While I avoid planning class for tomorrow, how about a cool #Physics demo? ⚛️🧲 Let's talk about Faraday's Law. What's Faraday's Law, precious? Well, it's the reason that a sheet of aluminum can bring a swinging magnet to a screeching halt. - ThreadSky

joeyneilsen.bsky.social • 131 days ago

While I avoid planning class for tomorrow, how about a cool #Physics demo? ⚛️🧲

Let's talk about Faraday's Law. What's Faraday's Law, precious? Well, it's the reason that a sheet of aluminum can bring a swinging magnet to a screeching halt.

Comments

drsnit.bsky.social•129 days ago

📌

hookean.bsky.social•103 days ago

It works if you reverse the materials too 🙂
https://www.youtube.com/watch?v=Qm7ZxfXpXs4&list=PLDqG5Fbl71JhHcB4n3qZ6878YS__nP8CS&index=42

firehorsep.bsky.social•130 days ago

😲

markswitzer.bsky.social•130 days ago

Will it also stop angular momentum?

I expect so.

joeyneilsen.bsky.social•130 days ago

Not as effectively... you can see that the magnet is still twisting at the end. It's because that motion only induces a tiny field in the aluminum, and even then it's mostly because the magnet is tilted. A perfect, level magnet should be able to twist freely in this setup.

rational.bsky.social•130 days ago

I would expect that if the B field is perpendicular to the axis of rotation (and the field is strong enough) the induced field would oppose the direction of spin?

(Now I have to try it)

markswitzer.bsky.social•130 days ago

I would think so too.

rational.bsky.social•130 days ago

First one is a quarter. 2nd is a neodymium magnet

joeyneilsen.bsky.social•130 days ago

Ah yes it depends on the axis of the magnetic field. Mine is oriented vertically, so spinning it doesn’t really change the magnetic flux through the aluminum.

rational.bsky.social•130 days ago

Yeah 100%

shamonki.bsky.social•130 days ago

I just heard that electronic devices kept inside an old (unplugged!) microwave oven, will not be protected from an EMP (solar flare or nuclear, eg.)

Perhaps he meant not safe from anyone spying on your device illegally without a warrant, such as the NSA ?

Only a proper Faraday Cage ?

joeyneilsen.bsky.social•130 days ago

Yeah I think the microwave cavity is designed to block microwaves most efficiently. It obviously doesn't block visible EM radiation because you can see inside!

captainmilk.bsky.social•131 days ago

Are you sure thats not a babybel cheese?

joeyneilsen.bsky.social•131 days ago

mmm forbidden neodymium babybel

shamonki.bsky.social•130 days ago

My son is obsessed with the vegan BabyBels.

A vibranium or adamantium babybel magnet, and he would go APE SHEET

adrian.weasel.net•130 days ago

Eddy Current and the Magnetics 🎵

anvraptor.bsky.social•130 days ago

I didn’t knew that, that’s so cool :0

absintheofcolor.bsky.social•130 days ago

diogeneslamp.bsky.social•130 days ago

When I teach magnetism, I start by showing the video of ICP rapping "F**in magnets, how do they work?" (the title of my lecture).

Students today don't get the reference

sugarrush33.bsky.social•130 days ago

we love induced eddy currents!

darthbluesky.bsky.social•131 days ago

what the

joeyneilsen.bsky.social•131 days ago

darth this is a good demo if you like gasps from the audience. they also like the one where you swing a bucket of water over your head without spilling it

tarrant-the-hero.bsky.social•131 days ago

I love the bucket of water one

gravity-pike.bsky.social•131 days ago

Even though the aluminum isn't attracted to the magnet, the moving magnet induces eddy currents in the metal sheet. These currents produce an electric field, which produces a magnetic field that exactly opposes the motion of the magnet.

gravity-pike.bsky.social•131 days ago

An (imo) even more dramatic demo is dropping a magnet through a copper tube.

https://youtu.be/H31K9qcmeMU?si=2taThgjL3jg39n7V

joeyneilsen.bsky.social•131 days ago

Yep I show that one farther down in the thread!

aloxite.bsky.social•130 days ago

Better than seeing the laws only in equations by a long mile.

Before I knew Faraday or Lenz's, I wondered how regenerative braking actually caused vehicles to slow down. Ofc Conservation of Energy was there, but the mechanism is pretty neat for regenerative braking.

kidarwolf.bsky.social•131 days ago

Thanks for this, you've helped me understand a little better how back emf works in an electric motor.

