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Homework Help: Quick question about uncertainty calculation

  1. Sep 29, 2008 #1
    1. The problem statement, all variables and given/known data
    lets say we have A * B
    A=12.8 + 0.2
    B=3.75 + 0.08
    how would we calculate the new uncertainty value
    ive seen soo many ways of doing this im a bit confused
    thanks in advance

    2. Relevant equations
    any equation used to calculate uncertainty


    3. The attempt at a solution
    the 2 main ways ive seen is one involving partial dervatives and the other is taking the square root of the answer times the (uncertainties/value)2
    im a little confused about which one to use
     
  2. jcsd
  3. Sep 29, 2008 #2

    rock.freak667

    User Avatar
    Homework Helper

    If C=AB, then

    [tex]\frac{\delta C}{C}=\frac{\delta A}{A}+ \frac{\delta B}{B}[/tex]


    this is obtained using differential calculus.
     
  4. Sep 29, 2008 #3
    (A+ε1)*(B+ε2)
    AB+Aε2+Bε11ε2

    so if we disregard the final term the incertainty would be Aε2+Bε1

    partial derivative of AB with respect to A is B
    partial derivative of AB with respect to B is A

    does that help?
     
  5. Sep 29, 2008 #4
    whats that called? I want to look that up. it looks interesting.
     
  6. Sep 30, 2008 #5
    Last edited by a moderator: Apr 23, 2017
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