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Errror uncertainty for ln(x)

  1. Nov 28, 2013 #1
    1. The problem statement, all variables and given/known data
    What would be the error uncertainty when you take ln of a number. For example ln(10) and the error uncertainty for 10 is ± 1


    2. Relevant equations



    3. The attempt at a solution
    Is the error uncertainty just (1/10)*2.3? (2.3 is the answer to ln(10))
     
  2. jcsd
  3. Nov 28, 2013 #2

    rude man

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    d/dx ln x = 1/x
    what is d(ln x) in terms of dx?
    That's for small changes in x.

    For larger changes in x, say x → x+a, you can Taylor-series-expand ln (x + a). The first term is of course the above = ln x + a/x , then the higher terms give you better accuracy.

    In your case, x = 10 and a = 1, the small-change approximation above gets you to within 0.9980 of the correct answer. Adding just 1 extra term in the series gets you to within 0.9999 of the exact answer. Etc.
     
  4. Nov 28, 2013 #3

    NascentOxygen

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    So the correct answer lies between ln(9) and ln(11)?
     
  5. Nov 28, 2013 #4

    rude man

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    Those are the extremes, yes, but the fractional error distributions differ slightly between x and ln(x).

    For small δx/x the fractional change in ln(x) is δ[ln(x)]/ln(x)] ~ [1/ln(x)] δx/x = 0.4343.

    E.g. for δx = +1 and x = 10,
    fractional change in ln(x) = [ln(11) - ln(10)]/ln(10) = 0.04139 for fractional change in x = 1/10 = 0.1000. So δln(x)/ ln(x) = 0.4139 δx/x.


    but if δx = 0.1, fract. change in ln(x) is [ln(10.1) - ln(10)]/ln(10) = 0.004321 for a fractional change in x of 0.1/10 = 0.0100. So δln(x)/ln(x) = 0.4321 δx/x, pretty close to 0.4343.

    Which as I said is not a big difference. So with little error, you can say that the fractional error in ln(x) is proportional to the fractional error in x, that ratio being 0.4343, if |δx/x| < 0.1.

    But say δx = 5, then
    fract. change in ln(x) = [ln(15) - ln(10)]/ln(10) = 0.1761 for δx/x = 0.5, so that ratio is 0.1761/0.5 = 0.3522 which is quite a ways from 0.4343.
     
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