  The effects of a magnetic field on the energy and on the spin of free
electrons are computed in the framework of quantum field theory. In the case of
a constant moderate field and with relatively slow electrons, the derived
formulae are particularly simple. A comparison with the approaches of classical
physics and of quantum mechanics shows essential differences and important
analogies. The relevance to the magnetic effects of the initial polarization
components of the electron states and the possible existence of special values
of these quantities are discussed in the final conclusions, which might be
useful to explain recent experiments on quasi-free electrons in chiral systems
in biology.
