# Dropping a Magnet Through a Copper Pipe

This simple demonstration of eddy current braking (Wikipedia) will probably be familiar to many of you, but this video from YouTuber JamesRB1995 is a short, well-shot, impressive documentation of the effect. Keep in mind that copper is not ferromagnetic, and there is no direct magnetic attraction going on here.

### 21 Responses to Dropping a Magnet Through a Copper Pipe

1. dude super cool that pipe. go get some dry ice hurry!!!

2. Any difference is velocity can be tapped for power. Do we actually have an infinite power source here?

• Anonymous on said:

Nope. If you draw energy from the copper pipe, it steals energy from the magnet. The magnet will actually get weaker. Think of the magnetic energy in the magnet like a battery.

• Ray Dzek on said:

So then permanent magnet motors aren’t?

• Anonymous on said:

Permanent magnet motors do get weaker after use. The more mechanical load you put on the motor, the more the electrical magnetic field tries to de-polarize the magnet.

• JammitTimmaj — are you trying to fool people, or do you actually believe that?  No energy is stolen from the magnet.  The energy is provided by his hand when he lifts the magnet away from the earth.  That potential energy is then spent: (1) mostly heating up the walls of the copper tube via eddy currents, (2) heating up the air very slightly as the magnet disturbs it, (3) another tiny bit goes to making clanging noises, and (4) the rest is work against his hand when he catches it.

• Anonymous on said:

You’re partially true. The magnetic domains in a magnet can be knocked loose. In fact, that’s how they were lined up in the first place. His hand did put energy into the system when he lifted the magnet, and energy is lost through resistive heating of the pipe, but there still is a “dragging” magnetic field being induced in the pipe. This “dragging” will eventually randomize the magnetic domains in the magnet. If this was done for a few hundred years I’m quite certain the magnet will probably be dead, with it’s magnetic domains truly randomized. Magnets will even loose their strength over time just sitting in a drawer, and shouldn’t be stored without attaching a steel bar “shorting” the poles together.

• @ Ray Dzek – there is no “loss of magnetism” from running a permanent magnet motor; “Anonymous” is displaying his/her ignorance (fortunately, ignorance is curable).

The power on the shaft comes from the electric current input; Or, you’ll get current output if you manually turn the shaft (generator mode). Many garden tractors use a single unit as both a starter, and as a generator to recharge the battery.

3. No infinite power source, you have to lift the magnets to the top of the pipe each time, this requires work.

4. Solution:

Install copper pipes around the tires of exercise bicycles, and place magnets on the tires of said bicycles.  Infinite energy!

5. nick caruso on said:

Another use for a discarded hard disk drive — extract the magnet assembly, and drop pieces of (non-ferrous) metal through the gap between the magnets.  those aluminum spacer rails used to mount smaller hard drives in older PC cases work well, and you can feel the interaction if you pull the metal up and down through the field.

• sandie on said:

Why can’t you use ferrous metals for the tube? I know what happens when you do but I don’t know why.

6. Anonymous on said:

Not familiar to me. That is cool. Real cool.

7. Very cool! Thanks for the demo!

8. That has to be one heck of a magnet.

9. This effect is due to what is commonly called “back emf” or back electro motive force. It is also present when you are spinning a generator. It is caused by the magnet creating a magnetic field (due to induced currents) in the copper pipe that are facing in the opposite direction to the magnetic field in the magnet. They work against each other and in effect apply a brake to the motion either from dropping a magnet through a copper pipe or from turning a generator. I would assume that the same effect would occur in the pipe were made of aluminum or any conductor like for instance lead. Someone should try that.

• Mike’s answer makes most sense to me. The falling magnet creates a changing magnetic field. A changing magnetic field creates a electric field in the metal. The electric field causes a current to flow. The flowing current creates another magnetic field which opposes the original one (Lenz’s law). As these two magnetic fields are opposing each other, they slow the magnet down as it falls. It would be interesting to measure the voltage between the top and bottom of the tube as the magnet falls.

10. also noticed that the thicker the copper tube the slower the magnets drop rate, It appears the thickness of the copper has an affect on the strength of the field then