Laplacian in Cylindrical and Spherical Coordinatesaxmann/Math714/Misc/...In spherical coordinates ,...
Transcript of Laplacian in Cylindrical and Spherical Coordinatesaxmann/Math714/Misc/...In spherical coordinates ,...
Laplacian in Cylindrical and Spherical Coordinates
By
David Bosworth
Cylindrical Geometry
We have a tube of radius a, length L, and they are
closed at the ends. We have from the
Homogeneous Dirichlet boundary conditions at the
tube of the surface and separation of variables, so
our eigenmodes have the following form:
where the part in Z we have the constant 2k− , for θ
we have the constant 2m− , and for the radial part r is
given by the substitution of the two separation in
the equation:
Now we have our partial differential eigenvalue
problem into three ordinary differential eigenvalue
with solutions:
Where the boundary condition is homogeneous,
that is and
For the radial dependence part we make a change
of variable:
So we Bessel’s equation:
and the solutions are the J and Y Bessel functions:
where we can get rid of the Y Bessel functions
because they are singular at the origin. Also, the
boundary condition is homogeneous Dirichlet, so
where is nth zero of .
Thus, we have and finally the
eigenfunctions of the Laplacian in the cylindrical
coordinate are
and the eigenvalues are
with
Also, we know that these eigenmodes form a
complete orthogonal set because they are
eigenmodes of a Hermitian operator. Notice that
the inner product is the triple integral in the
cylindrical coordinates,
Since the Bessel Functions, the complex
exponentials and the sines are orthogonal
functions, then
as expected because the Laplacian is Hermitian for
these eigenmodes with homogeneous boundary
conditions.
Example
Let be an inhomegeneous
boundary conditions and consider the base of the
cylinder only, where the rest of the cylinder is
grounded, we have
Because of the inhomogeneous boundary
conditions, first we need to have the Poisson’s
equation in standard form by choosing a function u
that will satisfy the inhomogeneous boundary
conditions, for example:
Then we can write:
so where
so the Poisson’s equation in standard form is:
.
We can start computing:
The Theta integral gives zero unless m = 0. So we
must take m = 0 for nontrivial solutions, meaning
the potential, like its eigenmodes, will have
cylindrical symmetry (no theta dependence). This is
due to fact that the charge density we started from
also had cylindrical symmetry. Let’s solve the
remaining double integral with Mathematica:
Next, we compute the denominator, We
have seen that
Now, we can write the general solution in
Mathematica:
Next, we can plot the solution as a contour by
choosing:
Spherical Symmetry
Now we can consider the case of homogeneous
boundary conditions on a spherical shell of radius a.
In spherical coordinates , the eigenmodes
satisfy: . For review the spherical
coordinates are defined by the following
transformation from the Cartesian coordinates:
Thus, the Laplacian becomes:
Again we separate the variables:
Thus, the equation becomes
And can be turned into three ODEs for
We have already solved the first two equations:
where
are the spherical harmonics, and the associated
Legendre Functions.
The Spherical Bessel Functions
The boundary conditions for the radial modes are
and is finite. With Mathematica
we can find the solution and change the sign of
We have previously encountered the Bessel
functions for cylindrical geometry. However, those
Bessel functions were of order Integers. Now
we have found Bessel functions of order
and Since we the boundary condition
that R(0) is finite, and the function
is singular at the origin, we
keep only the function that
is not singular at the origin, as we can see from the
plot
and
So simply the radial mode is:
But we also have another boundary condition: R(a)
= 0, so where is the
nth zero of
As you can see from Mathematica:
For fixed l these eigenmodes are orthogonal with
respect to the radial inner product:
because
Thus, the final solution is:
where (n gives the
radial dependence of the modes –it “counts” the
zeros of the spherical Bessel functions),
and runs from 0 to infinity (l gives the
theta dependence of the mode), and
(m gives the phi‐dependence).
The Laplacian operator is Hermitian so these
eigenmodes are orthogonal with respect to the
usual inner product, that is now given by the triple
integral in spherical coordinates,