§3.3.1 Separation of Cartesian variables: Orthogonal functions Christopher Crawford PHY 416 2014-10-22
Outline Analogies from Chapter 1 Inner, outer products Projections, eigenstuff Example boundary value problem – square box Separation of variables Solution of boundary conditions Numerical methods for BVPs Relaxation method Finite difference method Finite element method
Analogies from Chapter 1 Inner vs. outer product – projections & components Linear operators – stretches & rotations
Separation of variables technique Goal: solve Laplace’s equation (PDE) by converting it into one ODE per variable Method: separate the equation into separate terms in x, y, z start with factored solution V(x,y,z) = X(x) Y(y) Z(z) Trick: if f (x) = g (y) then they both must be constant Endgame: form the most general solution possible as a linear combination of single-variable solutions. Solve for coefficients using the boundary conditions. Trick 2: use orthogonal functions to project out coefficients Analogy: the set of all solutions forms a vector space the basis vectors are independent individual solutions
Example: rectangular box Boundary value problem: Laplace equation
Example: rectangular box Boundary value problem: Boundary conditions 6