- #1
Justin LaRose
- 17
- 3
Homework Statement
I am trying to solve a problem from a popular quantum mechanics text. I am learning on my own. I am trying to calculate the variance, which is <x^2>-<x>^2 = variance in x.
I posted a photo of the problem as a picture that is linked below as well as the solution, I simply do not understand the solution. It is on page 14 of the Griffith's QM text (2nd edition)
Homework Equations
The relevant equations are in the photographs, it would be very onerous and take a long time to type them here, I do not want to post the screen shots because then they would need to be downloaded, and I am not trying to waste anyones time.[/B]
The Attempt at a Solution
Here is what's confusing me a great deal. There are a lot of things that have nothing to do with the physics that are throwing me off...
In part a) when normalizing psi(x,0), it is a simple u substitution, and if you look at the solution, I have absolutely no idea where the heck this 2 is coming from. Maybe I brain fried from looking at it for too long. If someone could please explain where this 2 comes from or if this is a typo, that would be much appreciated, it's a simple calculus problem.
The next problem I am having is this.
in b) the solution to determine the expectation value of x
Ah! I've forgotten, the integral from - infinity to infinity = twice the integral from o to infinity, of course! Okay so I've answered my own question for part a), sorry about that!
Jeeze I guess I've answered my own questions here. I am going to keep this text because perhaps it will be educational to illustrate that sometimes by writing out a problem a light flickers in your head! (I won't be posting an online link to the photos of the solution because I've figured it out)
There is one thing I am kind of confused about, if someone could please refer me to some text or break it down for me, is it always permissible (excluding the much talked about non physical pathological functions) to have an integral like this...
Integral from - infinity to infinity (some function) = 2 integral 0 to infinity (same function), or does the function need to have an odd or even property, I simply forgot, I am kind of rusty?
Alright thank you.
Justin[/B]