Parallel vs Radial Magnetic Fields

This is part of the HSC Physics syllabus under the topic Applications of the Motor Effect.

HSC Physics Syllabus

  • investigate the operation of a simple DC motor to analyse:

- the functions of its components

- production of a torque `\tau = nIAB_(_|_) sin \theta`

- effects of back emf  


Why Do DC Motors Use Radial Magnets?

This video investigates the operation of a simple DC motor. It focuses on the effect of using a radial magnet and why this is preferred over parallel magnets.


Radial Magnets vs Parallel Magnets

Parallel magnetic fields are uniform as the magnetic field lines are always parallel and equidistant to one another. Radial magnetic fields produce a non-uniform radial field. In a radial magnetic field, the field lines are not parallel. 


Recall that `\tau = nIAB \sin \theta`. As the coil rotates in a parallel field, the angle `\theta` between the area vector and the magnetic field lines will change. This causes the magnitude of torque to oscillate between a maximum and 0 N m in the form of a rectified sinusoid.


However, the radial magnetic field lines are able to be perpendicular to the area vector more consistently throughout its rotation. This leads to torque being more constant at its maximum value.


Previous section: Operation of a Simple DC Motor

Next section: Back EMF in a Simple DC Motor