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Electromagnetic Induction Demonstration

Marsha M. Hobbs
Jackson Preparatory School
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This material is replicated on a number of sites as part of the SERC Pedagogic Service Project

Summary

This simple demonstration shows the interaction between electricity and magnetism. It is suitable for all levels of physics, from conceptual courses to calculus based introductory courses. Two coils of wire are held close to each other, but not touching. One is attached to a music source, such as a small radio or iPod, and the other is attached to an external speaker. Students can hear the music through the speaker even though there is no direct connection.
The level of explanation for the demonstration can vary from a very qualitative discussion of magnetic fields to a quantitative discussion of transformer theory.



Learning Goals

At the end of this activity the students will understand there is a relationship between the electric and magnetic fields. Specifically, they will verify that a magnetic field is created in a current carrying coil of wire and a changing magnetic field can induce a current in another coil of wire.
More advanced students will learn the relationship between the induced current and the spacing of the coils or the number of turns in each coil.

Context for Use

Conceptual or introductory high school class:
Materials:
1 small radio or ipod
1 small external speaker (Radio Shack Model: 277-1008 )
1 mini stereo cable (Radio Shack Model: PH62125 )
1 mini stereo cable cut in half with coils of magnet wire on each end

Description and Teaching Materials

The demonstration is set up by:
1. Turning on the radio and the speaker and connecting the cable between them
2. Removing the cable and replacing each end with the modified cable (coils of wire on each end)
3. Slowly bring the coils close together until the music is heard.
The students will be amazed and ready for an explanation. Several concepts can be discussed:
1. A current carrying coil of wire creates a magnetic field through it. Equations can be discussed if desired.
2. A changing magnetic field induces a current in a coil of wire as stated by Faraday's law of induction
3. A basic transformer is two coils of wire that can transfer electrical energy without touching.

For a more detailed discussion, the spacing of the coils can be changed, a piece of iron can be place through both coils, or coils with varying numbers of turns can be used.

Teaching Notes and Tips

The coils can be made by stripping the ends of a long piece of magnet wire (about 20 gauge for strength(, forming a coil with about 5 loops and soldering the ends to the two leads of the mini-stereo cable.

Assessment

White boards can be used to have students sketch the coils and fields after the demonstration and before the discussion.

References and Resources

http://faraday.physics.uiowa.edu/em/5k10.51.htm