|Let's take a step back. The most common type of electric motor is the brushed dc electric motor. This is the kind that you'll find inside essentially everything that moves (or shakes) and runs on batteries. This type of motor attracts an electromagnet towards a permanent magnet. When the two are close enough,the polarity of the current through the electromagnet is reversed, so that it now repels the permanent magnet, and thus keeps turning. It's quite easy to build a working model.|
A simpler yet motor (sometimes sold as the sold as the "world's simplest motor") just switches off the current for half of the cycle, letting the angular momentum of the spinning motor armature carry it through.
None of these is really the simplest motor. The real champion is the homopolar motor.
Ready to build one? Let's get started:
|Set the screw on the magnet, bend the wire.|
|Press and hold the top end of the wire to the top end of the battery, making an electrical connection from the top battery end to the wire.|
Short movie (25 s): spinning up the motor (Choose your format)
|Download high-res quicktime movie (1.6 MB)|
|Watch the low-res version on YouTube|
Wondering what to try next? You may also like this project, which is about building a similar sort of motor that spins water instead of a magnet.
How does this work?
When you touch the wire to the side of the magnet, you complete an electric circuit. Current flows out of the battery, down the screw, sideways through the magnet to the wire, and through the wire to the other end of the battery. The magnetic field from the magnet is oriented through its flat faces, so it is parallel to the magnet's axis of symmetry. Electric current flows through the magnet (on average) in the direction from the center of the magnet to the edge, so it flows in the radial direction, perpendicular to the magnet's axis of symmetry. If you took physics at some point, it's possible that you'll remember the effect that a magnetic field has on moving electric charges: they experience a force that is perpendicular to both their direction of movement and the magnetic field. Since the field is along the symmetry axis of the magnet and the charges are moving radially outward from that axis, the force is in the tangential direction, and so the magnet begins to spin. Neat!
It's called a homopolar motor because you never need to reverse the polarity of any motor component during operation, unlike the other types of motors that we've described. I first learned about this type of motor in an article by David Kagan, in the magazine The Physics Teacher, It turns out that it's been around longer than that: it was invented in 1821 by Michael Faraday. Somewhat surprisingly, this is more than just a curiosity: motors of this design are currentlybeing developed for quiet, high-power applications.
|Final note: How do we measure the rotational speed of the motor?|
You can get an optical tachometer for $20 or less, intended for use with model airplanes. I have model LXPT31 from Tower Hobbies, which is expecting to see an airplane propellor with two blades. I added two wide black stripes to the magnet with a Sharpie, which allow the tachometer to read the rotational speed of the motor. Pointing the tachometer at the magnet and spinning up the motor, we were able to clock a speed above 10,000 RPM after spinning up for about fifteen seconds. Spiffy.