Motors exploit the force exerted on a a conductor carrying current in a magnetic field
This force is very weak so must magnify it by arranging for there to be a strong magnetic field and many conductors each carrying lots of current
Excitation (magnetic field) and energy conversion are separate functions sometimes performed by separate components and sometimes all in one
Separating excitation and energy conversion is helpful for understanding how all types of motors operate
Magnetic Field and Magnetic Flux
Current carrying conductor placed in magnetic field experiences a force
The magnitude of force depends directly on the amount of current and the strength of the magnetic field
Force is greatest when mag field perpendicular to the conductor
Magnetic flux lines are continuous loops with direction
section of a wire is like a bar magnet
The cause of magnetic flux in a permanent magnet is something to do with the atomic-level circulating currents within the magnet, but it isn't important for the physical level or for understanding.
Magnetic Flux Density
Flux plots can show direction but also the intensity of the magnetic field
between every pair of lines is the same amount of magnetic flux
magnetic flux is a mysterious abstract quantity which is practically useful for understanding
magnetic flux density is B
lines close together indicate the tube of flux is squashed into a smaller space
flux density B is the flux in the tube Phi divided by the cross sectional area A of the tube