The Motor Principle
When two magnets are brought near, they will either attract or repel based on their
poles (like poles repel, unlike attract).
A current carrying wire generates a magnetic field around it. (Remember RHR #1:
point your right thumb in the direction of the current and your fingers curl in the
direction of the magnetic field.)
The magnetic field generated by the current carrying wire can interact with other
external magnetic fields (as another magnet would) and result in an attraction or
repulsion. This would generate a force on the current carrying wire.
The Motor Principle:
When a current-carrying conductor is located in an external magnetic field
perpendicular to the conductor, the conductor experiences a force that is
perpendicular to both itself and the external magnetic field.
To determine the direction of the force acting on a wire, another right-hand rule
was developed:
RHR #3: Electromagnetic Force:
Point your thumb in the direction of the POSITIVE current flow and your fingers
in the direction of the external magnetic field (pointing North). The direction of
the force will come out from the palm of your hand.
The DC Motor
The direct current motor consists of a helix wound on a permeable core, external
magnets, and a device called a split-ring commutator that allows current to flow in
and out of the coil even when the coil is rotating.
A single loop is located between two opposite magnetic poles. Each side of the
loop is connected to a segment of the split-ring commutator. These segments are
insulated from each other. For most of the time, each segment of the split ring is
in contact with a carbon block, called a brush. The brush allows current to flow
from an external circuit through the commutator to the loop. The current flowing
through the coil produces a magnetic field (RRH #1) which interacts with external
magnets producing a magnetic force (RRH #3) which causes the coil to turn.
When the loop is in its vertical position, the commutator has rotated far enough so
that he carbon brushes are now in contact with the insulation that separates the two
halves of the split ring. No current flows through the loop, so no electromagnetic
forces act on it. However, because of inertia, the loop continues rotating.