Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B What Magnets are best to demonstrate deceleration?

  1. Dec 3, 2018 #1
    I'd like to demonstrate to students how a magnet is slowed when falling through a copper pipe due to generation of magnetic force. What type of magnet should I use? ShouId it be cylindrical or flat? Which way shouId it be magnetized? I have 3/4 inch pipe.
  2. jcsd
  3. Dec 4, 2018 #2


    User Avatar
    Science Advisor

    Any that don't jam or tumble in the pipe work. The stronger the field per unit mass the slower it goes. For instance a magnetized nail is not going to be very impressive, whereas a rare earth magnet is.

    As part of the demonstration, try it with two identical magnets. Drop one thru the pipe while the other is simultaneously dropped in free air. Keep them far enough apart (arms length) that they don't interfere with each other.

    To get their interest you could call it a curiousity with minimal introduction and no explanation; then ask for explanations. Of course if it is college level someone will know why, whereas grade schoolers probably won't.

    Have Fun!
  4. Dec 4, 2018 #3


    User Avatar
    Science Advisor

    It seems that a cylindrical magnet with a diameter just slightly smaller than the Internal Diameter of the pipe would be best, because it won’t tumble. Also, if there is any way to cut a slot down the length of the tube on one side, it makes the demonstration more impressive, as the students will be able to actually watch the magnet fall in slow motion.

    Another nice addition would be to drop a cork or other lightweight object similar to your magnet in size and shape, so they can see that it drops fast.
  5. Dec 4, 2018 #4
    If you cut a slot down the length would it still work?
  6. Dec 4, 2018 #5

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    I don't see how. How does the current flow?
  7. Dec 4, 2018 #6
    I recommend an N52 cylindrical neodymium magnet that as said above is slightly smaller in diameter than the pipe is
  8. Dec 4, 2018 #7


    User Avatar
    Science Advisor

    The slots are a judgement call. They improve visibility, but the breaking effect is slightly diminished. So long as the slot does not go the entire length of the tube, a counter emf is still generated.

    But dropping a non-magnetic “control” object really brings the point home, IMHO.
  9. Dec 4, 2018 #8

    Dr. Courtney

    User Avatar
    Education Advisor
    Gold Member
    2018 Award


    This is a rare earth magnet that is fairly strong. We purchased 100 of them for experiments testing magnetoreception in fish and discovered it in three new species. They also work well for a number of other applications requiring a strong magnet for physics labs and demonstrations, including slowing when dropped through a non-magnetic conducting pipe. These have a hole in them, because we wanted to be able to connect them securely to fishing equipment. There is a slightly stronger magnet available without the hole:


    I tend to recommend the version with the hole. The hole has not been a problem for any experiments or demonstrations, but it is handy for attaching to non-magnetic stuff. Once you have some of these, you'll think of other neat things to do with them.
  10. Dec 4, 2018 #9


    Staff: Mentor

  11. Dec 4, 2018 #10


    User Avatar
    Science Advisor
    Gold Member

    I'm desperately trying to figure out how that image relates to a bar or cylindrical magnet being dropped lengthways down a tube ??
    It shows a magnet end on to some sort of "S" shaped surface. Doesn't really show a magnet going down a tube

    So the point is .... how does the eddy current formation of the above image vary from the magnet in a tube scenario, where the
    magnet sides are parallel ( not end) on to the tube



    A Vs B ........ your quoted image infers a B setup

    My initial thought was the same as V50's

    I would suggest that "A", is more stable as in "B" the magnet is more likely to want to tumble

  12. Dec 4, 2018 #11


    User Avatar
    Science Advisor
    Gold Member

    .... drop the second magnet down a plastic tube of the same internal diameter, rather than in free air, that way air resistance within
    a confined space ( the tube) can be truly accounted for.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?