Hands-on Science Carnival 2010 Activity Stations: Electricity & Magnetism

(Really) Simple Electric Motors

[Shopping List: neodymium disc magnets; insulated braided wire; bare solid copper wire; hex nuts; tape; steel screws; AA or AAA batteries; template or traced outline for bending the solid copper wire; small steel plate for motor stand (optional)]

Screw Motor

1. Place a small neodymium disc magnet on the head of a steel screw. Because steel contains some iron, it will be strongly attracted to the magnet and stick, magnetizing the screw as well.
2. Attach the opposite end of the screw to either side of the battery. The battery also contains steel, so the magnetized screw will stick to it, allowing it to dangle from the bottom as you hold the battery vertically.
3. Using a piece of braided (so it's easier to bend) insulated wire 1-2" longer than the battery, strip the insulation from at least 1/4" on each end.
4. Hold the battery (and the dangling screw with magnet attached) while pressing one end of the wire against the top terminal with your finger.
5. With your other hand, gently touch the opposite end of the wire to the side of the magnet. Because both the magnet and screw are coated with conductive metals, electricity will flow from one battery terminal through the wire, onto the surface of the magnet, through the screw and finally into the opposite terminal of the battery. As this happens, the magnet and screw should begin to turn, and if you're careful, it will spin very fast.

Tips: If the magnet and screw do not turn at first, try pointing the wire so that the side of the wire just brushes against the side of the magnet, barely touching. It may take a little practice to make it spin fast.

Egg-Beater Motor

1. Place a small steel hex nut over the + end of a AA or AAA battery (you may carefully tape it if you like, but be sure the hole in the top of the nut is not covered by the tape).
2. Attach a neodymium disc magnet to the - end of the battery. Because the battery is made of steel the magnet will stick, so don't use any tape on this end.
3. Measure and cut a length of solid copper wire, then strip any insulation completely, leaving it bare (these should be prepared in advance). [You could also use magnet wire, which is coated with an insulation layer, so you must sand or dissolve this layer from both ends and the center of the wire, i.e. anyplace the wire will touch a conductive surface.]
4. Using the provided template, bend the wire into a somewhat heart shape, with a sharp "V" bend at the top in the center and the two bottom ends horizontal and overlapping each other at least 1/2".
5. Lift the wire and bend gently to separate the bottom ends so that they still overlap horizontally by 1/2", but do not touch each other and are perpendicularly separated by a little less than the diameter of the magnet.
6. Stand the battery vertically on a table top. You may want to place the battery on a small flat piece of steel for a more stable base (a bottle cap works well as long as it was not bent by the bottle opener, and be sure the open end in pointed down (i.e. stick the magnet to the top of the cap).
7. Place the "V" bend of the copper wire into the hole of the hex nut on the top of the battery (it should cradle it nicely), and gently place each of the bottom ends of the wire on opposite sides of the magnet (180 degrees apart) so that the sides of the wire (not the very ends) brush against the side of the magnet. Because the magnet is coated with a conductive metal (nickel), electricity will flow from the top terminal of the battery in the same direction down both sides of the heart-shaped wire, onto the surface of the magnet and into the opposite terminal of the battery. As this happens, the entire copper wire should begin to rotate like an egg beater.

Tips: It may take some fine adjustments to get the motor to rotate smoothly. Be sure both ends of the wire are bent tangentially to the circumference of the magnet and just barely touching the edge, so that they can slide easily. Once you do have it turning smoothly, it should rotate very fast. The wire will also get very hot if you leave it running for too long, so be sure to disconnect it for a few seconds every couple of minutes.

What's Happening: In the first Electricity and Magnetism experiment we learned that electric current flowing through a wire induces its own magnetic field, and that a magnetic field produces a force (the magnetic force) that can cause another magnet to move. In both of these very simple motors, the battery causes an electric current to flow in a circuit through the wire, onto the surface of the neodymium (permanent) magnet and back into the battery. This induces a (temporary) magnetic field around the entire length of the wire, but most of the wire is too far from the permanent magnet for this to produce very much force. However, at the points where the wire touches the surface of the magnet the induced field from the electrical current is extremely close to the field of the permanent magnet, and produces a large enough force to actually move either the neodymium magnet or the copper wire. For the screw motor we are holding the wire and thus preventing it from moving, so it is the neodymium magnet (and attached screw) that moves. For the egg-beater motor, the magnet is stuck to the battery, so it is the copper wire that moves. The force produced is always perpendicular to both the permanent magnetic field and the direction of the electric current as it flows from the wire onto the magnet (we call this orthogonal), which causes the magnet (or the wire) to rotate.

Variations and Experiments: Try flipping the magnet over and repeating the experiment (with either motor). For the screw motor, try flipping the battery upside down (i.e. dangle the screw and magnet from the opposite terminal of the battery). How is the motor different now, and why?