2nd order diff eq, 3 dim. and wierd boundry condition

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Homework Help Overview

The problem involves a scalar field within a three-dimensional box defined by specific boundary conditions and a second-order differential equation, specifically the Helmholtz equation. The context includes concepts from differential equations and Fourier analysis.

Discussion Character

  • Exploratory, Conceptual clarification, Problem interpretation

Approaches and Questions Raised

  • The original poster seeks guidance on how to approach solving the differential equation given the boundary conditions. Some participants suggest using separation of variables as a potential method. There is a discussion about the nature of the equation, with questions raised regarding its classification as a wave equation and its implications for the boundary conditions.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the equation and its boundary conditions. Some guidance has been offered regarding the use of separation of variables, but there is no explicit consensus on the best approach or resolution of the problem.

Contextual Notes

Participants are navigating the complexities of the boundary conditions, particularly the non-standard condition on one face of the box, which raises questions about the nature of the solution. There is also a mention of the original poster's limited background in differential equations and Fourier analysis, which may affect their approach.

johnnymopo
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I need help. For the following problem, can someone suggest how I should start on this question. I only have one quarter of diff eq classes plus a few classes in Fourier analysis. I'm out of my league.

Consider a box with length, width and height given by L. The box encompasses the region
described by 0<=x<=L, 0<=y<=L, and 0<=z<=L. A scalar field inside the box satisfies the
differential equation:

∇^2 ψ = −aψ

Here a is a positive constant that is equal to 30/L2.
The field is 0 on the surfaces y=0, y=L, x=0, x=L and finally the surface z=L.
On the surface z=0, the field has the functional form:

ψ(x, y) = ( 1-|x-2/L|*2/L)(1- |x-2/L|*2/L)

Solve for ψ as a function of x, y, and z inside the box. Your final solution must be
an analytic expression, though it can involve an infinite sum.


What I don't understand is that ψ is time-independent wave, correct? and if so, how do I solve this when the boundary on one face is clearly not a wave. Please point me in the right direction to figure out how to do this. thanks.
 
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Well, this isn't a wave equation since it doesn't involve t.

Have you tried separation of variables: \Psi(x,y,z) = X(x)Y(y)Z(z)?
 
Is that not a Helmholtz equation, and is a Helmholtz not a time-independent wave?
 
johnnymopo said:
Is that not a Helmholtz equation, and is a Helmholtz not a time-independent wave?

Yes it is. And apparently it is referred to as a wave equation.
 

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