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sanitykey
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[SOLVED] Gaussian Form Integration
http://img153.imageshack.us/img153/9489/001km6.jpg
I'm trying to do the last part.
Stated in the question.
For (iii) i know the probability density is:
[tex]A^2e^{- \frac{x^2}{\sigma ^2}}[/tex]
Edit: Deleted my first attempt which I'm pretty sure was wrong and I tried using integration by parts which looked quite promising:
The integral I'm trying to do is:
[tex]\int_{0}^{\sigma} A^2e^{- \frac{x^2}{\sigma ^2}} dx [/tex]
(Take the [tex]A^2[/tex] out for now)
Which i can split into:
[tex]A^2\int_{0}^{\sigma} e^{- \frac{x^2}{2\sigma ^2}} e^{- \frac{x^2}{2\sigma ^2}} dx [/tex]
Let u = [tex]e^{- \frac{x^2}{2\sigma ^2}}[/tex] and dv = [tex]e^{- \frac{x^2}{2\sigma ^2}}[/tex]
then du = [tex]-\frac{x}{\sigma ^2}e^{- \frac{x^2}{2\sigma ^2}} dx [/tex] and v = [tex](0.68) \sqrt{2\pi} \sigma[/tex]
Using [tex]\int udv = uv - \int vdu[/tex] The integral becomes:
[tex]\left[e^{- \frac{x^2}{2\sigma ^2}}(0.68) \sqrt{2\pi}\sigma \right]_{0}^{\sigma}[/tex] - [tex](0.68) \sqrt{2\pi} \sigma \int_{0}^{\sigma} -\frac{x}{\sigma ^2}e^{- \frac{x^2}{2\sigma ^2}} dx [/tex]
= [tex]\left[e^{- \frac{x^2}{2\sigma ^2}}(0.68) \sqrt{2\pi} \sigma\right]_{0}^{\sigma}[/tex] - [tex]\left[e^{- \frac{x^2}{2\sigma ^2}}(0.68) \sqrt{2\pi} \sigma\right]_{0}^{\sigma}[/tex]
Which i think = 0 and then multiplied by [tex]A^2[/tex] still = 0
Is this right?
Thanks
Richy
Edit 2: Woops i put xs instead of es that was probably really confusing to anyone who read this before my edit, sorry!
Edit 3: Actually when i think about the graph for this which would generally be a gaussian shape and it wants me to integrate or find the area between 0 and [tex]\sigma[/tex] i don't see how the result can be 0, i think i might have went wrong somewhere but i can't see where.
Edit 4: Ok i just found these answers in a different paper apparently:
[tex]\int_{0}^{\sigma} e^{- \frac{x^2}{\sigma ^2}} dx = (0.421)\sigma \sqrt{\pi} [/tex]
and
[tex]\int_{-\infty}^{\infty} e^{- \frac{x^2}{\sigma ^2}} dx = \sigma \sqrt{\pi} [/tex]
From this i can see that the probability of finding the particle is 0.421 but why didn't my integration work? How would i work this out without this knowledge?
Homework Statement
http://img153.imageshack.us/img153/9489/001km6.jpg
I'm trying to do the last part.
Homework Equations
Stated in the question.
The Attempt at a Solution
For (iii) i know the probability density is:
[tex]A^2e^{- \frac{x^2}{\sigma ^2}}[/tex]
Edit: Deleted my first attempt which I'm pretty sure was wrong and I tried using integration by parts which looked quite promising:
The integral I'm trying to do is:
[tex]\int_{0}^{\sigma} A^2e^{- \frac{x^2}{\sigma ^2}} dx [/tex]
(Take the [tex]A^2[/tex] out for now)
Which i can split into:
[tex]A^2\int_{0}^{\sigma} e^{- \frac{x^2}{2\sigma ^2}} e^{- \frac{x^2}{2\sigma ^2}} dx [/tex]
Let u = [tex]e^{- \frac{x^2}{2\sigma ^2}}[/tex] and dv = [tex]e^{- \frac{x^2}{2\sigma ^2}}[/tex]
then du = [tex]-\frac{x}{\sigma ^2}e^{- \frac{x^2}{2\sigma ^2}} dx [/tex] and v = [tex](0.68) \sqrt{2\pi} \sigma[/tex]
Using [tex]\int udv = uv - \int vdu[/tex] The integral becomes:
[tex]\left[e^{- \frac{x^2}{2\sigma ^2}}(0.68) \sqrt{2\pi}\sigma \right]_{0}^{\sigma}[/tex] - [tex](0.68) \sqrt{2\pi} \sigma \int_{0}^{\sigma} -\frac{x}{\sigma ^2}e^{- \frac{x^2}{2\sigma ^2}} dx [/tex]
= [tex]\left[e^{- \frac{x^2}{2\sigma ^2}}(0.68) \sqrt{2\pi} \sigma\right]_{0}^{\sigma}[/tex] - [tex]\left[e^{- \frac{x^2}{2\sigma ^2}}(0.68) \sqrt{2\pi} \sigma\right]_{0}^{\sigma}[/tex]
Which i think = 0 and then multiplied by [tex]A^2[/tex] still = 0
Is this right?
Thanks
Richy
Edit 2: Woops i put xs instead of es that was probably really confusing to anyone who read this before my edit, sorry!
Edit 3: Actually when i think about the graph for this which would generally be a gaussian shape and it wants me to integrate or find the area between 0 and [tex]\sigma[/tex] i don't see how the result can be 0, i think i might have went wrong somewhere but i can't see where.
Edit 4: Ok i just found these answers in a different paper apparently:
[tex]\int_{0}^{\sigma} e^{- \frac{x^2}{\sigma ^2}} dx = (0.421)\sigma \sqrt{\pi} [/tex]
and
[tex]\int_{-\infty}^{\infty} e^{- \frac{x^2}{\sigma ^2}} dx = \sigma \sqrt{\pi} [/tex]
From this i can see that the probability of finding the particle is 0.421 but why didn't my integration work? How would i work this out without this knowledge?
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