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Percentage uncertatainty

  1. Sep 10, 2012 #1
    1. The problem statement, all variables and given/known data

    In a simple electrical circuit, the current in a resistor is measured as 2.5 ± 0.05 mA. The resistor is marked as having a value of 4.7 Ω ± 2%. If these values were used to calculate the power dissipated in the resistor, what would be the percentage uncertainty in the value obtained?

    2. Relevant equations

    none

    3. The attempt at a solution
    please help me, what is the solution for this one? thanks

    i'm sorry, i post this in the advanced physics too, i'm confused where to post this.. hope you can help me...
     
  2. jcsd
  3. Sep 10, 2012 #2

    PeterO

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    Homework Helper

    I think that when multiplying two quantities, you merely add the percentage errors??

    In this case you are multiplying 3 things I x I x R as in P = I2R; so you would add all three percentage errors.
    You are given the percentage error in R, and can calculate the percentage error in I from the values given.

    Peter
     
  4. Sep 10, 2012 #3
    Can you show me the solution? i'm confused... thanks peter...
     
  5. Sep 10, 2012 #4

    PeterO

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    If you wish to calculate the area of a rectangle with length 5.0 ±0.1 cm by 10.0 ± 0.1 cm, and give a percentage uncertainty, then

    A = l x w so Area = 50 cm2

    Now the percentage error.
    5.0 ± 0.1 means an uncertainty of 0.1 in 5 or 1 in 50 or 2%
    10.0 ± 0.1 mans an uncertainty of 1 in 100 or 1%

    SO the uncertainty in the answer is 3% [add them together]

    So Area is 50 cm2 ± 3%

    That is as close to the solution you seek I will give,
     
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