Want to hear your magnets? We’re going to use electromagnetism to learn how you can listen to your physics lesson, and you’ll be surprised at how common this principle is in your everyday life. This project is for advanced students.


We’re going to invert the ideas used when we created our homemade speakers into a basic microphone. Although you won’t be able to record with this microphone, it will show you how the basics of a microphone and amplifier work, and how to turn sound waves back into electrical signals. You’ll be using the amplifier and your spare audio plug from the Laser Communicator for this project.


[am4show have=’p9;p48;p77;’ guest_error=’Guest error message’ user_error=’User error message’ ]


Materials:


  • magnet wire
  • drill (or wind by hand)
  • nail
  • magnet
  • amplifier (see shopping list for info on where to get one)
  • audio plug


Download Student Worksheet & Exercises


An amplifier’s job is to take small electrical voltages (AKA the ‘input) and make them bigger (amplify them). Then, we usually plug a speaker or headphones into the amplifier and those turn the bigger electrical signal (AKA the ‘output’) into sound.  So any small voltage that we plug into the amplifier’s input will get larger and then turn into sound through the built-in speaker.


One way to show this is to use a coil of wire and a magnet. If you take a coil of wire and move a magnet past, around, or through it, you will create a small electrical voltage (and current) in the wire.  In fact, if you have enough wire and a big enough magnet, and move the magnet fast enough, the electricity coming out of the coil of wire can light up a light bulb (this is how an electric generator works).


So back to the amplifier: if we take the voltage from our little coil/magnet generator, and we put it into the amplifier, we’ll hear the sound from the speaker each time it makes a voltage. If we move the magnet back and forth really fast, we’ll hear a fast clicking sound. And if we were to move it super-incredibly-fast (faster than you could with your hands), then those clicks would blend together into a tone.  Tones like this are what all sounds are made of.


In fact, this is exactly what a microphone does.  Many microphones have a magnet and a coil of wire attached to a very thin piece of plastic or metal that vibrates when sound waves hit it.  The plastic (or metal) in turn moves the coil of wire next to the magnet super-fast. Then this causes the electric voltage to come out of the coil and if you plug it into an amplifier it will make the same sound that the microphone heard, only louder.


Exercises


  1. Why does the electromagnet make sound when you bring the permanent magnet close to it?
  2. How is this like a microphone?
  3. What did the aluminum do to the electromagnet?

[/am4show]