Sharing thoughtful approaches to teaching physics with the wider world.
What are your favourite Lenz's law
16 August 2017 at 14:17 #48114
A tweet on Twitter by @TakeOnPhysics
What are your favourite Lenz's law demonstrations? pic.twitter.com/kvwrlmhpl2
— Stimulating Physics (@TakeOnPhysics) August 15, 2017
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.
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.
17 August 2017 at 10:07 #48118
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.
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).
17 August 2017 at 21:53 #48121
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.
18 August 2017 at 12:46 #48124
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.
18 August 2017 at 13:01 #48125
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.
29 August 2017 at 16:08 #48171
Thanks for the video. It is great! I embedded it directly into your post. Hope that is ok!
18 August 2017 at 13:53 #48126
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
20 August 2017 at 20:47 #48133
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.
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.
7 September 2017 at 14:00 #48260
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.
2 October 2017 at 20:16 #48629
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.
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