In quantum mechanics, orbital angular momentum is a conserved property of a system of one or more particles that are in a centrally symmetric potential. If the radius of particle k with respect to the center of symmetry is rk = (xk, yk, zk) and if the momentum of the same particle is pk, then the orbital angular momentum of particle k is defined as the following vector operator,
where the symbol × indicates the cross product of two vectors. The total angular momentum of a system of N particles is
In the so-called x-representation of quantum mechanics, the vector rk is a multiplicative operator and
The components of the orbital angular momentum satisfy the following commutation relations,
The fact that L is a conserved quantity is expressed by the commutation with the Hamiltonian (energy operator)
It can be shown that this condition is necessary and sufficient that the potential energy part of H be centrally symmetric.