The Eddy Current Magnet Copper Pipe Trick, Explained

Drop something non-magnetic down a metre of copper pipe and it arrives in under half a second. Drop a neodymium magnet of the same size and you can count to three. Nothing is touching it, copper is not magnetic, and the magnet is not stuck. The eddy current magnet copper pipe demonstration is the most reliable piece of physics theatre in this entire collection, it takes two minutes to set up, and the explanation is genuinely satisfying rather than hand-waved. It also contains one result that nearly everyone predicts backwards.

Key Takeaways

• You’ll drop a magnet and a non-magnetic slug down the same copper pipe and time both.
• It teaches induced currents, Lenz’s law, energy turning into heat, and the idea of a drag force that grows with speed.
• Works from about age 9. Nothing is powered, nothing gets hot enough to notice.
• What usually goes wrong: a magnet that is too weak or too loose. Clearance of 1 to 2 millimetres and a neodymium magnet are both essential.
• The result that surprises everyone: two magnets stacked fall slower than one, not faster.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Copper pipe	1 m, 15–22 mm inside diameter	Plumbing merchant or hardware store, cut to length	Offcuts from a plumber, usually free
Neodymium disc magnet	One that drops through with 1–2 mm clearance	Hardware or craft shop, or a dead hard drive	Two thinner discs stacked to the same size
Non-magnetic slug of similar size	One	A short piece of brass rod, a coin that fits, a ball of Blu Tack	Anything of similar size that is not iron or steel
Aluminium tube, same length	One	A curtain rail, a tent pole	Optional, but it makes the comparison much better
Plastic pipe, same length	One	Hardware store, or a poster tube	Optional
Stopwatch or phone	One	Pocket	Slow-motion video is better than a thumb on a button
Folded towel	One	Cupboard	Catches the magnet at the bottom

Time: prep 5 min, experiment 25 min, cleanup 2 min Cost: around 10 to 15 dollars, mostly the pipe and the magnet Mess level: low

Do the Experiment: The Eddy Current Magnet Copper Pipe Drop

Step 1 — Prove the copper is not magnetic

Before anything drops, hold the magnet against the outside of the pipe and let go. It falls off. Try picking the pipe up with the magnet. Nothing. Put a steel paperclip on the magnet and it clings instantly. Copper is not attracted to magnets at all, and establishing that first is what stops the entire demonstration being written off as magnetic attraction.

Step 2 — Hold the pipe vertical and drop the non-magnetic slug

Have someone hold the pipe upright with the towel underneath, and drop the brass rod or coin in. Time it, or film it.

What you should see: it is through almost before you register it, in roughly half a second for a metre, which is very close to free fall.

Step 3 — Ask for the prediction, carefully

Will the magnet fall faster, slower or the same? And will it stick to the wall? Most people say the same, since copper is not magnetic. A few say it will stick. Almost nobody predicts the real answer, which makes this the best prediction question in the collection.

Step 4 — Drop the magnet

What you should see: a long pause. For a strong disc magnet with a couple of millimetres of clearance in a metre of copper pipe, expect two to three seconds. You can watch the pipe, say “any moment now”, and still beat it. If your pipe is clear you can see the magnet drifting down the bore at a steady walking pace.

Step 5 — Check that the speed is constant, not slowing

Mark the pipe at the halfway point. Film the drop in slow motion and compare the time for the top half against the time for the bottom half.

What you should see: the two halves take almost the same time. The magnet reaches a steady speed within a few centimetres and then holds it. That is a genuine terminal velocity, and it appears within an inch or two rather than after a long acceleration.

Step 6 — Compare pipe materials, then stack two magnets

Repeat with the aluminium tube and the plastic pipe. Then stack two magnets together and drop them down the copper.

What you should see: plastic gives free fall. Aluminium slows the magnet noticeably but less than copper does, because aluminium conducts less well. And two stacked magnets fall roughly half as fast as one, which is the opposite of what almost every child predicts. More mass should mean faster. It does not, and working out why is the best twenty minutes in this experiment.

What’s Actually Going On

As the magnet moves down the pipe, the magnetic field passing through any given ring of copper in the pipe wall changes: it grows as the magnet approaches and shrinks as the magnet recedes. A changing magnetic field through a conducting loop induces a voltage round that loop, and the copper is a conductor, so current flows. Because the copper is a continuous tube rather than a wire, the current flows in circulating loops, which is what the name eddy currents describes (HyperPhysics on Faraday’s law).

Those currents produce their own magnetic field, and its direction is fixed by Lenz’s law: it always opposes the change that created it. Ahead of the falling magnet, the copper’s induced field pushes against it. Behind it, the induced field pulls back on it. Both effects resist the fall. The copper has become, briefly and locally, an electromagnet arranged to slow the magnet down.

