Forces and Torques in Magnetic Field Systems

 The Lorentz Force Law gives the force F on a particle of charge q in the presence of electric and magnetic fields.

F = q(E + v× B)                              (3.1)

 F : newtons, q : coulombs, E : volts/meter, B : telsas, v: meters/second

 In a pure electric-field system,

F = qE                                 (3.2)

 In pure magnetic-field systems,

F = q(v× B)                               (3.3)

Unlike the case in Example 3.1, most electromechanical-energy-conversion devices contain magnetic material.

Forces act directly on the magnetic material of these devices which are constructed of rigid, nondeforming structures.

The performance of these devices is typically determined by the net force, or torque, acting on the moving component.  It is rarely necessary to calculate the details of the internal force distribution.

 Just as a compass needle tries to align with the earth’s magnetic field, the two sets of fields associated with the rotor and the stator of rotating machinery attempt to align, and torque is associated with their displacement from alignment.

 In a motor, the stator magnetic field rotates ahead of that of the rotor, pulling on it and performing work.

 For a generator, the rotor does the work on the stator.