• How can the direction of the magnetic field of an electromagnet be determined? Similar poles on magnets repel, while dissimilar poles attract. Running current through a wire creates a magnetic field similar to a field of a magnet. That field's direction and magnitude depend on the magnitude of the current and the direction of the current. The direction of the magnetic field can be determined by using the right hand rule:
• Place the wire in your hand with your thumb in the direction of the current (point your thumb from positive (+) to negative (-)). Close your hand so that your fingers wrap around the wire. Your fingers are pointing in the direction of the magnetic field lines.
• How Speakers Work?

A sound is produced when a vibrating object moves the air particles around it, which in turn move the air particles around them. Our ears pick up these fluctuations in air pressure and translate them into signals the brain can process. A speaker takes the electrical signal and translates it back into physical vibrations to create sound waves. Traditional speakers do this with a driver. The driver includes a permanent magnet and an electromagnet called a voice coil. The driver produces sound waves by rapidly vibrating a flexible cone that is connected to the voice coil. The voice coil is a basic electromagnet. This electromagnet is under the influence of a constant magnetic field created by a permanent magnet. These two magnets interact with each other the usual way. The positive end of the electromagnet is attracted to the negative pole of the permanent magnet and repelled by its positive pole. A stereo signal constantly reverses the flow of electricity switching the north and south ends of the electromagnet. In this way, the alternating current constantly reverses the magnetic forces between the voice coil and the permanent magnet rapidly pushing the coil back and forth. When the coil moves it pushes and pulls on the speaker cone. This vibrates the air around the speaker and creates sound waves.

• Set out pairs of magnets at each table.
• Briefly review magnets and their polarities.
• Possibly bring out a speaker radio and have it playing some music.

1. When the students enter have them explore, if they have not already started on their own, the affects the magnets have on each other when similar poles are near each other i.e. either North to North or South to South. Notice the affects the magnets have on each other when dissimilar poles are near each other i.e. North to South or South to North.
2. Distribute the remaining supplies. Have the students create an electromagnet by winding roughly 6 to 15 feet of the wire around a cylindrical object such as a C or D cell battery. Two inches of wire should be left hanging off of each end.

Winding the wire click for copyright

1. While keeping the wire in the shape of a coil, carefully remove the wire from the cylindrical object and tape the coil so it doesn't unravel.
2. Rub a piece of sand paper on each free end of the wire to remove the enamel insulation. Remove about 1 inch of insulation from each end.
3. Hold the ends of the wire on opposite ends of the battery to make a series circuit with the battery and the coil. Move the coil close to the magnet and observe its motion. Hook the battery up differently and see what happens to the magnets. (When the coil is connected to the battery in one way, one side of the coil is the north pole and the other side - the south pole of the electromagnet. The north pole of the electromagnet will be attracted to the south pole of the permanent magnet. When the battery is turned around, the poles of the electromagnet are reversed.)
4. Attach the coil and a permanent magnet to the bottom of the container with either tape or hot glue. Attach the coil and magnet in such a way that the coil and magnet are next to each other. There are many different creative ways to do this that will work. Examples are shown in Image 2 and Image 3.

The attached magnet click for copyright

click for copyright

1. Connect the ends of the speaker wire to the speaker output of the radio. On the back of the radio there should be plugs to connect the speaker wires to. These are usually holes that the wires are stuck into. Choose the plugs for one speaker (the left, for example) and insert one end of the wire from the yogurt speaker into one hole, and the other end of the wire into the other.
2. Turn the radio on and adjust the volume. When there is more current flowing through the wire coil, the electromagnetic force increases. As the radio changes the current very fast, the changing electromagnetic force causes the plastic cap to vibrate. The vibration creates sound waves in the air, which are heard.) (See Image 4.)
3. The wires of the speaker may get hot. When electric current flows through the wire, some of the electrical energy is converted to heat energy due to the résistance inside the wire. If you touch the wire, the heat will transfer to your skin. When the volume of the radio is turned up, more electricity flows through the wire and more heat is generated. If the speaker gets too hot, turn down the volume.

The yogurt speaker in action click for copyright