What are your favourite Lenz's law

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    • #48114
      Profile photo of Andrew Normand

      A tweet on Twitter by @TakeOnPhysics

      Asking what are your favourite Lenz’s law demonstrations got me thinking.

      Lenz’s law is probably the phenomenon I have the most amount of demonstrations for.
      This year I was lucky enough to do a talk to a group of trainee teachers on the IOP Teacher Training Scholarship programme.

      If you know anyone who is considering becoming a physics teacher they really should look at the benefits to this scheme.


      Like all the events I attend, when talking to other physics teachers I learned some new ideas to enrich my teaching.
      One of them was an application of Lenz’s law in sorting out waste. My refuse is collected in various bins and one of these we put in glass, plastic and cans. Cans can be from ferrous metals or non ferrous metals. It was the idea that cans can be sorted by induced fields and Lenz’s law in non ferrous metals. The diagram below is from the literature of a company making such machines.


      Eddy Current Separator


      The non ferrous metal has an emf induced in it which then causes a current to flow in the metal. This induced current flows in such a direction as to oppose the increase in the magnetic flux that makes the induced emf that makes it.

      It is very similar to this AQA question
      AQA lenzs lawWe always set up this demonstration when discussing this question

    • #48118
      Profile photo of Andrew Normand

      I have a couple of demos that may be of interest.

      Firstly a analogue of the traditional speedometer


      Secondly an idea from an American YouTube channel based on a large rare earth magnet falling through a tube.

      https://www.youtube.com/watch?v=4OptIPWmOnc )

      Each LED  has one leg connected by 300 turns of  thin enamelled copper to the other leg. I also have these mounted on bobbins that fit an acrylic tube.


      (Edit: Links used as embed didn’t work for me).


    • #48121
      Profile photo of Andrew Normand

      Excellent David, I have not seen the speedometer idea before. The closest I have got to this is making an aluminium tray spin

      I also move around a floating Japanese Yen. Mainly so I can can float a coin on water.


      I have never got around to making the LED light up coil yet. I have seen the kit made and sold by Cochranes of Oxford.

    • #48124
      Profile photo of Andrew Normand

      The levitating ring question above comes from AQA spec A Jan 12 exam



      We have an old set of aluminium rings for this demonstration. A useful ring is the split aluminium ring. In AQA physics it is important for the students to understand that an emf is produced however a current will only flow if there is a closed loop. We have questions in our notes about the induced emf in aircraft wings when flying in different orientations through the Earth’s magnetic field. The split ring we have will show an induced emf if we crocodile clip a voltmeter on either side of the split. At the same time the ring does not levitate because no current is flowing to cause an induced magnetic field.

      You can purchase these in the Unilab catalogue.

    • #48125
      Profile photo of Andrew Normand

      A third video I have:


      Has a rare earth magnet rolling down an inclined aluminium bar.

      My interpretation: As the eddy currents get compressed at the edges this tends to realign the magnet toward the centre of the bar.

      If too great a speed or angle it does fall off.

      • #48171
        Profile photo of Andrew Normand

        Hi David,

        Thanks for the video. It is great! I embedded it directly into your post. Hope that is ok!


    • #48126
      Profile photo of Andrew Normand

      I like it, not sure how many more eddy current demos I can fit in my lesson. I do do a session on magnetism for the students that attended our HE plus sessions and magnetism is always a popular session at science club.

      I have spherical magnets and in this video below roll a spherical magnet and a non magnetic ball bearing down an aluminium plate

      A professor of physics I know got me to make him one of these to use in his lectures on particle interactions with the Higgs field causing inertial mass. My favourite demonstration is what I call my Yen slide. It is a steel ruler with alternate poles facing upwards of very flat magnets. The Japanese Yen is a very light pure aluminium coin. The poles have to alternating to cause a change in flux.

      We purchase our magnets from first4magnets

      The spherical ones https://www.first4magnets.com/sphere-magnets-c45

      The flat ones for the slide


      If you DM me David i’ll send you some Japanese Yen


    • #48133
      Profile photo of Andrew Normand

      I just read again the can sorter information above. An eccentric rotation allows the ferrous metal cans to be dropped quickly away from the changing magnetic field. This is because the eddy current heat the cans up quickly. It reminded me of something I had been told at several visits to NMR machines. Older tattoos had metal particles in the ink. Patient in MRI scanners fields with have bits of metal that are getting hotter and hotter.

      The first Lenz’s Law demonstration I saw as  student teacher was the one with an Al or Cu plate left to oscillate between the poles of a strong magnet. We made one in college from some old copper.

      copper plate

      The demonstration shows how when the eddy current are restricted there is a smaller magnetic force opposing the motion, This is good for explaining why cores are laminated.

