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Basic Electromagnetism and Electromagnetic Induction
12 questions By Tony R. Kuphaldt
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Question 1 of 12
When lightning strikes, nearby magnetic compass needles may be seen to jerk in response to the electrical discharge. No compass needle deflection results during the accumulation of electrostatic charge preceding the lightning bolt, but only when the bolt actually strikes. What does this phenomenon indicate about voltage, current, and magnetism?
Reveal answerThe presence of an electric current will produce a magnetic field, but the mere presence of a voltage will not. For more detail on the historical background of this scientific discovery, research the work of Hans Christian Oersted in the early 1820’s.
Notes:The discovery of electromagnetism was nothing short of revolutionary in Oersted’s time. It paved the way for the development of electric motors, among other useful electrical devices.
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Question 2 of 12
Just as electricity may be harnessed to produce magnetism, magnetism may also be harnessed to produce electricity. The latter process is known as electromagnetic induction. Design a simple experiment to explore the phenomenon of electromagnetic induction.
Reveal answerPerhaps the easiest way to demonstrate electromagnetic induction is to build a simple circuit formed from a coil of wire and a sensitive electrical meter (a digital meter is preferred for this experiment), then move a magnet past the wire coil. You should notice a direct correlation between the position of the magnet relative to the coil over time, and the amount of voltage or current indicated by the meter.
Notes:Many students improperly assume that electromagnetic induction may take place in the presence of static magnetic fields. This is not true. The simple experimental setup described in the “Answer” section for this question is sufficient to dispel that myth, and to illuminate students’ understanding of this principle. Incidentally, this activity is a great way to get students started thinking in calculus terms: relating one variable to the rate of change over time of another variable.
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Question 3 of 12
A large audio speaker may serve to demonstrate both the principles of electromagnetism and of electromagnetic induction. Explain how this may be done.
Reveal answerI won’t tell you how to set up or do the experiment, but I will show you an illustration of a typical audio speaker:
The “voice coil” is attached to the flexible speaker cone, and is free to move along the long axis of the magnet. The magnet is stationary, being solidly anchored to the metal frame of the speaker, and is centered in the middle of the voice coil.
This experiment is most impressive when a physically large (i.e. “woofer”) speaker is used.
Follow-up question: identify some possible points of failure in a speaker which would prevent it from operating properly.
Notes:Since not everyone has ready access to a large speaker for this kind of experiment, it may help to have one or two “woofer” speakers located in the classroom for students to experiment with during this phase of the discussion. Any time you can encourage students to set up impromptu experiments in class for the purpose of exploring fundamental principles, it is a Good Thing.
