DIANNA COWERNE (VOICEOVER): This episode
is supported by 23andMe.
DIANNA COWERNE: I'm so excited.
I didn't think there was going to be an opportunity for,
like--
you get to handle liquid oxygen. [LAUGHTER]
JOSH WOLFE: Oh, yeah.
DIANNA COWERN: Throw the switch down.
Pour the liquid oxygen.
OK.
JOSH WOLFE: And then, lift the switch.
DIANNA COWERN: I feel nervous.
[GIGGLE] I don't know why.
DIANNA COWERNE (VOICEOVER): I'm here
at MIT with one of the technicians from the physics
demo group, preparing to pour liquid oxygen
over a magnetic field created by some super strong electricity.
Let me repeat that-- liquid oxygen.
I've never seen liquid oxygen, and they let me pour it.
By the way, I'm Dianna, and you're watching Physics Girl.
Gently.
OK, OK.
And then-- uh.
Ah!
Oh, my gosh.
I can't-- I can't believe this demo.
JOSH WOLFE: It is really cool.
DIANNA COWERNE (VOICEOVER): I was freaking out,
because the liquid was just hanging there between two
magnetized pieces of metal.
Liquids aren't supposed to do that, in my experience.
But this liquid was just hanging there
in a magnetic field created by electricity.
And as soon as you turn off the electricity,
let's see what happens.
DIANNA COWERNE: There's no surface tension effect here.
That's amazing.
JOSH WOLFE: Nope.
And as soon as I turn it off--
DIANNA COWERNE: Oh, my gosh!
Can I do it again?
DIANNA COWERNE (VOICEOVER): In all my time at MIT,
no professor ever did this demo for us.
So disappointing, because the physics here is really cool.
First thing, there are electromagnets,
which are made by running electricity
through coils of wire.
Two coils that, when we pass a DC current through them,
is going to create a magnetic field between these two
giant pieces of metal.
These are what are becoming magnetized--
OK.
--by the DC current running through these coils.
DIANNA COWERNE (VOICEOVER): The result
is a strong magnetic field around the metal pieces--
but most importantly, in the gap between them.
It's just like a strong magnetic field
between two permanent magnets, except you can turn it off.
Now, if you've poured liquid nitrogen, water, or alcohol
in between the two pieces of metal,
you wouldn't get any of it hanging around and partying.
You might get a few droplets of water sticking
to the surface because of surface tension,
but though the liquid oxygen has surface tension,
it's not what's causing the liquid to stick to the metal
here.
Because the oxygen boils away too quickly at the interface.
It's the Leidenfrost effect.
Then why is the oxygen hanging out?
Well, it's because different materials
have different responses to external magnetic fields.
Your face is pretty unresponsive to a strong magnetic field.
Iron is visibly responsive to an external magnetic field.
I don't recommend doing that with your neodymium magnets.
Hot iron acts differently, as we'll
see when we heat up this piece of metal with a blow torch.
Yeah.
Stay tuned for that.
But liquids don't usually visibly react
to magnetic fields.
And yet, liquid oxygen clearly does.
By the way, liquid oxygen is oxygen cooled down
below 90 Kelvin.
In Fahrenheit, that's when your spit freezes way
before it hits the ground.
So liquid oxygen has this property called paramagnetism.
It reacts mildly to an external magnetic field,
and all the little mini-magnets that
make up the oxygen align with the external magnetic field.
These mini-magnets I'm talking about
are basically the little magnetic moments
of the electrons and the oxygen's atoms.
You can really think of a magnet being made up
of tiny, tiny, tiny, tiny little magnets,
all aligned in the same direction.
But in a paramagnetic material, as soon
as you turn off the external magnetic field,
all those little mini-magnets go back
to pointing in random directions.
Now, iron in turn has ferromagnetic properties,
so all of its little mini-magnets
are aligned strongly with an external magnetic field
and even keeps them of that alignment after the field is
removed.
But there is a point when even the mini-magnets
in ferromagnetic materials won't align,
and it's when you get them hot enough.
JOSH WOLFE: Yup.
So this is just a little magnet.
And then, we have a thermally insulated plate
so we don't damage our magnet.
DIANNA COWERNE: Oh, that's--
OK.
That's not stuck on there that hard.
JOSH WOLFE: No.
Yup, just loosely.
DIANNA COWERNE: Yeah, OK.
Ah!
Amazing!
DIANNA COWERNE (VOICEOVER): We're
heating the iron metal here, past what's
called the Curie point.
There's too much energy, too much
random motion of the magnetic moments for them to align.
And so the iron no longer sticks to the magnet.
This is just like how it's impossible to get
a bunch of preschoolers to line up
if they have a ton of energy.
But if you freeze them--
I mean, metaphorically, if you let the iron cool back down--
JOSH WOLFE: --cools, it will actually
be redrawn to the magnet.
DIANNA COWERNE (VOICEOVER): The Curie point of iron
is around 1043 Calvin, which is incidentally around
the temperature when iron starts to get hot enough
to emit visible light, which is why it's glowing here.
I really enjoy these demonstrations,
and we tried some other demonstrations
and some other stuff with these demos.
DIANNA COWERNE: Can you-- can you just, like--
oh, wow, there's a big spark there, too.
We did wheel that out.
I'm going to save that demo for next week, though.
Thank you to the technical services group at MIT.
They were amazing.
They made all of this possible.
Thank you guys, for watching this video.
Subscribe if you want more physics,
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