I intend to place the following lab in a sequence of labs dealing with electromagnetism. This will not be the first of the series. I intend to begin with a study of DC electromagnets. After discovering the relationships between N and I on F, and comparing cores of various m, I plan to ask students their thoughts about AC.


I expect that some students will predict that AC will not work as a power source for electromagnets. I think they will assume that the polarity reversal (at 60 Hz, for example) will cancel itself over time, like electric polarities cancel in a crystal.


We will then investigate the advantages and limitations of each. Finally, we will see some of the many applications of AC electromagnets. I hope to diverge into the exotic with a coil gun and/or can crusher. But the ubiquitous solenoids, relays, and loudspeakers would protest if not included in this study. We'll look at speakers in this brief conceptual study.


Prelab Questions:


1.         Make a schematic sketch of a loudspeaker. The sketch should be simple and illustrate the basic components of a speaker.


2.         Make a schematic sketch of a microphone. The sketch should be simple and illustrate the basic components of a microphone.





Physics ### lab

Electromagnets in Loudspeakers




Purpose: The purpose of this experiment is to investigate the use of AC electromagnets in loudspeakers. We will make a simple speaker, and construct a simple telephone system.


Equipment: For this experiment, you will need:


*          24" of 30 AWG enamel magnet wire (from Radio Shack, p/n 278-1345B)

*          1.5" brass-coated steel brad (or equivalent)

*          Ceramic magnet, 0.5" diameter, 3/16" thick (from Radio Shack, p/n 64-1883)

*          A small piece of 100 grit sandpaper.

*          A Styrofoam cup

*          A mini plug (from Radio Shack, p/n 274-284)

*          1-1/4" Alligator Clips (from Radio Shack, p/n 270-380)

*          22-gage Hook-Up Wire (from Radio Shack, p/n 278-1244)




Background: Students generally assume that things like speakers are so complicated that construction would require especially skilled technicians and sophisticated equipment. (Gone, it seems, are the days of the crystal set.) This experiment will show how simple it is to make a speaker, and will illustrate the underlying concept.




Wrap the magnet wire (enameled finish to keep from shorting) around the brad near the head. Keep the coil length under 3/16", the thickness of the ceramic magnet. Leave a few inches at then ends in order to make connections. See picture below:


Place the ceramic magnet under the head of the brad and press the brad into the bottom of the Styrofoam cup. Separate the two tines of the brad and press them against the inside bottom of the cup as shown in the pictures below.


The magnet will stick to the brad (folded sheet steel).


You can see the tines, spread out and pushed against the bottom of the cup.



Finally, clean the ends of the magnet wire with sandpaper. In order to make an electrical connection, you need to remove the enamel finish.


Now solder the mini plug and alligator clips to the ends of a convenient length of hook-up wire. And that's it! Plug it in to a headset jack and listen to the music (or news).




Follow up Questions for Students:


1)         What is the purpose of the ceramic magnet?


2)         If you doubled the electrical power to the speaker, how should that affect the sound intensity?


3)         If you connect two speakers together with wire, will they work as a telephone system? Try it.







Instead of a brad, I cut a thin piece of sheet steel and folded as shown. It works the same.


I got good results from the headphone jack. However, I got nothing when I plugged into the regular speaker jack in the back of the computer. Hmmm... I will investigate this result.


My speaker was clear, but not very loud. Of course, it is a small set up; small coil, small permanent magnet. But I would like to drive it harder with a more powerful amplifier to hear the results.


I intend to include an activity for students to build a simple telephone system with only speakers and connected wire. Some pictures are below.


This is all that's required.

(And some wire. I placed the speakers about 50' apart)


I got the speakers out of an old broken computer.


The speakers I used were quite small. I'm sure that bigger would be better. I could hear tapping but not voice.




Cup speaker:


Simple Telephone:


HDD Speakers: