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A simple motor/generator demonstration for use in interactive lecture

Peter Bohacek, Henry Sibley High School
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This material is replicated on a number of sites as part of the SERC Pedagogic Service Project

Summary

This image shows a simple motor/generator apparatus that can be used to demonstrate Faraday's Law of Induction and the Lorentz force.

This apparatus is easy and inexpensive to construct and provides a clear and compelling demonstration of Faraday's Law of Induction and the Lorentz force. Two magnets are suspended from springs so that they are free to oscillate vertically. These magnets are placed inside two solenoids (coils of wire). The solenoids are connected. Moving one of the magnets induces a current in the solenoid. This current flows to the other solenoid where is generates a force on the other magnet, causing it to move. Connecting the solenoid leads in different ways produces different results. The instructor can ask students to predict the results of each connection.

Learning Goals

This activity is intended to demonstrate an example of Faraday's Law of Electric induction (generators) and the Lorentz magnetic force (motors). Students often find these to be abstract concepts. As such, multiple simple demonstrations help students comprehend these phenomena.

If this is used as part of an interactive lecture demonstration, students can also practice using physics reasoning to make predictions of the outcome of the experiment.

Context for Use

This activity is intended for use in an introductory physics course covering electricity and magnetism.This activity can be used as an interactive lecture demonstration. It can be used as an introduction to Faraday's Law and Lorentz force, or after students have studied these concepts.

Description and Teaching Materials

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A video showing the demonstrations (this can be used in lieu of the actual apparatus)

To download the video for use in class, use this Quicktime version: movie of motor/generator demonstration (Quicktime Video 6.7MB Aug20 10)






Teaching Notes and Tips

A careful explanation of each part of the apparatus will help students grasp how it works. Ask students to think through and explain each section. For example, ask them to describe what happens if a magnet is dropped through a coil of wire. Similarly, ask them to describe the magnetic field generated by a current flowing through a coil of wire, and the effect this field would have on a magnet in the coil. As they get closer to understanding, show them what happens if one magnet is moved while the coils are connected in phase (top of one coil connected to the top of the other).

Next, ask them to predict what will happen if the coil wires are switched, that is, if they are connected out-of-phase (top of one coil connected to the bottom of the other). Ask then to use physics reasoning to defend their position and to persuade others to accept their prediction. The instructor should encourage discussion before giving in to students' requests to reveal the answer by performing the demonstration.

Assessment

Perhaps the best assessment of what students gained from this demonstration is to ask them to predict the outcome of similar experiments and see whether there understanding of this demonstration helps them understand similar ones. For example, ask students what would happen if one of the magnets was flipped upside down, so that its north/south pole was positioned the opposite way to the other magnet.

References and Resources