The Motor Principle: when a current-carrying
conductor is located in an external magnetic field perpendicular to the
conductor, the conductor experiences a force perpendicular to itself and to the
external magnetic field.
The right-hand rule for force on a conductor
can be used to determine the direction of the force experienced on the
conductor: if the right thumb points in the direction of the current
in the conductor and the fingers of the right hand point in the
direction of the external magnetic field, then the force on the
conductor is directed outward from the palm of the right hand.
The motor principle is used to form a precise definition of
ampere. 1 ampere is the amount of
current flowing through two straight parallel conductors
1 meter apart in a vacuum which produces a
force 2 × 10-7 newtons per
meter of conductor.
Analog electric meters (i.e., galvanometers, ammeters, voltmeters) operate
on the motor principle. Electric motors are an important application of
the motor principle. An electric motor consists of a permanent external
field magnet (stator) and a coiled conducting ammeter (rotor) which is
free to rotate within the field magnet. Brushes and a commutator
(designed differently if A.C. or D.C. current is supplied to the
armature) connect the armature to an external voltage source.
The speed of rotation of a motor depends on the amount of current
flowing through it, the number of coils on the armature, the strength
of the field magnet, the permeability of the armature, and the
mechanical load connected to the shaft.