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Circuit resistance question

  1. Dec 20, 2016 #1
    1. The problem statement, all variables and given/known data
    I understand part a, but not part B.
    circuit-jpg.110596.jpg

    2. Relevant equations


    3. The attempt at a solution
    For part b) the PD across the resistor is 2V, but we don't know the current so we can't calculate using ohm's law eqn.

    And because the new resistor's resistance is unkown, how do we actually calculate it?
     

    Attached Files:

  2. jcsd
  3. Dec 20, 2016 #2

    Jonathan Scott

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    You've not actually said what the question is for part b, but if it's to determine the resistance R that isn't difficult. You can calculate the currents through meter V1 and through V2, so the current through the unknown resistance will be the difference between those.
     
  4. Dec 20, 2016 #3
    shoot.

    the question is: Calculate the value of R.
     
  5. Dec 20, 2016 #4

    cnh1995

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    For part b), what can you say about the current flowing through the circuit?
     
  6. Dec 20, 2016 #5
    okay if V1 has voltage of 4v and its resistance is 1,000,000 ohms, then it's current must be 0.000004.

    And this current must also flow through the resistor. Since total voltage is 6v, then this component must have voltage of 2V, so its resistance must be (2/0.000004) = 500,000 ohms, right?
     
  7. Dec 20, 2016 #6

    cnh1995

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    Right.
    But that's not the final answer. You need to find R.
    Edit: Resistance of V1 is 10MΩ. You have taken it as 1MΩ..
     
  8. Dec 20, 2016 #7
    okay the mistake I just realised is that the Mohms = 10,000,000 ohms so the current is I = 0.0000004 so resistance R is (2/0.0000006) = 5,000,000

    but you're saying that's not the value of R?? But isn't the new component R??
     
  9. Dec 20, 2016 #8

    cnh1995

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    Yes I had edited my post immediately after posting..
    R is the resistance used in place of the 10 ohm resistor. What about the resistance of voltmeter V2?
     
  10. Dec 20, 2016 #9

    CWatters

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    Is V2 removed for part b) ?
     
  11. Dec 20, 2016 #10
    I'm still confused about what v2 has to do with the resistance of the component R??? (and no, v2 is not removed)
     
  12. Dec 20, 2016 #11

    cnh1995

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    What is the current flowing through the component resistance R? Is it same as that flowing through V1?
     
  13. Dec 20, 2016 #12

    Jonathan Scott

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    You need to consider the current flow. From the information available, you can calculate the current through V1 and the current through V2. The total of the currents through R and V2 must be equal to the current through V1, so the current through R is given by the difference of the two calculated currents, as I mentioned above. You then know both the potential across R and the current through it so you can calculate R.
     
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