Dimensional analysis of the SED

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Discussion Overview

The discussion revolves around the dimensional analysis of the Schrödinger equation (SED) and the process of rewriting it in a dimensionless form. Participants are exploring the implications of substituting variables and how derivatives change in this context, with a focus on understanding the mathematical transformations involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about how to write the SED in dimensionless form and questions the substitution of variables, specifically x = x'x0.
  • Another participant notes that the second derivative changes and questions how a factor k relates to x0.
  • A different participant suggests that the derivative introduces a factor of 1/x0² but is uncertain about the overall process and the implications of switching variables.
  • One participant asserts that multiplying by m x0²/ħ² will yield the required coefficient in front of the second derivative, indicating a possible path to the solution.
  • There is a request for clarification on the procedure and the rationale behind the transformations, highlighting a lack of understanding among participants.
  • Some participants express frustration with the responses received, indicating a breakdown in communication and a desire for more constructive engagement.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the correct approach to the problem. There are multiple competing views on how to handle the variable substitution and the implications for the derivatives, leading to ongoing confusion and debate.

Contextual Notes

Participants mention specific factors and transformations without fully resolving the mathematical steps involved. There is an indication that assumptions about the variable substitutions and their effects on the equation may not be fully articulated.

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I have an exercise at the moment where I am supposed to put the Schrödingerin dimensionless form (the exact exercise is attached). I must admit that this idea of dimensional analysis is quite new to me. I don't understand how you can write the SED in the dimensionless form described. Therefore I could use some hints from one of you :) They introduce this new x' = x/x0. To put the SED in the given form are we then supposed to substitute x = x'x0 in the SED? If so I don't see how the h^2/2m disappears.
 

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The second derivative also changes:
[tex] \frac{\partial^2}{\partial x^2} \stackrel{?}{=} k \, \frac{\partial^2}{\partial x'^2}[/tex]
How is k related to x0?
 
I guess the derivative somehow would throw a factor of 1/x02 in but I am overall unsure about what is done in this exercise.
Do we switch variables from: x-> x'/x0? In that case everything that is named x should just be changed to x' and I can't see what sense that would make. On the other hand we could subsitute x= x'x0 but I don't see that going anywhere. Can you in detail explain what the idea is?
 
I'm not here to do your homework. You are right that you get a factor of [itex]1/x^2_0[/itex] in front of the second derivative. Now, if you multiply by
[tex] \frac{m x^2_0}{\hbar^2}[/tex]
the coefficient in front of the second derivative becomes [itex]-1/2[/itex], as required in the problem. You can read off what [itex]\tilde{V}_0[/itex] and [itex]\tilde{E}[/itex].
 
you are not here for my homework? No, I guess not but you are here to help me, and I think you can trust me on the fact that I have really tried to think this over but can't make sense of it:
So I ask again, in a more elaborative way, what is that is done:
Do we switch variables from x-> x'? In that way I don't see how the factor of 1/x02 comes in since you would basically just replace every x by x'? You could I suppose plug in x = x'x0 but that doesn't seem to make sense either. Can't you see the problem? A student pointed out the problem today too and the teacher agreed in a way and said something I didn't really get.
If you really don't want to help me fine, but please don't reply to my posts in future times :)
 
Last edited:
Please read through what has been already written carefully, instead of rambling on things that do not make sense.
 
I have! But your comments merely state that the derivative changes which throws in a factor of 1/x02. So in principle I can solve the problem yes. But I would like to understand what it is you do to obtain this. Why this aggressive attitude? I asked a question - if you don't want to help me then simply don't respond. And if things I say doesn't make sense maybe that should hint you at the fact that I don't understand the general procedure in this exercise.
 

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