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henpen

What's wrong with this infinite square well?

  • one year ago
  • one year ago

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  1. henpen
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    |dw:1360436634776:dw|\[φ''=-\frac{2mE}{ℏ^2} φ\] \[ \varphi= A \sin(\frac{\sqrt{2mE}}{ℏ}x)\] \[\varphi(a)=\varphi(-a)=0\] \[ \pm\frac{\sqrt{2mE}}{ℏ}a=n \pi\] \[ \frac{\sqrt{2mE}}{ℏ}=\pm\frac{n \pi}{a}\] \[\lambda= \frac{2 \pi}{\pm\frac{n \pi}{a}}=\pm\frac{2a}{n}\] Which, for n=1, is shorter than the real ground-state lavelength (4a). What have I done incorrectly?

    • one year ago
  2. JamesJ
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    You haven't written down the correct general solution. You can also have terms \[ ... + B \cos(\omega x) \] with \( \omega = \sqrt{2mE}/\hbar \).

    • one year ago
  3. JamesJ
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    This will give you the 4a, as cos(omega.x) = 0 => omega.x = pi/2 +- n.pi

    • one year ago
  4. henpen
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    Aren't you assuming A=0 for that solution? \[Asin(\pi/2 \pm n\pi) \ne 0\]

    • one year ago
  5. JamesJ
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    The general solution of y '' + w^2.y = 0 is y(x) = A.sin(wx) + B.cos(wx) or y(x) = A.cos(wx + phi0) Either way, there are two parameters. You are applying one of the boundary conditions. You actually are going to apply another boundary condition to y'(x) in order to completely nail down the solution.

    • one year ago
  6. JamesJ
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    Alternatively, the two conditions you are using phi(-a) = 0, phi(a) = 0. From this, you get two families of solutions. One with sin, the other with cos. This equivalent to using the second general solution with the phi0 alternating between 0 and pi/2

    • one year ago
  7. henpen
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    Ah, that's it! Thanks. What are the boundary conditions? That \[\varphi'(-a)=\varphi(a)=0\] Because \[\varphi(x)=0\] for \[x>a, x<-a\]?

    • one year ago
  8. henpen
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    *\[\varphi'(a)=\varphi'(-a)=0\]

    • one year ago
  9. henpen
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    That guess came from avoiding making the first derivative jump about at +- a

    • one year ago
  10. JamesJ
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    You have two conditions, which solving for the two parameters in the general solution. I recommend you start with A.cos(wx + phi0) and you'll get what you want.

    • one year ago
  11. henpen
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    \[A \sin( \omega a)+B \cos( \omega a)\] It is harder to make this expression = 0 in this form than yours, as both summands cannot be made = 0 or the negative of the other by eyeballing it, so yours is simpler. Thanks, I think I've got it.

    • one year ago
  12. henpen
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    @JamesJ , sorry to tag you, but I have a (very) brief question: the general solution is \[ \varphi= A e^{\pm i \omega x}\] What is done with \[\Im(\varphi)\]? Is only the real part considered, as is often done in classical mechanics? Thanks again

    • one year ago
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