In quantum mechanics
, the case of a particle in a one-dimensional ring
is similar to the particle in a box
. The particle follows the path of a semicircle from
where it cannot escape, because the potential from
is infinite. Instead there is total reflection, meaning the particle bounces back and forth between
. The Schrödinger equation
for a free particle
which is restricted to a semicircle (technically, whose configuration space
is the circle
Using cylindrical coordinates on the 1 dimensional semicircle, the wave function depends only on the angular coordinate, and so
Substituting the Laplacian in cylindrical coordinates, the wave function is therefore expressed as
The moment of inertia for a semicircle, best expressed in cylindrical coordinates, is . Solving the integral, one finds that the moment of inertia of a semicircle is , exactly the same for a hoop of the same radius. The wave function can now be expressed as , which is easily solvable.
Since the particle cannot escape the region from to , the general solution to this differential equation is
Defining , we can calculate the energy as . We then apply the boundary conditions, where and are continuous and the wave function is normalizable:
Like the infinite square well, the first boundary condition demands that the wave function equals 0 at both and . Basically
Since the wave function , the coefficient A must equal 0 because . The wave function also equals 0 at so we must apply this boundary condition. Discarding the trivial solution where B=0, the wave function only when m is an integer since . This boundary condition quantizes the energy where the energy equals where m is any integer. The condition m=0 is ruled out because everywhere, meaning that the particle is not in the potential at all. Negative integers are also ruled out.
We then normalize the wave function, yielding a result where . The normalized wave function is
The ground state energy of the system is . Like the particle in a box, there exists nodes in the excited states of the system where both and are both 0, which means that the probability of finding the particle at these nodes are 0.
Since the wave function is only dependent on the azimuthal angle
, the measurable quantities of the system are the angular position and angular momentum, expressed with the operators
Using cylindrical coordinates, the operators and are expressed as and respectively, where these observables play a role similar to position and momentum for the particle in a box. The commutation and uncertainty relations for angular position and angular momentum are given as follows:
- where and
As with all quantum mechanics problems, if the boundary conditions are changed so does the wave function. If a particle is confined to the motion of an entire ring ranging from 0 to
, the particle is subject only to a periodic boundary condition (see particle in a ring
). If a particle is confined to the motion of
, the issue of even and odd parity becomes important.
The wave equation for such a potential is given as:
where and are for odd and even m respectively.
Similarly, if the semicircular potential well is a finite well, the solution will resemble that of the finite potential well where the angular operators and replace the linear operators x and p.