Mind the Magnetic Gap
A swinging aluminum plate stops dead in the presence of a strong magnetic field. Then, with one small change, it swings with ease.
- 10- or 12-inch (25- or 30-cm) wooden hand-screw clamp
- Two 1 x 1 x 1/2-inch double countersunk rare-earth magnets (these should have cone-shaped depressions on both sides)
- Drill bit for pilot holes
(Note: Typically, countersunk rare-earth magnets have holes for #6, #8, or #10 screws. A #6 screw will require a 7/64-inch [2.75-mm] pilot hole; a #8 screw will require a 1/8-inch [3-mm] pilot hole; a #10 screw will require a 9/64-inch [3.5-mm] pilot hole.)
- Pencil (not shown)
- Electric drill
- Safety glasses
- Polyurethane glue, such as Gorilla Glue
- Two stainless-steel flathead screws that fit the magnets above
- Screwdriver or screwdriver bit to fit screws
- Scrap piece of Styrofoam about 2 x 2 x 1/2 inches (5 x 5 x 1 cm)
- Masking tape
- Perforated aluminum plate, 6 x 12 x 1/8 inches (15 x 30 x 0.3 cm), with holes of approximately 1/2 inch (1 cm) in diameter
- Steel or aluminum rod smaller in diameter than the holes in the aluminum plate and at least 12 inches (30 cm) long
- Two 1 1/2 x 1 1/2 x 3/4-inch (4 x 4 x 2-cm) wooden blocks
- Drill bit the same diameter as the steel or aluminum bar
- Ring stand
- Right-angle clamp
This Snack is constructed in three parts. In Part I, you’ll put together the variable-gap magnet assembly. In Part II, you’ll prepare the perforated aluminum plate. In Part III, you’ll make the wooden bushings that help the plate swing and complete the assembly.
Part I: Make the Variable-Gap Magnet
In this part of the assembly, you’ll be screwing the two magnets to the inner tips of the hand-screw clamp so that they come together, facing (and attracting) one another when the clamp is closed.
- Open the wooden hand-screw clamp as wide as possible.
- Place one magnet against the inside tip of the wooden hand-screw clamp, along the top edge (see left photo below). Put a drill bit through the hole and tap with a hammer to mark a spot to drill (see photos below). Set the magnet aside.
- Drill a pilot hole at the pencil mark. (As noted above, a #6 screw will require a 7/64-inch [2.75-mm] pilot hole; a #8 screw will require a 1/8-inch [3-mm] pilot hole; a #10 screw will require a 9/64-inch [3.5-mm] pilot hole.)
- Cover one side of a magnet with polyurethane glue and place it over the pilot hole. Insert a screw and tighten the magnet firmly in place (see photos below).
- Cover the magnet with a piece of thick Styrofoam and tape in place (see photo below). This is to protect both the magnet and you.
- Determine which side of the remaining magnet should go against the wood on the other side of the clamp. The two magnets should be arranged so that they attract each other (see left photo below). Then repeats Steps 2, 3, and 4 to secure the second magnet in place (see right photo below).
- Allow the glue to dry overnight.
Part II: Prepare the Aluminum Plate
In this step, you’ll cut slits into the top of the perforated aluminum plate.
- Hold the aluminum plate vertically.
- Working down from the top, use the hacksaw to cut slots between the centers of the holes going down one edge of the plate (see photo below). Stop when you’ve made a vertical strip about 4 inches (10 cm) long.
- Repeat until slots have been added to each column of holes on the plate (there should be about nine in all). Then set the plate aside for final assembly in the next section (click to enlarge the photo below).
Part III: Make the Wooden Bushings
These wooden bushings will help the plate swing in a straight line.
- Find the diameter of the steel or aluminum rod. Drill a hole through a wooden block that is the same diameter as the rod. Repeat with the second block.
- Using the right-angle clamp, support the rod crosswise on the ring stand.
- Slide one wooden block a short distance down the rod.
- Hold the aluminum plate so the slots are on the bottom. Using a hole near the top, slide the plate onto the rod so that it sits against the wooden block.
- Slide the second piece of wood onto the bar and sandwich the plate between the two wooden bushings. Adjust the spacing of the bushings so that the plate can swing freely, but also swings straight.
Pull the aluminum plate back until it’s at about a 45-degree angle, then let go and allow it to swing through the gap between the magnets. You’ll probably need to adjust the gap and the positioning to get a clean swing. Carefully watch the motion of the plate.
Remove one wooden block and pull the plate off the rod. Flip the plate so that the cut side is now up and the uncut side swings through the gap. What happens to the plate’s motion?
Raise the plate by differing amounts. How does that change the motion, if at all? Grab the plate and push it through the gap. What does it feel like?
That strange feeling that the plate is moving through molasses is caused by eddy currents. Eddy currents are electric currents that flow in circles every time you move a metal object in a magnetic field.
At the atomic level, eddy currents can be explained by a phenomenon known as the Lorentz force. When charged particles in the aluminum plate move through a magnetic field, they experience a sideways deflecting force—the Lorentz force—that causes them to move in circles in the plane of the plate. These circular currents create magnetic fields of their own, which interact with the external magnetic field to resist motion.
On the side of the plate with the cuts, however, the circular currents are largely blocked, and so less effective at generating magnetic fields. That’s why the plate can easily swing through the gap between the magnets.
The strength of the eddy currents generated depends on the speed of the charged particles moving through the magnetic field—so the faster the plate moves, the stronger the magnetic field produced, and the greater the braking force. You can feel this when you pull the plate through the gap by hand. Pulling the plate through slowly is easy, but the faster you pull, the greater the resistance. You can also see this by releasing the plate at different angles.
The strength of the force moving the charged particles is also dependent on the strength of the magnetic field. The closer the magnets are to each other, the stronger the field, so the effect is the largest when the gap is the smallest.
In the real world, eddy currents can be useful or a nuisance, depending on the circumstances. In the Drop Zone amusement park ride, which drops riders down from the top of a tall tower, eddy currents help slow the falling passenger cars when they reach the bottom. Metal parts in electric motors, however, sometimes have slots cut into them to prevent unwanted eddy currents from slowing them down.
You can make a similar plate out of plywood. Since wood has charged particles in it, just as aluminum does, it may not be obvious why wood doesn’t stop in the magnetic field. The difference is that the electrons in wood aren’t free to move, so they can’t support the currents that make a magnetic field.
It’s pricey, but you can also buy a perforated copper plate that’s the same size and shape as the aluminum. Copper is a better conductor, so you might imagine that it stops just as well or better—but it doesn’t. Though the stopping force is, in fact, a bit higher, the mass of a copper plate is more than three times greater, so the copper plate takes longer to slow than an aluminum plate.