  The physics involved in the fundamental conservation equations of the spin
and orbital angular momenta leads to new laws and phenomena that I disclose. To
this end, I analyse the scattering of an electromagnetic wavefield by the
canonical system constituted by a small particle, which I assume dipolar in the
wide sense. Specifically, under quite general conditions these laws lead to
understanding how is the contribution and weight of each of those angular
momenta to the electromagnetic torque exerted by the field on the object, which
is shown to consist of an extinction and a scattering, or recoil, part. This
leads to an interpretation of its effect different to that taken up till now by
many theoretical and experimental works, and implies that a part of the recoil
torque cancels the usually called intrinsic torque which was often considered
responsible of the particle spinning. In addition, I obtain the contribution of
the spatial structure of the wave to this torque, unknown to this date, showing
its effect in the orbiting of the object, and demonstrating that it often leads
to a negative torque on a single particle, i.e. opposite to the incident
helicity, producing an orbital motion contrary to its spinning. Furthermore, I
establish a decomposition of the electromagnetic torque into conservative and
non-conservative components in which the helicity and its flow play a role
analogous to the energy and its flux for electromagnetic forces. I illustrate
these phenomena with examples of beams, also showing the difficulties of some
paraxial formulations whose fields do not hold the transversality condition.

Optics