If you spend much time browsing home-science channels on youtube, you’ve probably seen videos of what happens when you drop a magnet through a metal tube. If not, enjoy this mind-bending display of Eddy Current Braking.
While it might be difficult to get your hands on such a large magnet and pipe, the phenomenon will occur with a tube of any size, as long as it’s made of a sufficiently conductive material and your magnet is powerful enough. While magnets will ordinarily only stick to metals that contain one of the three ferromagnetic elements, Iron, Nickel, or Cobalt, this effect has nothing to do with that “stickiness”; it arises from a different property of electromagnetism known as Lenz’s law, which is related to Faraday’s law of induction. Faraday’s law states that a changing electromagnetic field creates an electric current, and vice versa; a changing electric current creates a magnetic field.
If you’ve ever used a shake-powered flashlight, you’ve seen these two laws in action; the motion of a magnet back and forth through the spiraling copper coils inside the device creates an electric current, which can charge a battery or power a small LED. It may be tempting to think of it as the magnet “dragging” a small number of electrons through the metal back and forth with it, but the reality of the situation is slightly different. The change in magnetic field strength as you shake the flashlight back and forth induces a circular current perpendicular to the direction of the magnet’s motion; that is, around the barrel of the flashlight. This is why the spiral shape (and insulation) of the conductive coils is necessary; in trying to travel around the magnet, the current is also forced toward one end of the wire.
If you apply this principle to a copper tube, though, the electrons can make a full circuit without moving one way or the other. As the magnet falls and its field grows stronger in the portion of pipe below it, a “ring” of current is generated around the tube. That increase in electric current, correspondingly, creates a magnetic field, which repels the magnet and slows its fall. Trailing the magnet, the current grows weaker, but since it’s the change in electric current that creates magnetic effects, this too acts to slow the magnet’s fall, this time by attracting it from above.
All this amounts to a striking distortion of the apparent effects of gravity, and makes an excellent tool for sparking interest in science or just showing off to your friends. Here at PhysicsCentral, we created a fun way to drive home the power of this effect, using nothing but a bit of plumbing and a few office/classroom supplies:
All you’ll need for this project is a few magnets (rare earth magnets work best; fridge-style magnets usually aren’t strong enough to see the effect), scissors, clear tape, transparency sheets, and a metal tube that’s slightly larger than your magnets, cut into sections. Most hardware stores will cut pipe when you buy it, if you ask. Otherwise, you may also need a handheld pipe cutting tool.
Start by cutting the transparency into strips roughly the same length as your sections of pipe, and a little wider than the interior circumference of the tube.
Then roll each strip into a tube, insert it into the pipe, and tape it so that you have a transparent tube which fits snugly into the metal. This may take a few tries, especially if you’re using a small tube; the plastic can be springy.
It helps to leave just a small bit of plastic exposed when forming the end of the tube to maintain shape, then tape it along the seam as you pull it out, bit by bit.
Once you’ve taped the entire strip into a tube, slide a piece of pipe over each end. If done properly, you may not even need to tape them together; the transparency will be flush with the inside of the tube or close to it. This is important to prevent the magnet from getting caught on the ridges that occur otherwise.
Repeat this with all your sections of pipe, drop a magnet down it, and watch the bizarre interplay of electricity and magnetism at work! The person holding the pipe should be able to feel the apparatus get heavier when the magnet is in the copper sections, and lighter when it free-falls through the plastic parts.
If you have your own pipe cutter, a miniature version can be constructed with significantly less tape and time by using small (~3-4 cm) sections of pipe, and fitting them around a single sheet of transparency. Happy Experimenting!
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