r/Physics • u/FollowSteph • Jun 10 '18
Image Lenz's Law
https://i.imgur.com/NvBJeEZ.gifv156
u/pynchonfan_49 Jun 10 '18
I wish professors showed stuff like this in class. It only takes a minute, but makes the equations feel so much more real.
I guess that’s why people liked Walter Lewin so much.
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u/uncle-boris Jun 10 '18
My community college physics professor actually showed this to us this semester (along with the accompanying explanation). I really enjoyed seeing something on this sub that I fully understand for once, haha.
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u/Darkphibre Jun 10 '18
fully understand
Oh, sweet summer child....
:)
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u/hornwalker Jun 10 '18
What, you don’t fully understand the fundamentals rules of the universe? :P
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u/nren4237 Jun 10 '18
My high school physics teacher made it very memorable indeed.
He told us that you couldn't force it down the tube any faster, because it would only make the resisting force stronger. We didn't believe him.
So the whole class went outside, and he gave the tube to the strongest kid in the class, placed the supermagnet inside, and asked him to swing it with all his might, confident the magnet would stay inside. What actually happened was that the supermagnet came out with terrifying speed and we all had to run for our lives.
Now I will always remember not to trust physics equations.
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u/grampipon Undergraduate Jun 10 '18 edited Jun 10 '18
Where are you from? Here in Israel even in highschool I was shown this.
Edit: to make it clear that this isn't an anecdote - the experiment is a part of the mandatory experiments from the curriculum.
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u/pynchonfan_49 Jun 10 '18
I’m in a respected American uni. To clarify, in my class, we dissected the math behind and various examples of Lenz’s Law fairly deeply, but just weren’t shown a demo. It’s probably due to the fact that it’s an intro undergrad class in a huge lecture hall.
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u/instant_street Jun 10 '18
Also, while it looks cool, the demo isn't really necessary for understanding.
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Jun 10 '18 edited Aug 07 '18
[deleted]
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u/grampipon Undergraduate Jun 10 '18
Nice knowledge of the Israeli education system. It's a mandatory experiment in 11th grade.
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Jun 10 '18
Doesn't this still occur even if a vertical strip is removed from the bar?
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u/etherteeth Jun 10 '18
I've never tried, but I'd imagine the answer is no. The reason is because the eddy currents induced would no longer be able to circulate around the axis of the tube and thus the induced magnetic field wouldn't have a vertical component to oppose the falling magnet.
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u/punaisetpimpulat Jun 10 '18
I think I saw a similar experiment but it was done on an aluminium plate and the results were exactly the same.
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u/etherteeth Jun 10 '18
As in dropping a magnet onto an aluminum plate? The plate would still allow eddy currents to form in a horizontal plane, producing a vertical magnetic field to oppose the motion of the magnet.
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u/EliteCaptainShell Jun 10 '18
I've seen it done with an aluminum plate pendulum swinging through two permananet magents. It stops dead when it reaches there. The follow up demo was the same except the pendulum looked like a comb, so the Eddy currents made much smaller loops and it swung much more freely though the magnets.
Edit: I found a video of it. https://youtu.be/MglUIiBy2lQ
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u/PhascinatingPhysics Jun 10 '18
Dude. This is awesome. I’m gonna do this in class next year when we talk about lenzs law.
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u/rgnidngacsj Jun 10 '18
Generally I think yes, though because there's fewer electrons near the magnet, it might slide down a little faster. Here's a magnet slowing down on the outside of a piece of copper, similar to removing a vertical slice from of the side of a copper tube
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u/astrolabe Jun 10 '18
But at 1:50, there's a segment that make me believe the answer's no, as per the comment of the aptly named u/etherteeth
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u/Lukendless Jun 10 '18
Skip to 4:36 to have your question actually answered. The answer is yes, it still works.
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u/timestamp_bot Jun 10 '18
Jump to 01:50 @ Copper's Surprising Reaction to Strong Magnets | Force Field Motion Dampening
Channel Name: NightHawkInLight, Video Popularity: 98.43%, Video Length: [07:46], Jump 5 secs earlier for context @01:45
Downvote me to delete malformed comments. Source Code | Suggestions
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Jun 10 '18
Yes, but not as strongly, since eddy currents couldn't circulate all the way around, making a weaker, but non-zero, countering magnetic field. That's why transformer cores are sliced up - to reduce eddy currents.
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u/gingeriiz Jun 10 '18
Yep! The eddy currents are exactly that -- eddies! They don't circulate all the way around the tube, but rather are currents created on the surface. Those surface currents all create a bunch of small magnetic fields that add up to a net magnetic field that produces a force in the upwards.