The key quantitative feature is that the induced voltage grows with speed, so the current grows with speed, so the retarding force grows with speed. Start the magnet from rest and gravity wins, accelerating it. As it speeds up, the drag force rises until it exactly balances the magnet’s weight, and from then on the speed is constant. That is why your halves take equal times, and it is why a falling magnet in a copper tube is a textbook case of a velocity-proportional drag force. Wiederick and colleagues worked out a simple theory for exactly this kind of magnetic braking and tested it (Wiederick et al., 1987), MacLatchy’s group made it a quantitative student experiment (MacLatchy, Backman & Bogan, 1993), and Hahn and colleagues modelled the tube geometry specifically (Hahn et al., 1998).

Now the two-magnet puzzle. Doubling the magnets doubles the weight pulling down, which should make it faster. But the braking force depends on the square of the magnetic field strength, because the field induces the current and then acts on that current. Double the field and the drag quadruples, while the weight only doubles. The net result is that a stack of two falls at roughly half the speed of one. That squared dependence is the single most useful thing in this article, because it explains why the effect is so dramatic with neodymium and nearly invisible with a fridge magnet: a field ten times weaker gives a hundred times less braking.

Finally, the energy. The magnet should have arrived at the bottom with a certain amount of kinetic energy, and it did not. That energy went into driving currents through the resistance of the copper, which means it became heat. The temperature rise is far too small to feel, but the accounting is exact, and it is the same accounting that makes eddy-current brakes on roller coasters and trains work. Those brakes have no pads, nothing wears out, and they cannot grip at a standstill, which is exactly what the velocity-proportional force predicts.

How to Explain It to Your Kid

Ages 5–8: The pipe pushes back

Copper is not sticky for magnets at all, and you can prove that first by trying to hang the magnet on the outside. But when a magnet moves past copper, the copper wakes up and starts pushing back, and it pushes harder the faster the magnet goes. So the magnet speeds up a little, gets pushed back more, and ends up drifting slowly instead. Have them drop the magnet and the coin again and again, and let them hold the pipe so they can feel that nothing is touching the magnet.

Ages 9–12: Moving makes current, current makes a magnet

Introduce the chain in three links. One: a magnet moving past metal makes electricity flow in the metal. Two: electricity flowing in a loop makes a magnet. Three: that new magnet is always arranged to oppose what is happening, so it fights the fall. Then have them test link one by swapping copper for aluminium and plastic. Plastic does not conduct, so no current, so no braking. That is a complete three-step explanation they can give to someone else, which is the real test.

Ages 13+: Velocity-proportional drag and the squared field

Give them the full chain, then set the measurement. Film drops with one, two and three stacked magnets and extract the terminal velocity from the slow-motion footage. If the drag goes as the square of the field and the weight goes linearly, the velocity should fall roughly in proportion to one over the number of magnets. Test it. The fit will be imperfect, because stacking magnets does not double the field at the pipe wall exactly, and discussing that discrepancy is better physics than a clean result.

The question to ask: “If I cut a slot all the way down the length of the pipe, would the magnet fall faster or slower?”

Why Teaching This Early Pays Off

This experiment attacks a specific wrong model in a way almost nothing else does. Children sort materials into magnetic and not-magnetic, and copper is firmly in the second box. Then the second box does something dramatic, and the model has to get more complicated: it is not about what sticks, it is about what conducts and what is moving. Research on how learners revise ideas suggests that such incorrect categorisations are usually built from real, partially valid knowledge, so the productive move is to extend the model rather than declare the old one wrong (Smith, diSessa & Roschelle, 1994).

It also introduces something children rarely meet: a force that depends on speed. Gravity does not care how fast you are going, and friction barely does, so most of a child’s intuitions about forces are speed-independent. A drag force that grows with velocity is the mechanism behind terminal velocity, air resistance at low speeds, car dampers and electrical braking, and meeting it in a form this vivid is a real head start. None of this will change a school grade. What it does is make “copper is not magnetic but it still slows magnets down” a sentence a child can say and defend.

Before You Start: What Parents Should Know

Best age	9 and up. The stacking result lands from about 11, and the squared-field argument from 13
Supervision	Light, but the adult owns the magnet at all times and counts it in and out
Real hazards	The magnet, not the physics. Neodymium discs snap together hard enough to pinch fingers badly and to chip into sharp fragments, they wipe bank cards and interfere with pacemakers, and two or more swallowed magnets attract through tissue and often require surgery, which the US Consumer Product Safety Commission states plainly. Cut pipe ends can be sharp: file them or tape the rims
Don’t	Don’t let a child under six anywhere near the loose magnet, and don’t use small magnet balls or cubes from a desk toy, which are the specific products behind most paediatric magnet-ingestion cases. Use one disc large enough that it could not be swallowed, kept in an adult’s hand between drops. There is nothing electrical in this experiment and nothing should ever be plugged in for it