      Here the the full copper plate and is heavily damped

      With slits cut in the copper the plate oscillates for a lot longer

      I made a version with my neodymium square. This is relatively cheap to make using these magnets


      Watch how the orientation of the pendulum so that the plane of the face is at right angles to the motion for the magnetic breaking.

      If you have a magnetic sqaure of alternative poles up magnets, then you really should invest in some pyrolitic graphite. The levitation below is not due to a field being created in the graphite but is to do with diagmagnetism. Still worth seeing, I find levitation is always enjoyed by students.

      It is definitely levitating as you can see in this video. Watch the second pass of the banner. I seem to have lost a magnet. It was found later on a radiator.

      I get my pyrolitic graphite here


      This seller made the one in this video for Grand Illusion, notice how the graphite has been cut into the characters from flatland. https://www.youtube.com/watch?v=oNRDvDIqD1M

      Going back to Lenz’s Law the demonstrations like the Yen slide and the oscillating plates have a real world application. It is used in rollercoasters. Rita Queen  of speed at Alton towers uses this system. It is how electric drills stop so quickly.

    • #48260
      Profile photo of Andrew Normand

      Not exactly on-message but related enough for me to hopefully get away with and a great anecdote to add to any lesson with this stuff.

      Something I read the other week in a book about astrology  that I feel is too good not to share.

      The short version is that Europa, the moon of Jupiter is not magnetic but the magnetic field of Jupiter is so large that as it orbits, an induced magnetic field is created in Europa probably because of a massive salty ocean. Amazing.

      Europa Magnetic

    • #48629
      Profile photo of Andrew Normand

      This is excellent information to add to my lessons.  I love things that lead me into an evening of reading around a topic. I found  a great article here,

      Magnetic fields of moons induced by the changing Jovian magnetic field acting on a conducting medium on or below the moons surface. In some moons this is salty water. This magnetic phenomena was used to first suggest these liquid water lakes existed under the surface on these moons. I was initially thinking Lenz’s law demonstrations, however the more I think of it you cannot really teach Lenz’s law without teaching Faraday’s law of electromagnetic induction.

      Just one more Lenz’s law demonstration I like to perform.

      Such an observable time difference.

      Not really talked about safety procedures for magnets in this thread yet. One of the main reasons I would use a reputable dealer like first4magnets.com is the detailed safety information. For example this leaflet came with the magnets we have purchased.


      My favourite demonstration of Faraday’s law of electromagnetic induction is my homemade guitar pick up.

      The guitar pickup is very easy to make. I wound enamelled copper wire around a stack of a few magnets from the Westminster motor kits.

      The wire around the magnets needs to be connected to a small guitar amp or something similar. The tuning fork needs to be ferromagnetic. This will then oscillate the B field and the rate of change of flux will induce and EMF in coil. The karimba on the left my son made. He has played it while gigging with his band.
      The technology was originally used in more code transmission, to make a carrier wave for heterodyne mixing. Hammond used the technology to generate tones for the organ. Also a Fender Rhodes electric piano uses tuning forks about coils around magnets to make the tones.


      Apparently in some BMWs a sensor in the wheel is constructed in a similar way.


    • #61269
      Profile photo of Andrew Normand

      We had some dry ice left over so I did the levitating ring. The ring on the right had been in dry ice for a while. It lowered the resistance and hence a larger current creating a larger opposing magnetic field.




    • #61270
      Profile photo of Andrew Normand

      I like the one I pinched from Bernard Rand that I believe he pinched from an edexcel paper, the masses on springs with neodyms under neath going in and out of a coil. We used it in an SHM lesson recently. He also did a really neat shaded pole motor with a drinks can which is the only use I’ve ever seen for our 25V supply.

    • #61592
      Profile photo of Andrew Normand

      We have added another Lenz’s law demonstration to our set. We spin a copper disk using an electric motor. A magnet is then lowered to be above the copper disk. This slows the disk down.



    • #61597
      Profile photo of Andrew Normand

      We have an eddy current brake from a bike trainer to do that.

    • #61730
      Profile photo of Andrew Normand

      I have got all the kit at home to set up Bernard’s coupled pendulums. I saw the idea here on Twitter


      If you use Twitter Bernard is definitely worth following for ideas

      Here is the video if you do not use Twitter


      These are the files for the AQA old spec A EMPA




    • #61734
      Profile photo of Andrew Normand

      I have always like coupled pendulums. I came across them in the old AQA EMPA.

      A great demonstration of the principle is given by sixty symbols

      I have always found this next setup difficult to arrange. Luckily others have done it

      It was an old EMPA question that has a lot of good questioning about data loggers and sample rate. Things that could come up on the new practical paper.







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