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Homework Help: Triangle wavefunction - position uncertainty and probability amplitude

  1. Sep 30, 2013 #1
    1. The problem statement, all variables and given/known data
    Hey guys. Basically I have a wavefunction that looks like this
    http://imageshack.com/a/img843/3691/22r3.jpg [Broken]

    I have to find:
    (a) The normalization constant N, of course by normalizing it
    (b) Find <x> and <x^2> and use this to find Δx.

    2. Relevant equations
    I'm just gona insert images, as i'm writing it all with my tablet:
    http://imageshack.com/a/img33/5705/czbs.jpg [Broken]

    3. The attempt at a solution
    Okay, so I've found the normalization constant, N, but I've got 2 different answers depending on the way in which I solve the problem, and I want to know which is correct (and why, if possible). Please view the image below:
    http://imageshack.com/a/img11/7232/0hmb.jpg [Broken]

    So...which is correct? :S

    I need to know the explicit expression for the probability density to proceed further.

    Thanks a lot guys!
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Sep 30, 2013 #2


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    Homework Helper

    The square of a function that is defined in parts, is still a function defined in parts, eg.
    [itex]f(x) = \left\{\begin{array}(g(x); x<0 \\ h(x); x\geq0\end{array}\right.[/itex]
    It follows that
    [itex](f(x))^2 = \left\{\begin{array}((g(x))^2 &; x<0 \\ (h(x))^2&; x\geq0\end{array}\right.[/itex],
    that is to say
    [itex](f(x))^2 \neq (g(x))^2 + (h(x))^2[/itex]

    This is a notational problem: your [itex]|\Psi|^2[/itex] is incorrect, although you do manage to integrate it correctly in parts. Your first approach is correct as it is the integral of the square that you want to have as 1 (probability of the particle being anywhere is 1).
  4. Sep 30, 2013 #3
    Great, thanks a lot dude! But my next question is this - how do i go about finding <x> ? i mean I need a single expression for psi mod squared, right?
  5. Sep 30, 2013 #4


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    Homework Helper

    I'm not exactly sure what you mean by a single expression. Could you elaborate?

    There should be no problem integrating over functions that are piece-wise defined.
    For example if
    f(x) = 1 for x < 0 and f(x) = 2 for x > 0, the integral [itex]\int_{-1}^1f(x)dx = \int_{-1}^0 1 dx + \int_0^1 2 dx = 1 + 2 = 3[/itex].

    Start by writing out the function [itex]\Psi^* x \Psi[/itex]. It will be a piece-wise defined function. Then proceed to integrate it, and you will get <x> by definition.
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