Magnets affect materials in one of three ways, attraction, non-attraction (neutral)
and replusion. In a homopolar motor the flow of electrons through
a wire creates a magnetic field that is in oppostion to that of
the disc magnet and thus is repulsed by it. This repulsion creates
a tangential force called the Lorentz force that is reponsible
for creating the spinning motion. Dependent on which item is fixed
the opposite item will spin. So, in this case the magnet causes
the screw to become temnporarily magnetized and stick to the battery
and the wire is held in place while the screw allows the magnet
to spin along its axis.
This simple Motor is composed of simply a magnet, a battery, a piece of wire and a screw.