1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Calculating Experimental Error

  1. Oct 5, 2014 #1
    1. The problem statement, all variables and given/known data

    I am writing a lab report for an X-ray diffraction. I have been attempting to come up with an equation for the error using formulas some people from college gave me and also some I found on wikipedia but I am quite sure I am doing it wrong. The only variable is the angle where the maximum intensities are found. I am using Bragg's law to calculate the spacing between the atoms.

    2. Relevant equations

    D = (N*wavelength)/(2*sin(x))

    As there is no error in N, wavelength, or "2", we can let that equal A.

    D = A/sin(x)

    Some equations I was given:

    Z = aX
    dZ = adX

    Z = X^a
    dZ/z = |a|dx/x

    Z = SinX
    dZ = dX CosX

    3. The attempt at a solution

    D = Z = A/sin(x) = A (sin(x))^-1 = A f(y)^-1

    I have tried loads of ways of calculating this but I keep getting silly answers. Any help, ideas or links would be really appreciated.
  2. jcsd
  3. Oct 5, 2014 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    Take it one step at a time. You might find it helpful to introduce new variables. For example, let w=1/sin(x). Then you have ##D = Aw##, so applying your first rule, you have ##\delta D = A \delta w##. (I'm using deltas instead of d because dD looks weird.) Now your job is to find ##\delta w##. If you let ##v=\sin x##, then ##w=1/v = v^{-1}##. Using the second rule, you can find ##\delta w## in terms of ##\delta v##. Then you need to find ##\delta v## in terms of ##\delta x##, and then put it all together.
    Last edited: Oct 5, 2014
  4. Oct 5, 2014 #3
    Hey thanks for the reply, it is very concise and logical, I actually tried that but assumed I must have made a mistake as the value I was getting for the error seemed to large ~80%.

    The final equation I have is:

    dD = A (dx cosx)/(sinx)^2

    This equation seems to give a value for error of about 80%. x ranges from 3 to 35 and dx was 0.1. ie the beam angle ranged from 3 to 35 degree in 0.1 degree steps.
  5. Oct 5, 2014 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    You need to use radians, not degrees. That's probably where the issue lies.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: Calculating Experimental Error
  1. Experimental error (Replies: 2)