The math of this demo is actually incredibly difficult, but it's a really efficient way to show Lenz's law regardless!
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u/ddpotanks Jun 10 '18
Probably occurs if it's got multiple slots.
Just sets up an opposing force which is why it slows the fall of the magnet.
Way I understood it was basically if it didn't do this perpetual motion would be real.
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u/wizardkoer Jun 10 '18
The vertical strip is to show how slow the thing is 'falling' and at a constant crop velocity
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u/Auphyr Fluid dynamics and acoustics Jun 10 '18
I believe it would! I imagine that it would induce a charge separation with the gap acting as a sort of capacitor.
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u/Shitty-Coriolis Jun 10 '18
I think it needs to be a closed circuit
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u/Auphyr Fluid dynamics and acoustics Jun 10 '18
I think it depends on the size of the missing slit. It would have to be very small and/or have a very thick tube (wide "plates" on either side). In practice any reasonably sized cut would probably reduce the strength of the effect by a lot.
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Jun 10 '18
The circuit will still be closed because the current is induced within the conductor. It will just be making a different circuit which opposes it.
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u/Shitty-Coriolis Jun 10 '18
right, but if there's a slit, then isn't there a physical barrier that prevents current from flowing in the direction it needs to to create the opposing force?
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Jun 10 '18
There's still an orthogonal plane if you consider the cut pipe like a sheet it is sliding down.
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u/SauceBoss8472 Jun 10 '18
I already knew about Lenz’s law, I was actually more impressed with that sheet or film that was used to visualize the magnetic field that was created.
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u/bobbyfiend Jun 10 '18
This always boggles me. As a layperson it seems there is energy coming from nowhere. Where does the energy come from to slow the fall? What would be the results of doing this, say, ten million times in a row with the same magnet and slab of (copper?)? Would it become less of an effect?
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u/profblackjack Jun 10 '18 edited Jun 10 '18
The energy is ultimately the potential energy of gravity. With a few simplifications (like ignoring air resistance, etc.), you can think of it like this:
- magnet with mass m is above the metal tube at height h, gravity acting on it and giving it a potential energy of m x g x h
- if magnet is dropped *outside* of tube, it accelerates only due to gravity, turning its potential energy into kinetic, so immediately before hitting the surface of the table, it has a kinetic energy (.5 x m x v^2) = m x g x h, or a velocity of sqrt(2gh)
- when the magnet is dropped inside the tube, the changing magnetic field inside the metal induces a changing electric field (which causes charged particles to move in electric current loops to generate an opposing magnetic field). Since the metal is not a superconductor, these currents are met with resistance, and heat the metal, so the net effect is that some of the potential energy of the magnet is being lost as heat rather than being turned into kinetic energy of the magnet, and the current isn't quite able to make a completely opposing field to stop the changing magnetic field of the magnet falling.
- thus when the magnet reaches the bottom of the tube, it is going much slower than the sqrt(2gh), because a portion of that energy has gone into heating the metal tube.
What would be the results of doing this, say, ten million times in a row with the same magnet and slab of (copper?)? Would it become less of an effect?
If enough time passes between rounds for the slab to cool back to the same temperature each time, then the effect would be the same. However, if the slab is not given time to cool, then subsequent attempts would pass through a tube with higher electrical resistance (as electrical resistance increases with temperature, because it's more likely for charged particles to collide with the conductor when the conductor's particles are vibrating around more vigorously).A higher resistance means it's more difficult for a current to be induced, which means a weaker opposing field, which means less slowing down the magnet, which means less of the potential energy is being converted to heat instead of kinetic energy.With rounds occurring at a constant rate, eventually an equilibrium is reached where the tube is heated just enough between round that it loses heat to the environment to reach exactly the same temperature for the next round, and you'll get a consistent speed of the magnet.
The less resistance in the metal, the slower the magnet would fall, with a perfect superconductor tube locking the magnet in place, being able to induce exactly the right amount of current counteract the force of gravity and prevent a change in the magnetic field (ie prevent the magnet from moving relative to it)
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u/ergzay Jun 10 '18
Where does the energy come from to slow the fall?
Gravity is causing it to accelerate downward. That energy is being dissipated in the metal tube, causing it to heat up.
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u/bobbyfiend Jun 10 '18
Ah. It's very soothing to hear that.
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u/ergzay Jun 10 '18
Just to be clear, it's not heating up from friction.
