Magnetic Pendulums

Copper coils become electromagnetic swings.

The current generated when one copper coil swings through a magnetic field will start a second coil swinging, showing some of the ways that electricity and magnetism interact.

Note: This Snack may be difficult to build properly or may not clearly show the desired results even if it is built correctly. This may be due to the effects being very subtle and/or hard to observe. The construction technique may require exceptionally fine tuning and tinkering, or it may just be inherently difficult to execute properly. That being said, if you're up for a challenge, go for it!

Subjects:

Keywords:

magnet

pendulum

feedback

exhibit-based

Tools and Materials

One 1 x 12-inch board, 24 inches (60 cm) long
Two 2 x 4-in boards, 18 in (45 cm) long
One 1 x 4-in board, 24 in (60 cm) long
One 9/16 in (14 millimeter) drill bit (for cow-magnet holes) and drill
Carpenter's tools: Ruler, pencil, hammer or screwdriver, saw
Nails or screws
Magnet wire (#22 gauge or finer) for the coils
Masking tape
2 cow magnets (usually 1/2-in [1.3-cm] diameter, available at feed stores)—be sure to get the cylindrical ones with the north and south poles at the ends, not the rectangular ones with north and south poles on the faces
An electrical lead wire with alligator clips at both ends
A partner, preferably an adult

Assembly

Use the 9/16-in (14-mm) drill bit to drill one hole through each 2 x 4 for the cow magnets. Make four saw cuts in the 1 x 4-in (2.5 x 10-cm) board. Nail or screw the boards together.
Make two coils of wire, each with at least 50 turns and an inside diameter of approximately 1 in (2.5 cm). Leave leads of approximately 3 feet (90 cm) at both ends of each coil. (Prewound spools can be used, but the second lead is not accessible, so the wire must be unwound from the coils and then wound back again to provide the leads on each end. This process doesn't take long and is worth doing, since the plastic spools provide convenient forms for the coils.)
Insert the magnets in the holes in the 2 x 4 boards. Insert the coils' leads into the slots in the 1 x 4 in (2.5 x 10 cm) board. Adjust the position of the coils so that they are centered right over the cow magnets. When the coils are properly positioned, bend the leads toward the middle of the top board, and tape them firmly in place near the holes. You want the coils to hang freely so that they are at the very ends of the protruding cow magnets, since this is where the magnetic field change is greatest.
Cut the leads so that the wires will reach the center of the top board with enough left over for each to be twisted together with a lead from the other coil. Scrape the insulation off the end of each wire, and twist them together as shown in the diagram to make a good electrical connection. The wires now form a continuous loop.
Tape the wire firmly to the top of the board. Make sure the wire on top of the board doesn't move when you swing the two coils.

To Do and Notice

Pull one coil back and then let it swing back and forth over a magnet. Notice that the second coil begins to swing.

Change the polarity of the magnet by flipping it 180° and reinserting it into the 2 x 4. Swing the pendulum again and notice what happens.

Remove the tape at the top of one of the coils, and reverse the coil. Retape the leads. Swing the pendulum again and notice what happens.

Connect a clip lead from one place where the copper leads are twisted together to the other. Swing a coil and watch what happens.

What's Going On?

When you start the first coil swinging on and off the end of the first magnet, a current is induced in the coil. Since the two coils are part of the same continuous circuit, this current also flows through the second coil.

A current-carrying coil of wire behaves like a magnet. The magnetic field around the second magnet attracts or repels the second coil, setting the second coil in motion. (Alternatively, you could say that the magnet exerts a force on the current flowing through this second coil. However, the electromagnet explanation is simpler.)

When the second coil swings, it becomes a generator too—that is, a current is induced in the coil. The resulting current in the two connected loops is a result of both coils swinging through both magnetic fields.

Reversing the coil or the magnet's polarity changes the direction of current induced in the coil. This in turn changes the direction in which the other coil swings.

The clip lead short-circuits the coils. The electric current generated by the coil you are swinging will not flow through the second coil, so the second coil will not move.

Going Further

You can monitor the current in the circuit by placing an ammeter (100 microamperes) in series in the circuit. You can also monitor the voltage in the circuit by placing a voltmeter (1 volt) in parallel in the circuit: Clip it to the two twisted wire junctions. Try to observe the phase relation between the swinging coils and the voltage and current measured by the meters.