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B Significant Figures

  1. Nov 26, 2016 #1
    I measure some quantity with an instrument that it is precise to two decimal points.
    So maybe I get 8.84 V. Then I do some changes in my parameters and get 0.01 V.

    The two measured values are precise to two decimal points. But the first one has three significant figures, while the second one has only one significant figure.

    Why is that? Should not both of them have the same number of significant figures?

    Why are the leading zeros not considered significant?
     
  2. jcsd
  3. Nov 26, 2016 #2

    jbriggs444

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    Significant figures are a way of dealing with "relative error". That is, error as a fraction of the actual value.

    If the actual value is smaller, the relative error is larger.
     
  4. Nov 26, 2016 #3

    Bystander

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    How many "leading" zeroes can you write?
     
  5. Nov 26, 2016 #4
    For a particular unit that I chose and is Volts, only two leading zeros.
     
  6. Nov 26, 2016 #5
    But what is the connection of significant figures to the precision? By precision I mean the number of decimal points to which a measured value is known. For ex, a precision of 0.1 or 0.01.
     
  7. Nov 26, 2016 #6

    DrGreg

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    And what if you chose millivolts? Or kilovolts?
     
  8. Nov 26, 2016 #7

    8.84 in mV would be 8850 and 0.01 in mV would be 10.

    So 3 significant figures vs 1 significant figure. The 0 at the end should not count.

    That's the thing. I do not get the connection between significant figures and the precision. An instrument with a certain precision (two decimal points for a particular unit) gives of two numbers with different number of significant figures. Or maybe there is a connection for numbers larger than 1 and another connection for numbers between 0 and 1.
     
  9. Nov 26, 2016 #8

    jbriggs444

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    log(value/error).
     
  10. Nov 26, 2016 #9
    Value being the true value or measured value? Error being the relative error or absolute error? And log(value/error) representing what?
     
  11. Nov 26, 2016 #10

    jbriggs444

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    As long as the absolute error is small compared to the the true value (or to the measured value) then it does not matter which.
    The absolute error. For instance, if the accuracy of the measuring device is +/- .001 then the error is .001.
    If you know what the value is and what the error is, what do you think the log(value/error) is?
     
  12. Nov 26, 2016 #11
    Maybe the connection is this.
    Let's say we have an instrument which has a precision of 2 decimal points for a particular unit V.
    Then the number of significant figures can be: 3 for numbers of the form 4.55, 2 for numbers of the form 0.85, 1 for numbers of the form 0.03. That's it if the numbers are below 10. And if the the numbers can be whatever, then the number of significant figures can be from 1 to n, whatever n is.

    So a significant figure is a digit that adds to the precision, but I thought there was a connection between the number of decimal points and number of significant figures. Anyway.
     
  13. Nov 26, 2016 #12

    jbriggs444

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    What does this mean? What numbers and what n are supposed to be whatever?
     
  14. Nov 26, 2016 #13
    The numbers being the measured values. If the measured value if 88.85 then I have 4 sign figures.
    For 4544.34 I have 6 sign figures.

    The precision is always at two decimal points, but the number of signif figures can be from 1 to n depending on what the measured value is.
     
  15. Nov 26, 2016 #14
    The number of significant figures? The integer part at least plus 1.
     
  16. Nov 26, 2016 #15

    jbriggs444

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    Again, what is "n" supposed to denote?
     
  17. Nov 26, 2016 #16
    The number of significant figures.
     
  18. Nov 26, 2016 #17

    jbriggs444

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    So it is your position that the number of significant figures can range from one to the number of significant figures.
     
  19. Nov 26, 2016 #18
    Yes. n can vary from 1 to whatever.

    I understand that it is a mistake to say that x can vary from 1 to x. But you got my point.
     
  20. Nov 26, 2016 #19

    jbriggs444

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    If n is the number of significant figures then the number of significant figures is n. But that statement tells no one anything about what the number of significant figures is.
     
  21. Nov 26, 2016 #20
    I do not get where you are going with this.
     
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