I have a question about how my book derives a formula. It starts with: We have an electron free too move on a cylinder and from the cylinder there is an electric field pointing radially outwards. Now for an electron moving in an electric field it sees local magnetic field given by: B = 1/c^2 v x E (1) And from spin pertubation theory we now get the following pertubation of the Hamiltonian: H' = -μ [itex]\bullet[/itex] B = C L[itex]\bullet[/itex]S I don't understand the last equality. I know μ(the magnetic moment) is proportional to the spin but how do we get L from (1)? - and how does it even make sense to use "v" the velocity in classical electrodynamics in a quantum mechanical system?