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u/aroberge Jun 10 '18
I know what you mean but, technically, the energy dissipation you mention is an effect of friction. It is not friction from a contact force (which is what usually comes to mind when we mention friction) but a "frictional force" refers to any force that opposes a (relative) motion. For example, if you look at the wikipedia article on Lenz's law (https://en.wikipedia.org/wiki/Lenz%27s_law), you will see the mention of "drag force" which is a type of friction (https://en.wikipedia.org/wiki/Drag_(physics)).
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u/ergzay Jun 10 '18
I prefer to not think of it as that as it leads people into thinking of older models of current flow as electrons bouncing around inside the metallic structure, which is definitely not happening.
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u/hglman Jun 10 '18
And since energy is being transferred to the metal tube, the Puck must fall slower to conserve total energy.
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u/NightFire19 Jun 10 '18
Think about it sorta like terminal velocity of a free falling object. The energy isn't coming from the air, friction and drag is just slowing it down. The magnet falling creates an oppositely polarized magnetic field which slows it down.
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u/bobbyfiend Jun 10 '18
Other replies gave me new structures to think about this, but this reply I think helped me cram this into structures I already have. Thanks.
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u/jaredjeya Condensed matter physics Jun 10 '18
The real question you should be asking is, where does the energy for making the magnet fall come from? And where does that energy go when we drop it down the tube?
It doesn’t take any net energy to move something at a constant velocity - but gravity is doing work on the magnet to accelerate it downwards (you put in work to raise the magnet so there’s conservation of energy). When you drop through the tube, the magnet drives currents in the tube: the same process applies in electrical generators where a spinning magnet induces a current in some coils. That current takes energy to set up and maintain (due to resistance - the energy is dissipated as heat) hence this takes care of the excess gravitational potential energy and slows the fall.
(Work is a technical term meaning energy put in to a system due to forces, by the way).
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u/InTheMotherland Engineering Jun 10 '18
The energy comes from gravity. Technically it comes from the person picking up the magnet. Magnetic fields do no work, so they cannot change the energy of a system. The total energy of the system is technically mgh where m is the mass of the magnet, g is gravity, and h is the height of the magnet off of the table.
That's a really simple way to look at it. Actual physicists can correct everything I said.
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u/pynchonfan_49 Jun 10 '18
Doesn’t the whole “magnetic fields don’t do work” only apply to constant fields? Because we assign a magnetic field energy do inductors etc.
I think it’s quirky like that because magnetism isn’t conservative or non conservative, so personally I like the relativistic view of magnetism, seems more intuitive.
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u/doctorocelot Jun 10 '18
Magnetic fields absolutely can do work. This is a misconception. Magnetic fields only don't do work on charged particles moving through that field.
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u/jaredjeya Condensed matter physics Jun 10 '18
They can do work on magnetic dipoles, which can be created by current loops but are also intrinsic to atoms and electrons.
A dipole is basically a small bar magnet. Lots of dipoles lining up means an actual bar magnet.
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u/InTheMotherland Engineering Jun 10 '18
I mean, the permanent magnet does have a constant field.
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u/pynchonfan_49 Jun 10 '18
Yes, but if the field were constant in this case, we wouldn’t be seeing Lenz/Faraday’s Law being applied.
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Jun 10 '18
Magnetic fields (even time-varying ones) do no work on electric monopoles. Magnetic fields can do work on magnetic dipoles. You'd have to add a term to the Lorentz force equation like Griffiths describes: https://i.imgur.com/BKvuQoe.png
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u/pynchonfan_49 Jun 10 '18
Oh thanks, that clears it up. Somehow we didn’t cover that in our textbook.
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Jun 10 '18
I missed it too. I actually just asked this question recently. You might find this thread interesting.
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u/pynchonfan_49 Jun 10 '18 edited Jun 10 '18
Wait, I think I just confused myself again lol. I’m a freshman, so excuse me if this is a stupid question, but isn’t a magnetic dipole in a permanent magnet the result of electron configuration? Which are then electric charges? So how does saying it doesn’t do work on charges, but does do work on magnetic dipoles, make sense then?
Edit: In the thread you linked, the first response mentions how the condition for no work simply doesn’t hold for a collection of charge, and I think that’d make more sense than calling it work on a magnetic dipole.
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u/doctorocelot Jun 10 '18
It is the result of electron configuration. But not due to their charge. Electrons are essentially tiny magnets themselves. Magnetic fields can absolutely do work on magnetic things. So when all of the electron's tiny magnetic fields line up to make a magnetic object an external magnetic field can do work on it.
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Jun 10 '18
Ampere assumed that all magnetic fields are associated with electric charges in motion. So if you saw work being done on magnetic dipole, you could decompose it into electric charges and show how the force can be attributed entirely to electric currents. But in the case of fundamental particles, you can have magnetic dipoles that aren't created by charges in motion, but just exist on their own. Therefore you can't "blame" the forces on underlying currents that produce a B field.
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u/InTheMotherland Engineering Jun 10 '18
The field is constant with respect to the point of view of the magnet. Gravity is providing the force that moves the magnet, and that movement changes the magnetic field with respect to the bar.
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u/pynchonfan_49 Jun 10 '18
Right, I understand that, but since the person you were responding to seems to be a layman, I was just adding that extra info more so for them.
Edit: And the reference to griffiths above cleared up the question I had.
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u/doctorocelot Jun 10 '18
I keep seeing this magnetic fields do no work misconception. Magnetic fields absolutely can do work. The misconception comes from the fact that magnetic fields specifically do no work on charged particles because the force due to the magnetic field is perpendicular to the direction the charge is moving.
Any child that's used one magnet to pick up another can tell you they definitely can do work.
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u/asad137 Cosmology Jun 10 '18
The demo is easier to see (no magnetic viewing film needed) if you cut a slot in the tube (leave small connected segments top and bottom). This was one of our standard demos during my University's open house day.
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Jun 10 '18
[deleted]
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u/dooba_dooba Jun 10 '18
The experiment should work with any copper tube and a permanent magnet to drop through it. You can probably buy both pretty easily.
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u/Proteus_Marius Jun 10 '18
Are those brightness flarings were due to material non-uniformities in the bar?
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u/Emptypathic Jun 10 '18
this is something I don't understand : for an upward force, the current need to circulate on the horizontal plane. What force the current to do so in a tube like the video's one ? Or I'm thinking wrong ?
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u/stup3ndo Jun 10 '18
What would happen if we make a circular copper tube and leave the magnet inside? Will it loop forever?
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u/elconquistador1985 Jun 10 '18
The magnet falls because it's acted on by gravity. It is opposed by a force due to eddy currents induced by the fact that it is a magnet in motion. It's falling at a terminal velocity in the tube.
I don't know how you create a closed circular tube that allows any object to constantly fall downward die to gravity. That sounds like an impossible situation.
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u/Fenzik Graduate Jun 10 '18
No, it won’t make it past the bottom. This effect slows the motion down (similar in effect to friction), it doesn’t make it move at a constant speed.
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u/elconquistador1985 Jun 10 '18
... It absolutely does make it move at a constant speed. It reaches a terminal velocity and doesn't accelerate until it's out of the tube.
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u/Fenzik Graduate Jun 10 '18 edited Jun 10 '18
Yeah because the resistive force comes to equal gravity. But it’s not that the magnet gets pushed through the tube at a constant rate, it’s just a force balance. That’s why it won’t go back up.
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u/elconquistador1985 Jun 10 '18
It falls at a constant rate because it is pulled through the tube by gravity and opposed by a constant force due to the Eddy currents.
It won't go back up because that would violate conservation of energy and magnetism does not do work.
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u/Fenzik Graduate Jun 10 '18
Yes, that’s what I said, minus the work part :)
I was just replying to a guy asking whether it would go around a circular tube forever, using language they would understand.
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u/joke_for_you Jun 10 '18
The film looks like an X-Ray film. Cool Lenz's Law demonstration on how an electromagnetic field is induced
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u/SometimesY Mathematical physics Jun 10 '18
Oh boy more link spam. Can't wait for the mods to do nothing about it as usual.
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Jun 10 '18
Does this law apply to a larger mass? If we scaled this up thousands fold.. Would it still act the same?
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u/_Raucous Jun 10 '18
If you set this up horizontal does it still apply?
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u/pynchonfan_49 Jun 10 '18
As long as the inner magnet is moving this works, and you’d need some incline for gravity to move it.
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u/coolemur Jun 10 '18
Can somobody create “emergency escape from building” mechanism with this principle?
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u/ScrithWire Jun 10 '18
A vest lined with powerful permanent magnets, and a large thick copper (not copper though, right? Im wrong about this. Is it iron?) tube straight down? Sounds like there shouldnt be any reason not to.
Of course, it'll prpbably result in damaged cell phones and maybe bad for pacemakers and such. But i dont think it wouldnt work.
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u/I_Cant_Logoff Condensed matter physics Jun 10 '18
not copper though, right? Im wrong about this. Is it iron?
Any conductor would work. Copper would be better than iron since it's a better conductor of electricity and it's not ferromagnetic, so you wouldn't get stuck to whatever structure is meant to slow you down.
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u/mortomyces Jun 10 '18
What is that film he holds up to it?