When It Doesn’t Work

• The magnet falls almost as fast as the coin. Either the magnet is not neodymium, or the clearance is too large. A fridge magnet gives essentially no effect because the braking force goes as the square of the field. Aim for a magnet that fits with one or two millimetres to spare all round.
• The magnet jams in the pipe. Clearance too small, or the pipe is dented or has solder residue inside. Roll the pipe on a flat surface to check for bends, look through it against a light, and step down one magnet size if necessary.
• It falls slowly in aluminium and not at all slowly in copper. Check that your “copper” pipe is copper. Chrome-plated or brass-look pipes are sometimes plated steel, in which case the magnet will stick to the wall rather than float down it. A magnet that clings to the outside tells you immediately.
• The two-magnet result goes the wrong way. Make sure the magnets are stacked pole to pole so they attract and act as one longer magnet, not forced together in repulsion. Also check they have not shifted apart inside the pipe during the drop.
• Timing by hand gives scattered results. Human reaction time is around a fifth of a second, which is a large fraction of these intervals. Film at the highest frame rate your phone offers and count frames instead. The scatter collapses immediately.

Take It Further

Wall thickness and material: if you can get two copper pipes of the same bore but different wall thickness, time both. Thicker walls carry more current and brake harder. Then add the aluminium tube and rank all three by terminal velocity. The ranking follows electrical conductivity, which you can look up and compare against your own data.

Terminal velocity against magnet count: film one, two, three and four stacked magnets and extract terminal velocity for each. Plot velocity against number of magnets, and separately against the reciprocal of that number. One of those two plots will look much more like a straight line, and deciding which tells you something real about how the force scales.

Where the Eddy Current Magnet Copper Pipe Demo Belongs in a Sequence

Put induction first if you can. Shaking a magnet through a coil to light an LED shows the same induced current as a voltage you can measure, and this experiment shows it as a force you can see, so the pair explains each other. Before either, mapping a magnetic field with iron filings gives a child the picture of a field to talk about. On the mechanics side, the terminal-velocity idea here connects directly to whether a heavier parachute load really falls faster, which is the same balance of weight against a speed-dependent drag in a completely different physical system. That pairing is unusually good, because seeing one idea in two unrelated places is what transfer actually looks like.

What to Watch For Over the Next 3 Months

• Week 4: they explain the trick to someone else and get the three links of the chain in the right order.
• Month 2: they notice an eddy-current brake somewhere, usually on a roller coaster or a gym exercise bike, and recognise it.
• Month 3: they ask whether the pipe gets warm, which means they have followed the energy and are asking where it went.

Frequently Asked Questions

Is copper magnetic after all?

No. Copper is not attracted to a magnet at all, which you can confirm by trying to hang the magnet on the pipe’s outside. What copper is, is an excellent conductor, and the braking comes from currents induced in it by the magnet’s motion rather than from any attraction.

Why do two magnets fall slower than one?

Because the braking force depends on the square of the magnetic field while the weight depends only on mass. Doubling the magnets doubles the weight pulling down but roughly quadruples the retarding force, so the net result is a slower fall, at about half the speed.

Would an aluminium pipe work?

Yes, and noticeably less well. Aluminium conducts electricity less readily than copper, so the induced currents are smaller and the braking is weaker. Comparing copper, aluminium and plastic in one session is the cleanest way to show that conductivity is what matters.

Does the pipe get hot?

In principle yes, and in practice far too little to feel. The kinetic energy the magnet does not gain becomes heat in the copper’s resistance, but a single drop involves a fraction of a joule spread through a metre of metal. The energy accounting is exact even though the thermometer cannot see it.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. Wiederick, H. D., Gauthier, N., Campbell, D. A., & Rochon, P. (1987). “Magnetic braking: Simple theory and experiment.” American Journal of Physics, 55(6), 500–503. https://doi.org/10.1119/1.15103
2. MacLatchy, C. S., Backman, P., & Bogan, L. (1993). “A quantitative magnetic braking experiment.” American Journal of Physics, 61(12), 1096–1101. https://doi.org/10.1119/1.17356
3. Hahn, K. D., Johnson, E. M., Brokken, A., & Baldwin, S. (1998). “Eddy current damping of a magnet moving through a pipe.” American Journal of Physics, 66(12), 1066–1076. https://doi.org/10.1119/1.19060
4. Nave, R. “Faraday’s Law.” HyperPhysics, Georgia State University. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html
5. U.S. Consumer Product Safety Commission. “Magnets.” https://www.cpsc.gov/Safety-Education/Safety-Education-Centers/Magnets
6. Smith, J. P., III, diSessa, A. A., & Roschelle, J. (1994). “Misconceptions Reconceived: A Constructivist Analysis of Knowledge in Transition.” Journal of the Learning Sciences, 3(2), 115–163. https://doi.org/10.1207/s15327809jls0302_1