ohm = 26.7 V
    is it correct ?
    but it seems to be wrong..
    so anyone can help ?
    thx
     
  2. jcsd
  3. Apr 9, 2009 #2
    Try breaking the "real" voltmeter, into an ideal voltmeter and the internal resistance of it. How would this configuration look like ;)
     
  4. Apr 9, 2009 #3
    actuallt the diagram is

    Code (Text):

    ┌------W (100k ohm) -------------┐
    │                                 │
    │                                 │
    ---                                      (200k ohm)  
     -                                              │
    ---                                            │
     -                                              │
    │                                 │
    │                                 │
    └---------------------------------┘
     
    well, the diagram is shown in series
    but the question is saying '' across'' ?
    so... i dunno ....
     
    Last edited by a moderator: Apr 10, 2009
  5. Apr 9, 2009 #4
    Well, you can measure voltage between the two nodes of the resistor, hence you have to put parallel with the resistor, thats what it means by across.

    Now how can you split the voltmeter into an ideal voltmeter and its internal resistance?
     
  6. Apr 9, 2009 #5

    Redbelly98

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    We need to see the actual circuit in order to answer the question.

    The circuit in post #3 is not displaying properly. Could you do it this way instead:
    [noparse]
    Code (Text):

    {post ciruit diagram here}
     
    [/noparse]

    Then we should be able to see what is going on.
     
  7. Apr 10, 2009 #6

    Redbelly98

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    kyip, please do all correspondence about this problem by posting here in this thread.

    If you have an online image of the circuit diagram:

    [noparse][/noparse][i][size=2]the URL of the image goes here[/size][/i] [Broken]

    For example, typing

    [noparse][PLAIN]https://www.physicsforums.com/customavatars/avatar122961_23.gif[/noparse] [Broken]

    will display this:
    https://www.physicsforums.com/customavatars/avatar122961_23.gif [Broken]
     
    Last edited by a moderator: May 4, 2017
  8. Apr 10, 2009 #7

    Redbelly98

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    Okay, now I can see the updated circuit diagram in post #3.

    When the 100 kΩ voltmeter is connected across (i.e., in parallel with the 200 kΩ resistor, that will change the net resistance of that component.

    So, first try solving the circuit with the extra 100 kΩ voltmeter there.
     
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