Make an Electric Guitar: Electromagnetism!

1 make an Electric guitar : electromagnetism ! An acoustic guitar works by physically amplifying the sound waves created by the strings vibrations moving the air around them. What is sound? What are the allowed modes of vibration of a fixed string, and what are the variables that you can change? [Could also discuss harmonics.] How does the resonance chamber work? The Electric guitar An Electric guitar works by converting the vibrating motion of the string into an electrical signal. That is done through the pickup. That electrical signal is then sent to an acoustic amplifier which amplifies the electrical signal and turns it into a sound wave.

2 What is the difference between an electromagnetic wave and a sound wave? The pickup does not depend on sound in any way, but on magnetism and electromagnetic induction. A simple Electric guitar consists simply of a taut string and a pickup. You can get multiple frequencies by changing the tension in the string or effective length of the string, or by having multiple strings of differing densities under different tensions. Equations! = Equation 1: Relationship between wave speed, frequency, and wavelength for all waves. = Equation 2: Relationship between wave speed, tension, and linear density (mass/length) for waves on a string.

3 The Pickup The pickup consists of a coil of wire around a magnet. In our design, we sandwich it between two pieces of cardboard in order to hold it together. You can experiment with the strength/size of the magnet and the number of coils, but a simple pickup can be built as follows: Step 1: Using a 2 inch long rectangular ceramic magnet (about inch thick) or a inch thick and at least inch diameter neodymium magnet, glue it between two pieces of thin cardboard that overlap all sides of the magnet by at least an inch (more if you are going to try a larger coil than suggested here).

4 Thus you should get something that resembles a spool for you to roll your coil on. Note that it does not matter if your magnet is not cylindrical, you can still wind a coil of wire around it! Step 2: Using magnet wire (28-32 gauge, typically enamel coated copper), wind at least 200 loops around your magnet (about 50 feet): Before you start, tape down the leading end of the coil or cut a slit in the cardboard to slide it into and hold it steady. make sure to leave a few inches or more extra so that you can attach alligator clips to it later. Wind 200-400 (or more if you want to try!)

5 Loops around the magnet, trying to keep the wire taught while you are winding. As you wind, the coil should typically spread itself out nicely on your spool . At the end, again remember to leave some extra to attach to. Put the end through the slit or tape it down to keep the coil tight. Step 3: Using fine sandpaper, carefully sand the enamel off both ends of the magnet wire sticking out of the pickup coil. You need to clear off the enamel so that the copper is bare and you can make electrical contact. If you are sanding on a table, be sure to protect the table so that you do not scratch it!

6 Attach a 1/8 phono jack to your pickup (to the bare wire) with the alligator clips as provided (see image above, right), and plug it into an amplifier. Do not turn on the amplifier yet. [For the greatest effect, use a full size guitar amp!] The String(s) An Electric guitar string is made of a material that can be magnetized (a ferromagnetic material): iron, nickel, cobalt. We will discuss why in a moment. [Define ferromagnetic, paramagnetic, diamagnetic.] A refrigerator is typically paramagnetic, which is why fridge magnets stick to them, but it is not itself a magnet.

7 A frog is diamagnetic, which is why it will levitate in a magnetic field [see levitating frog video on Youtube; explain that this is not a particular property of frogs, but of water, or of materials we tend to think of as being non-magnetic]. Diamagnetism, to a greater or lesser degree, is a property of all materials and always makes a weak contribution to the material's response to a magnetic field. For materials that show some other form of magnetism (such as ferromagnetism or paramagnetism), the diamagnetic contribution becomes negligible. (Wikipedia) Step 4: Attach your string (tie it) to something fixed in place (that can take some force without it moving) such as a table clamp.

8 Run the string across some portion of the table and over a pulley attached at an edge. Attach the hanging end of the string to a mass hanger, and hang enough mass so that the string is taut. Pluck the string and make sure you can hear the twang (as quiet as it may be) and that it is reasonably stable. Try adding more or less mass, or push down on the weight hanger to hear the change in frequency of the sound that is being created by the string moving the air. Step 5: Now put your pickup beneath the guitar string. You need it to sit as close to the guitar string as possible without touching, so you might need to put it on something, of you can have someone hold it up.

9 Turn on the amplifier and pluck the guitar string: you should hear the sound coming from the amp! Vary the tension in the string as you pluck it to hear the changes in frequency. How the Pickup Works: Ferromagnetism and Electromagnetic Induction The magnet is in the pickup coil simply to make the guitar string into a magnet. If you bring a magnet near a ferromagnetic material, the material will become magnetic itself if it is not already. [Becoming magnetic means that the domains line up in the external magnetic field and stay lined up, at least temporarily.]

10 Why do you think they might not stay lined up?]. When you move a magnet near a wire, it exerts forces on the charges in the wire (this is called the Lorentz Force). If the wire is part of a circuit, then the forces will make charge flow, and an electrical current is induced (this is called Faraday s Law of Induction, and might be one of the most important discoveries of modern time, do you know why?). So the magnetic wire is vibrating near the coil of wire in the pickup, and thus will induce an electrical current that varies with the same frequency as the wire.