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Question about Full scale deflection

  1. Dec 5, 2017 at 9:50 PM #1
    1. The problem statement, all variables and given/known data
    hello, just came across this type of question for first time.
    A voltmeter with a range of 0-30volts is to be used to measure a 120 volt circuit. calculate the value of the resistor to be placed in series with the meter. the sensitivity of the meter is 1000 ohms per volt ?

    2. Relevant equations

    im taking a guess its 120-30 = 90 vd x 1000 = 90k ohms ? any input would help thanks
    3. The attempt at a solution
     
  2. jcsd
  3. Dec 6, 2017 at 12:56 AM #2

    mfb

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    Don't guess. What does the sensitivity tell you about the resistance at 30 V at the voltmeter?
     
  4. Dec 6, 2017 at 6:10 AM #3
    its telling me at 30volts its 30k ohms since its 1k per volt ..needs 90volts to drop across the resistor .. 120volt x 1000 k = 120k - 30k = 90k x .001ma = 90vd ?
     
    Last edited: Dec 6, 2017 at 6:37 AM
  5. Dec 6, 2017 at 6:49 PM #4

    mfb

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    What do you multiply here and what are the units? k without a unit doesn't have a proper meaning.
    That equation is wrong.
    That equation is wrong as well.
    That equation has at least an issue with units.
     
  6. Dec 6, 2017 at 7:02 PM #5
    so what are the proper steps to find the answer ?
     
  7. Dec 7, 2017 at 12:40 AM #6

    mfb

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    If the voltmeter has 30 kOhm, what is the maximal current flowing through it?

    At this current, what do you need as additional resistance to get 90 V voltage drop there?
     
  8. Dec 7, 2017 at 6:52 AM #7
    my choices to pick from the homework are

    60k ohms 90k ohms

    90ohms 150k ohms

    if you take 90k x .001 you get 90vd
     
  9. Dec 7, 2017 at 8:21 PM #8

    mfb

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    With the right units this can be useful.

    Work with units. It helps spotting most errors you'll make if you don't work with them.
     
  10. Dec 7, 2017 at 8:59 PM #9
    what do you mean by units ?
     
  11. Dec 7, 2017 at 11:07 PM #10

    mfb

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    Ohm, Volt, Ampere and so on.
     
  12. Dec 8, 2017 at 12:54 AM #11
    how would you write out the problem using ohms law ?
     
  13. Dec 8, 2017 at 3:09 AM #12

    mfb

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    That's your homework task. You need a few simple steps in a row, going from the voltmeter to the additional resistor step by step.
     
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