The Motor Principle

The Motor Principle: when a current-carryingconductor is located in an external magnetic field perpendicular to theconductor, the conductor experiences a force perpendicular to itself and to theexternal magnetic field.

The right-hand rule for force on a conductorcan be used to determine the direction of the force experienced on theconductor: if the right thumb points in the direction of the currentin the conductor and the fingers of the right hand point in thedirection of the external magnetic field, then the force on theconductor is directed outward from the palm of the right hand.

The motor principle is used to form a precise definition ofampere. 1 ampere is the amount ofcurrent flowing through two straight parallel conductors1 meter apart in a vacuum which produces aforce 2 × 10^-7 newtons permeter of conductor.

Analog electric meters (i.e., galvanometers, ammeters, voltmeters) operateon the motor principle. Electric motors are an important application ofthe motor principle. An electric motor consists of a permanent externalfield magnet (stator) and a coiled conducting ammeter (rotor) which isfree to rotate within the field magnet. Brushes and a commutator(designed differently if A.C. or D.C. current is supplied to thearmature) connect the armature to an external voltage source. The speed of rotation of a motor depends on the amount of currentflowing through it, the number of coils on the armature, the strengthof the field magnet, the permeability of the armature, and themechanical load connected to the shaft.