Dropping a Magnet Through a Copper Pipe

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By Sean Michael Ragan
Posted 2011/12/14 @ 4:40 pm
Category Education, Science

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.

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21 Responses to Dropping a Magnet Through a Copper Pipe

Jesse Ransom on December 14th, 2011 at 5:48 pm said:

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

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Rahere on December 14th, 2011 at 6:14 pm said:

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

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- Anonymous on December 15th, 2011 at 10:04 am 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.
  
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  Ray Dzek on December 20th, 2011 at 1:42 pm said:
  
  So then permanent magnet motors aren’t?
  
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  Anonymous on December 21st, 2011 at 9:28 am 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.
  
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  frustum on December 21st, 2011 at 3:49 pm said:
  
  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.
  
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  Anonymous on December 22nd, 2011 at 1:24 pm 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.
  
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  darr247 on April 8th, 2012 at 5:21 am said:
  
  @ 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.
  
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Matthew Trey on December 14th, 2011 at 6:28 pm said:

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

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- Stephen Howard on December 15th, 2011 at 4:51 am said:
  
  Dark matter effecting fusion.. there is infinite power. We are the recipients of it by the simple fact of our being. http://www.cbsnews.com/video/watch/?id=4955212n
  
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- Manoj Mahalingam on December 20th, 2011 at 1:27 pm said:
  
  But make it long and into a ring Any law of conservation of energy means the magnetic energy will vanish sometime.
  
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Mr. i. Lulz on December 20th, 2011 at 2:16 pm said:

Solution:

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

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nick caruso on December 21st, 2011 at 8:13 am 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.

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- sandie on April 23rd, 2012 at 9:48 pm said:
  
  Why can’t you use ferrous metals for the tube? I know what happens when you do but I don’t know why.
  
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Anonymous on December 21st, 2011 at 8:15 am said:

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

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Skeptikat on March 24th, 2012 at 8:17 pm said:

Very cool! Thanks for the demo!

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Victor Blake on March 29th, 2012 at 5:30 am said:

That has to be one heck of a magnet.

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Mike Duncan on March 29th, 2012 at 6:42 am said:

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.

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- Shane Gibney on April 12th, 2012 at 6:02 pm said:
  
  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.
  
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Tony on May 25th, 2012 at 2:01 am said:

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

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- Tony on May 25th, 2012 at 2:15 am said:
  
  Here is a cool explanation of this effect
  http://www.coolmagnetman.com/magpipes.htm
  
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