What is the correct method for solving the infinite square well energy problem?

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Discussion Overview

The discussion revolves around the methods for solving the infinite square well energy problem in quantum mechanics. Participants are examining the validity of different approaches to finding the energy levels associated with the potential well, including the forms of the wave functions used in their solutions.

Discussion Character

  • Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant presents their solution to the infinite square well problem and questions its correctness compared to a textbook answer.
  • Another participant points out that the assumed wave function solution of the form ψ = cos(kx) is not the only option, as solutions of the form ψ = sin(kx) also exist.
  • A participant references the textbook, suggesting that it has demonstrated that the energy expression holds for both cosine and sine solutions.
  • One participant expresses doubt about their solution, noting that their choice of k*a/2 = (n + 0.5)π leads to a contradiction for n > 0.
  • Another participant suggests that if n ≥ 0 were considered, the initial solution would be valid.
  • There is a discussion about the alternation of sine and cosine solutions with respect to the quantum number n, indicating that the ground state corresponds to a cosine function while higher states alternate between sine and cosine functions.
  • A participant expresses confusion regarding the previous comments made by another participant.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the correct method for solving the problem, as there are multiple competing views regarding the validity of different wave function forms and the implications of the quantum number n.

Contextual Notes

There are unresolved assumptions regarding the boundary conditions and the implications of the chosen wave function forms on the energy levels. The discussion also highlights the dependence on the definitions of the wave functions used in the solutions.

rsaad
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Hi

I have attached my attempt of solving the infinite square well for Energy. The value I get is different from that of the book, also in the attachment,
Kindly explain if my answer is correct given the fact that I proceeded step by step and used no tricks.
Thank you.
 

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You assumed the solution is of the form ψ = cos kx. There are also solutions of the form ψ = sin kx.
 
yes, but as you see in the excerpt of the book, they have proved that the the given E holds for cos(kx) too
 
I think my solution wrong because I have taken k*a/2 = (n+ 0.5)*pi
For the given situation, for x=a/2 for cos((n+ 0.5)*pi*x)=0 is only possible if n=0, but we are taking n>0.
 
If we were taking n>=0 then my solution was right.
 
The sine and cosine solutions should alternate with n. The ground state is a cosine, the next higher state is a sine, the next one is a cosine, etc.

If you put the walls of the box at x = 0 and x = a, it's easier because all the solutions are sines.
 
Last edited:
I do not understand what you said jtbell
 

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