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

Homework Help: Expectation values

  1. Mar 9, 2008 #1
    1. The problem statement, all variables and given/known data
    A particle moves in a sequence of steps of length L. The polar angle [tex]\theta[/tex] for each step is taken from the (normalized) probability density [tex]p(\theta)[/tex]. The azimuthal angle is uniformly distributed. Suppose the particle makes N steps.
    My question is how do I find the expectation value (say [tex]<z^2>[/tex] for example).

    2. Relevant equations
    Usually for a probability density p(x) we have
    [tex]<x^m>=\int x^m p(x) dx[/tex].


    3. The attempt at a solution
    I think that I can get the values for one step. eg.
    [tex]<z^2>=\int_0^\pi (Lcos(\theta))^2p(\theta)d\theta={L^{2}\over 2}[/tex]
    Note: the density [tex]p(\theta)[/tex] is normalized.
    I just don't know how to treat N steps. Do I just multiply the one-step result by N?
     
  2. jcsd
  3. Mar 10, 2008 #2
    What is [tex]p(\theta)[/tex]? Is it given?
     
  4. Mar 10, 2008 #3
    Oh ya. Sorry. It is
    [tex]p(\theta) ={2 \over \pi}cos^2({\theta \over 2})[/tex]
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook