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May. 06, 2011

Make a Speaker

by Science Friday Education

Click to enlarge images
In everyday life, we use magnets in a variety of ways, from holding notes on a refrigerator to securely locking a safe or a cabinet door. Electromagnets are especially useful because the magnetic field can be controlled by turning an electric current on or off. When an electromagnet is placed next to another magnet, the attraction and repulsion between the two can cause movement or vibrations.
 
In this activity, students will learn how an electromagnet works by making a simple one. Using this knowledge, students will design a diagram to make a working speaker using household materials. Then students will follow instructions on one method of making a speaker, and test their own designs to compare results.
Grade Level: 6th – 8th grade
Subject Matter: Physical Science
National Standards: NS.5-8.1, NS.5-8.2
So you want to jam like a rock star, but you don't want to shell out for an electric guitar? Make one yourself. Sound artist Ranjit Bhatnagar explains the art of a DIY instrument he calls a "junk guitar." You can piece one together using little more than a plank of wood, some wire, a magnet and a guitar string. Forget those air guitar solos. Plug in. Rock out.
 
Activity Materials
Reel of insulated copper wire - available in electronics or hardware stores
Three-inch nails – one per student or group of students
Double A (AA) batteries – one per student or group of students
Paperclips – several for each student
Small round ceramic magnets – one per student or group of students. These can be bought from Carolina.com. Here’s a link to the type of magnet you need
Plastic cups – one per student or group of students
Boom box radio - must have speaker wire output port
Roll of masking or Scotch tape
Scissors – one pair per student or group of students
 
Vocabulary
Sound: vibrations transmitted through an elastic material or a solid, liquid, or gas.
Magnetic Field: the space surrounding a magnetized body or current-carrying circuit in which the resulting magnetic force can be detected.
Electromagnet: a type of magnet whose magnetic field is produced by the flow of electric current. The magnetic field disappears when the current ceases to flow.
Insulator: a material or device that prevents or reduces the passage of heat, electricity or sound.
 
What To Do
Begin the lesson by having students watch the Science Friday video, “How To Make A Junk Guitar.” Ask students to describe the materials used in the video to make a junk guitar. Why was it necessary to use a magnet? Tell students that they will explore a simple way to make a speaker with household materials.
 
Activity 1 – Make An Electromagnet
 
1. Tell students that before they make a speaker, they have to know how an electromagnet works. Review the definition of an electromagnet. Can they think of a way to make a simple electromagnet?

2. Have students tightly coil the insulated copper wire around half of the nail until that portion of the nail is covered twice. Make sure students leave free at least 10 inches of wire on each end of the coil.

3. Use the scissors to strip about an inch of the insulation from each end of the free wire. Ask students why the wire needs to be exposed. Have students tape one end of the wire to one end of the battery.

4. Have students connect the other free end of the wire to the other end of the battery. Make sure students do not tape this end, since the coil will get hot if it is connected for too long.

5. Place a few paper clips in front of each student. Ask students to predict what will happen if the nail comes close to the paperclips. Have students pass the tip of the nail next to the paperclips. What happened?

6. Have students connect and disconnect the wire from the battery while touching the paper clips. What happens to the paper clips? Tell students that they have just made an electromagnet. Why are electromagnets useful?

Activity 2 – Make A Speaker

1. Tell students that now they will create a speaker, using the same basic principles that they learned from making an electromagnet.

2. Hand out to each student a plastic cup, a round ceramic magnet, tape and insulated copper wire. Ask students to draw a diagram, to show how they would make a speaker using these materials. Compare and contrast the various diagrams and have students explain their design.

3. To begin building the speaker, have students tape the round ceramic magnet inside the bottom of a plastic cup.

4. Coil the insulated copper wire 25 times into 1½-inch diameter circles with 10 inches of wire hanging free from each end. If needed, students may use an appropriately-sized round object to wind the coil, and then slide it off when they’re done.

5. Tape the coil to the bottom of the cup and make sure to strip a few inches of insulation from the end of each wire.

6. Have students test their speaker by connecting the two free ends of the coil to the “speaker out” of the radio. Students can hold the cup or let it rest on the table. Ask students to predict what will happen if the radio is turned on.

7. Turn the radio on and have students observe and describe what happens. What happens when you increase the volume?

8. Allow students to rebuild their speaker according to their own diagrams, if they differ from the steps above. Discuss the results. Why did or didn’t their diagram work?

Note: The coils will get warm if left on for an extended period of time. Also, increasing the volume will increase the heat.
 
What's Happening?
An electromagnet is a magnetic field that can be turned on or off. A simple electromagnet can be made by coiling wire around a nail and connecting the ends of the coil to a battery. The battery produces an electric current that flows from the battery through the wire. As the current flows through the wire, it creates a magnetic field. Once the current ceases to flow, the magnetic field disappears. Electromagnets are useful for devices that need magnets that can be controlled. A speaker uses electromagnets to produce sounds or vibrations.
 
A speaker is a device that converts an electric signal into sound by using an electromagnet, magnet and speaker cone. The radio sends an electric signal to the speaker output port. When the coil of wire is plugged into the output, the electrical signal passes through the coil. As electricity passes through the coil of wire, it becomes an electromagnet. The ceramic magnet inside the cup is attracted or repelled by the magnetic field of the coil. The bottom of the cup vibrates from the force of the electromagnet and ceramic magnet attracting and repelling. The vibration produces the sounds that we hear.
 
Topics for Science Class Discussion
• What is needed to make the speaker louder?
• How could you improve the sound quality of the cup speaker?
• What would happen if you replaced the nail in the electromagnet activity with aluminum foil? Or with a plastic core, like a pen?
 
Extended Activities and Links
Experiment with different numbers of turns on the electromagnet from the first activity. What is the magnetic power of a single coil wrapped around a nail? Or 10 turns of the wire? 100 turns? How about changing the thickness of the nail? Have students measure and compare the electromagnet’s "strength" by how many paper clips it can pick up.
 
Learn more about the different parts of a speaker:
http://www.bcae1.com/speaker.htm
 
Make a real record player out of simple materials:
 
 
Try hooking up your cigar box guitar to your homemade speaker!
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