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Homework Help: Finding error in Calculation

  1. Dec 11, 2011 #1
    1. The problem statement, all variables and given/known data
    I'm in the middle of a project finding the e/m ratio of the electron by the use of the formula below. I know all required values and can find a result, I'm unsure how to calculate an error for the calculation.
    In the formula below, D, n, A and f are all given quantities (therefore no error/the error can be taken as 0, right?)

    2. Relevant equations

    [itex]\frac{e}{m}[/itex] = (32/D2)*(nA2∏)2*(Va/Is2)*(Is/Vs)2

    3. The attempt at a solution

    The last 2 parts of the equation are where the trouble arises. I have had to measure and record results in two different ways to find values and to plot the 2 graphs - Va/Is2 and Is/Vs - and the software used gives a percentage error in the line.
    Do I need to find the error in each measurement for Va and Is etc?

    The current way I know to calculate error would mean using the smallest value measured and half of the smallest possible measurement (half the equipment increment). This would result in a stupidly high error, eg:
    smallest V measurement was 5V, this is also the smallest increment on the equipment, therefore error is ±2.5V meaning an error in that set of measurements of 50%??
    I'm then taught to add the percentage errors... this leads to 350% error.
    However, because I know the gradients of the graph, is it acceptable to only use these errors? This would give a total are of ≈10%. Much better!

    Which is the correct way of going about this? And if the answer is neither of these, what is the correct way of going about this!?

    Thanks in advance!
  2. jcsd
  3. Dec 12, 2011 #2
    Any Help would be appreciated!
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