Since the current gets stronger as the magnet gets closer, is it fair to say that an electric motor with the magnets very close to the coil will produce back emf more quickly?

joeyneilsen.bsky.social•131 days ago

With a motor we're talking about AC current, so you'll get an AC induced current and emf. The stronger the magnet, the stronger the induced signal. But the rapidity of that depend on the motor frequency (or in the demo here, the speed of the magnet). Make sense?

kidarwolf.bsky.social•131 days ago

Yes, thank you so much. I've been struggling to wrap my mind around it with the way my dad (a mechanical and electrical engineer) explained it. You've just turned an intellectual thicket into a garden. ❤️

joeyneilsen.bsky.social•131 days ago

☺️

mayumithekimuras2.bsky.social•130 days ago

Nice

dreaminof18.bsky.social•103 days ago

Woah

joeyneilsen.bsky.social•103 days ago

Whoa!!

mementomoree.bsky.social•130 days ago

When I worked in Motorsport I was a specialist (among other things) of a Dyno.
We used this effect to brake/control the acceleration of a car and measure the HP at the wheel hub without interference of the tyre

mementomoree.bsky.social•130 days ago

I specify, it wasn't one of those Dyno with big rolls inside the floor, but we had for each driving wheel a ginormous electric brake to be bolted directly at the wheel hub after removing the wheel.

atransparentsoul.bsky.social•130 days ago

That’s really cool!

darmstadiym.bsky.social•129 days ago

Is this where nuclear power comes from

joeyneilsen.bsky.social•129 days ago

No thank goodness! I wouldn't guess homegrown nuclear experiments have a great track record.

diogeneslamp.bsky.social•130 days ago

That's a nice demo but it helps if the audience knows aluminum is not ferromagnetic, so the usual magnetic attraction to ferromagnets is not arresting the motion

I demonstrate Faraday's Law with a small induction range. Boil water, make the students hold a magnet over it, feel it vibrate

joeyneilsen.bsky.social•130 days ago

Yeah I show that part more clearly in some of the videos later in the thread. When I do it in class, I have a volunteer come up to attest to the fact that it's not a magnetic attraction

fold6tacoz.bsky.social•130 days ago

I took ap physics in high school, but the teacher insisted I do things his way, and my mind just couldn't function his way. I often wonder where I'd be if he'd been more flexible or understanding. Because even at 46, my eyes lit up seeing your post! Thank you for procrastinating.

btnkris.bsky.social•130 days ago

I know this is a cool magnet demo, but it do looks so much like you’ve created a Babybel pendulum

joeyneilsen.bsky.social•130 days ago

There is a lot of demand for this based on the replies!!

maniagnosis.bsky.social•131 days ago

So, what's the deal with the twist when the magnet stops?

joeyneilsen.bsky.social•131 days ago

I think there's probably a little bit of winding in the twine, because you can see it twisting when it's swinging freely. But since the magnet isn't perfectly level, its field won't be exactly vertical, so there might be a little bit of a torque on it at the end too....

maniagnosis.bsky.social•131 days ago

Hmmm. Do I have a magnet handy? 😁

thisisbossi.bsky.social•130 days ago

My brain can only see that as a Babybel hanging on a string & I'm just going to choose to stick to assuming it's magnetic cheese.

svenpetersen1965.bsky.social•130 days ago

Eddy currents. Train brakes work like that.

joeyneilsen.bsky.social•130 days ago

And elevator brakes, I believe!

jeanniemarschall.bsky.social•130 days ago

Whoop, I loved this! Ty 😁

starangel369.bsky.social•130 days ago

Cooking magnets steals their magical powers.

yuweiquan.bsky.social•130 days ago

As a one-time physics major, I knew exactly what was going to happen, but it’s still a sight to behold! Great demo!

jung-gt.bsky.social•130 days ago

📌

eyesceneeye.bsky.social•131 days ago

📌

welcome2absurdia.bsky.social•129 days ago

Is it: What happens in Faraday’s cage stays in Faraday’s cage?

(I’ll show myself out.)

rickinbaltimore.com•131 days ago

Damn physics is amazing

rxnnmrtn.bsky.social•130 days ago

Why does it stop? It shouldn't.

joeyneilsen.bsky.social•130 days ago

It produces a magnetic field in the aluminum that pushes backward on it… weird huh?

rxnnmrtn.bsky.social•131 days ago

That is cool!

youreaquim.bsky.social•130 days ago

Genuine question from someone not as smart, what would happen if you gently chucked that magnet over the sheet? Would it stop mid air and drop?

joeyneilsen.bsky.social•130 days ago

This effect is a lot stronger when the magnet is close to the sheet. If you managed to throw it just over the sheet, it would stop moving forward the same way it does on the string, but it would be hard to see because it wouldn’t stop falling downward.

spacewolflaika.bsky.social•130 days ago

That's pretty cool!

xangpow.bsky.social•131 days ago

What about the law of motion? Where is the energy going? Can it have enough energy to still be able to move past the aluminum?

diogeneslamp.bsky.social•130 days ago

The kinetic energy of the swinging magnet is converted to electrical current in the aluminum sheet and that energy dissipates as heat due to the resistance in the metal

Same principle as an induction stove except the magnet is an electromagnet making an oscillating field and the sheet is your pot

onbluskysku.bsky.social•131 days ago

The energy turns into current in the aluminum sheet, which ultimately turns to heat via friction. See: the induction cooktops example

joeyneilsen.bsky.social•131 days ago

There's also energy in the magnetic field itself!

xangpow.bsky.social•130 days ago

So could it be said that the faster the magnet is moving the more heat it will send into the aluminum?

onbluskysku.bsky.social•130 days ago

Assuming that the magnet comes to a complete stop, yes

joeyneilsen.bsky.social•130 days ago

Yeah the faster it's going, the more kinetic energy it has, and that has to get transferred to the aluminum and its electric and magnetic fields. The electrical energy eventually heats up the aluminum.

thundergoose.bsky.social•130 days ago

Why does physics get all the cool demonstrations? I’m teaching the cardiovascular system this week and I don’t get to demonstrate what happens if the heart valves were 2 way valves instead of 1 at valves!

doclt.bsky.social•131 days ago

📌

joeyneilsen.bsky.social•131 days ago

Here's a slow-motion view. The magnet swings on the string and comes to a sudden stop above the aluminum plate, without touching it.

joeyneilsen.bsky.social•131 days ago

Why? Well, Faraday's law says that if you change the magnetic field passing through some surface, you get a voltage around the border of that surface. In the metal sheet, that means that current flows, and the current has its own "induced" magnetic field!

joeyneilsen.bsky.social•131 days ago

Incidentally, this is the operating principle for an induction cooktop: the current that flows through the bottom of your pan heats it up, cooking your food!

phasmatic.bsky.social•130 days ago

Why does the cookware for an induction stove need to be ferrous in order to work, and why doesn’t it stick to the cooktop? Your aluminum plate wouldn’t cook food on such a stove, would it?

joeyneilsen.bsky.social•130 days ago

Good question. I think the key is that iron has a much higher resistivity than copper and aluminum, which means it should heat up more readily when current flows through it.

It doesn't stick bc there's no downward force. Example: the swinging magnet stops bc it feels a push in *reverse,* not down.

phasmatic.bsky.social•130 days ago

Interesting!

So the momentum of the magnet becomes heat in the metal right? It makes me wonder if the magnetic property of induction compatible cookware is part of why it works, or is it just the way the stove knows there’s an iron pan with sufficient resistivity to heat well?

bernardrand.bsky.social•103 days ago

I don’t think it’s just the resistivity. I suspect a decent amount of the heat generated is from hysteresis during constant magnetisation and demagnetisation rather than just the induced currents.

joeyneilsen.bsky.social•131 days ago

Faraday's Law (and Lenz's Law) also tell us that this "induced" magnetic field opposes the original change. Increasing the field through the metal sheet? The induced field will make it smaller. Trying to shrink the field? The induced field will strengthen it.

joeyneilsen.bsky.social•131 days ago

As the magnet swings toward the aluminum sheet, the sheet sees a stronger and stronger magnetic field. It develops a field that points in the opposite direction, and this field pulls on the magnet. The faster the magnet moves, the bigger the force on it!

joeyneilsen.bsky.social•131 days ago

You can see similar effects if you drop the magnet down a copper tube. The magnet gently floats down, while a non-magnetic ball bearing falls quickly.

jlohaylo.bsky.social•131 days ago

I love demonstrating Lorentz forces to my trainees in an MR suite

joeyneilsen.bsky.social•131 days ago

Ooh this looks like a blast

martonis.bsky.social•130 days ago

I would think a coil would creat a magnetic field. Not a sheet of aluminum foil

joeyneilsen.bsky.social•130 days ago

That’s what makes it so cool!

passitallong.bsky.social•130 days ago

It’s early here and for some reason I thought that was a donut and then THE GIANT HAND APPEARED.

Now that I’ve had a few sips of coffee and have recalibrated, yes, very cool. 😂👍

ashfl.bsky.social•130 days ago

📌

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