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Laplace-runge-lenz Vector




:
\mathbf{A}=\mathbf{p} imes \mathbf{L} - m k rac{\mathbf{r}}{r}

where:
  • \mathbf{r}\!\, is the position vector of the mass m\!\,,

  • \mathbf{L}\!\, is the Angular Momentum ,

  • k\!\, is a parameter that describes strength of the Potential .


The Laplace-Runge-Lenz vector is constant for the motion of a particle
acted on by a Central Force that varies as an inverse square (e.g.,
Gravity and Electrostatics ), i.e., for Potential s that vary
as -k/r\!\,.


PROPERTIES


By its definition, \mathbf{A} is perpendicular to \mathbf{L} (i.e.,

\mathbf{A} \cdot \mathbf{L} = 0
). (Recall that
\mathbf{r} \cdot \mathbf{L} = 0, because
\mathbf{L} \equiv \mathbf{r} imes \mathbf{p}).

The Laplace-Runge-Lenz vector can be used to derive the elliptical orbits of
the Kepler problem

:
\mathbf{A} \cdot \mathbf{r} \equiv Ar \cos heta =
\mathbf{r} \cdot \left( \mathbf{p} imes \mathbf{L} ight) - mkr


where heta is the angle between the position and
Laplace-Runge-Lenz vectors. Permuting the Triple Dot Product
\mathbf{r} \cdot \left( \mathbf{p} imes \mathbf{L} ight) = \mathbf{L} \cdot \left( \mathbf{r} imes \mathbf{p} ight) = L^{2}
and rearranging yields the formula for an Ellipse

:
rac{1}{r} = rac{mk}{L^{2}} \left( 1 + rac{A}{mk} \cos heta ight)


The vector points toward the Pericenter , from the geometric center of the orbit to the attracting, central body. The magnitude for a Periodic Orbit with Eccentricity e\!\, is given by:




A simple prescription for quantizing a classical system is to set
the Commutation Relation s of the quantum mechanical operators
equal to the Poisson Bracket of the corresponding classical
variables, multiplied by i\hbar. For brevity, we
define a reduced Laplace-Runge-Lenz vector

: