Episode 410: Preparation for electromagnetism topic
This topic develops a formal description of magnetic fields and moves on to electromagnetic forces and induction.
The scheme described follows a fairly standard approach that is similar to the order in which the topics appear in most exam specifications. The three main electrical machines, motor, generator and transformer, are covered at the end so that all the relevant physics will have been covered before they are discussed. It would be possible to introduce motors after Episode 412 but there would be problems in discussing back emf and eddy currents at this stage. Transformers can be used as a way of introducing the ideas of Episode 415.
An alternative approach might be to introduce the idea of flux via the transformer and generator, offering a different way of thinking.
Episode 411: Describing magnetic fields
Episode 412: Force on a conductor in a magnetic field
Episode 413: Force on a charged particle (F = Bqv)
Episode 414: Electromagnetic induction, flux and flux linkage
Episode 415: Electric motors
Episode 416: Generators and transformers
Looking ahead
Magnetic fields will have to be shown; iron filings are a standard way to do this, but you may also find a magnaprobe useful – this is a small magnet mounted in a universal joint so that it pivots freely in three dimensions. Fields will also have to be measured both for permanent magnets and with simple geometries of wires and coils. A Hall probe, especially one that is calibrated, is helpful but search coils can also be used; the latter will need a mains supply that can produce a current of a few amperes and/or a signal generator.
Some form of electron beam tube (‘Teltron tube’) is helpful in Episode 413 but an old black and white TV or oscilloscope plus a magnet can show the deflection of charged particles.
One or more sets of 120/240 turn coils and C-cores are useful when discussing flux while physically larger coils can help to develop ideas. If C-cores are not available, then retort stands (uprights) can be used for some work but the magnetic circuits will not be as good. A demountable transformer is great for some spectacular demonstrations.
'Westminster motor kits' can show both motor and dynamo principles but there are many cheap motors available which will also operate as dynamos when turned by hand or attached to a falling weight.
Main aims
Students will:
Describe the magnetic fields of permanent magnets and current-carrying conductors using lines of flux.
Use the terms flux and flux density in connection with magnetic fields.
Determine the force (magnitude and direction) on a current-carrying conductor in a magnetic field.
Explain the operation of a simple electric motor.
Determine the induced current or emf (magnitude and direction) when there is relative motion between a conductor and a magnetic field.
Explain the operation of a simple generator and a transformer.
Prior knowledge
Students should have met ideas relating to permanent magnets, the magnetic field due to a current, and motors. They will probably have done some electromagnetic induction, probably including transformers but their ideas in this area are likely to be much hazier.
There are links to be made with other fields. It is helpful, but not essential, if both electric and gravitational fields have been studied. When considering the force on charged particles, particle accelerators etc., students will need to be familiar with the equations governing circular motion.
Where this leads
An understanding of the motion of charged particles in electric and magnetic fields is vital when studying particle physics.
You may wish to touch on the relationship between electromagnetic fields and electromagnetic radiation (and the work of James Clerk Maxwell), but this idea is rather tenuous for students at this level.
Electromagnetic theory is one of the cornerstones of physics, and a grasp of these ideas is fundamental for students who are going on to future studies of physics, chemistry or engineering.
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