Substitution into the Schrödinger equation gives the Pauli equation. This Hamiltonian is similar to the classical Hamiltonian for a charged particle interacting with an electromagnetic field. See Lorentz force for details of this classical case. The kinetic energy term for a free particle in the absence of an electromagnetic field is just p 2 2 m where p is the kinetic momentum, while in the presence of an electromagnetic field it involves the minimal coupling Π Π = p − − q A , where now Π Π is the kinetic momentum and p is the canonical momentum.