Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I am doing a project analysis and need some help with errors

  1. Oct 22, 2009 #1
    Basically, I have used microwaves in a wave guide to measure the width of the guide via standing waves. This led me to plot a graph of 1/wavelength in the guide^2 vs 1/wavelength^2 in air.

    The graph showed the following formula; y = 1.0741x + 455.08 with an R^2 value of 0.9979 from 5 points.

    This enabled me to calculate the width of the guide using the following formula; 1/2*width^2 = c. which came out as 2.34e^-2m.

    So far a very good result, as when I measured the guide it had a width of
    2.3e-2m +/- 0.05e^-2.

    The problem is that I now need to work out the error in my experimental value of a. Which is obviously a product of the accuracy of the slope of the graph. How should I proceed? Any help would be appreciated.
  2. jcsd
  3. Oct 25, 2009 #2
    You need Gauss' error laws:

    Given a formula [tex]z=x^{\pm a}[/tex] ([tex]a[/tex] a constant) then the error in [tex]z[/tex] is [tex]\frac{\Delta z}{z}= |a|\frac{\Delta x}{x}[/tex]

    and given a formula [tex]z=b.x[/tex] ([tex]b[/tex] a constant) then the error in [tex]z[/tex] is [tex]z=a.\Delta x[/tex]

    You can combine both of those to get the error. To get the error in the slope and x-intercept in your graph (provided you've added some kind of linear trendline to it) then you need to use a regression package (one comes as standard with MS Excel) and that will give you the data you need.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook