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Internal resistance Question

  1. Jul 28, 2016 #1
    1. The problem statement, all variables and given/known data
    See image attached.

    I am currently stuck on part b.

    2. Relevant equations
    (Rt) (Rint) / Rt + (Rint)

    3. The attempt at a solution
    I thought that Rt would be 26.7 kohms, but my answer is not matching up with the answer in the book. I am unsure why since the resistors are in series, so I figured I would just add the 2 values. Once I find that, I just plug in total resistance and internal resistance into the formula above. Then I would have to convert Mohms to kohms.
    Does that sound right?

    Thank you in advance

    Attached Files:

  2. jcsd
  3. Jul 28, 2016 #2


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    Can I suggest you make a sketch circuit showing the DMM modelled as an ideal DMM and an 11M resistor.

    You don't say what Rt is?
    You mention resistors in series but don't say which ones you are talking about.
  4. Jul 28, 2016 #3
    Im not 100% sure by what you mean by making a sketch. I thought 26.7 kilohms would be the total resistance within the circuit (by adding the 2 resistors together). If not, how would you do so?

    Here is my sketch, but am unsure it this is what you were suggesting.

    Attached Files:

  5. Jul 29, 2016 #4
    Part (b) of the question asks you to measure V2 assuming an internal impedance of 11M in the meter. Your sketch shows the meter across the wrong resistor.

    I would suggest you calculate the current in the circuit in the usual way, then calculate the voltage (theoretically) across R2. Once you have done that, measuring the voltage with the meter effectively places two resistors in parallel. So what do you think would happen to the circuit current if you change the effective resistance of R2? And if the current is different, what would happen to the voltage drop across R2, would it be the same as you calculated in part (a)?
  6. Jul 29, 2016 #5


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    Yes your sketch is exactly what I meant but as Numbskull said you have the meter across the wrong resistor.
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