DIANA: This episode is brought to you by Curiosity Stream.
Hi.
I'm Diana, and you're watching "Physics Girl."
There's something I've been wanting to show you for, like,
a couple days.
My last video had liquid oxygen and it hovered
between two electromagnets.
And that kind of stole the show.
And whoever you were in the comments
that questioned my choice of physics classes
because I had never seen liquid oxygen,
and I get excited by freshman level physics demonstrations.
Who killed your joy?
Anyway, the real hero of my easily amused physics
loving heart was the tiny spark that
forms when you turn off the electricity
to the electromagnets.
Because Josh, the gentlemen from the MIT Technical Services
Group showing me these demos, promised a demo
with an even bigger spark.
He did not disappoint!
Side note, pyrotechnics is sometimes
a side effect of demos meant for this pesky thing called
learning.
So we'll get that out of the way real quick.
This demo had two giant coils of wire
with another wire laid over the top.
When you flip the switch to allow electricity
to run through the coils, the wire on top jumps off.
Meh.
It's kind of wondrous.
It shows induction, which is a really
counter-intuitive and awesome phenomenon in physics.
So that's cool.
Induction happens when a changing electrical current
induces a current in a nearby wire.
Induction is like the bad influence kid of physics.
It induces you to have a current too.
But there can only be one bad kid, so it pushes you apart.
That made no sense.
DIANA: It's really just because of the magnetic fields produced
by the currents that push them apart.
But, ack, learning.
Now when we turn off the current things get real saucy.
DIANA: OK, that jumped off and then--
aah!
Oh my gosh!
That spark is from turning off the current.
This demo is the universe telling you
it is not happy with change.
"Sesame Street" must be in my life foreverrrr!
But, really.
Inductors don't like it when you turn off their current.
What?
Oh, right.
What's an inductor?
Those giant coils of wire, or this thing on a circuit
diagram, or these things if you do electronics.
But basically, an inductor is just a slinky
connected to electricity.
And it doesn't like it if you turn off the electricity
running through it.
You can think of it like momentum.
The electricity has some oomph as it's
going through the inductor.
So as you open the switch to turn off the power,
the current doesn't just stop.
It keeps flowing toward the switch,
building up charge until you get a huge voltage
across the switch.
And at some point the voltage will be
so high it can jump across air.
Or, in our case, across the small part of the switch
that's still partially touching.
So you get a ton of current running
through a tiny piece of metal and it
gets really hot, really fast.
This aspect of induction used to be used in some car ignitions,
because you'd need a really high voltage.
So once you opened a switch connected to an inductor
you got a really high voltage that
could turn on the ignition.
Induction was also used to create the earrings
that I'm wearing, which are shrunken dimes.
We used a giant electromagnet to shrink coins.
I made a video explaining how that works.
So check it out.
Why the electricity doesn't like to turn off?
That's an interesting question.
And I find it really intuitively difficult to understand.
Because it's a result of Faraday's law of induction,
which says that if you change the magnetic flux--
and I love the idea of flux.
Because it's the amount of something through an area.
So in our case, it's the amount of magnetic field
through the area of the coil.
And we're changing that by turning off the electricity.
Electricity is what's creating the magnetic field.
And when you turn off the electricity,
the magnetic field shrinks.
Which means the magnetic flux goes down.
Now this change in magnetic flux causes an electro motive force,
which acts like a voltage.
And that pushes back against the change in current
that changed the magnetic flux.
But what does this all mean?
The math is there, but what does it mean?
Now, the best way I can find to understand it
is that it's kind of like a conservation of energy.
When you run current through the inductor
it builds up a magnetic field.
And it took energy to create that magnetic field.
So the magnetic field is storing energy.
When you turn off the current, the energy
can't just disappear.
It has to go somewhere.
Energy conservation, bru.
Yeah, that energy doesn't just disappear.
So it goes into a flowing current that
ends up raising the voltage.
I went down the rabbit hole of trying to understand induction
intuitively, and I got to special relativity, the way
that magnetic fields relate to electric fields.
And let me tell you, that's fascinating.
But it's beyond the scope of this video.
Here's another metaphor.
This is all kind of like how a water hammer works.
If you've ever turned off a valve in your house really fast
you hear this like, this knocking.
That's called a water hammer, and it
happens because you've got water flowing through the pipe.
And as soon as you shut the valve you're blocking the flow
but it's still going.
It's got that actual momentum.
And it'll smack into the valve and you
get kind of this sloshing of movement back and forth.
But to me, the interesting stuff is what's actually happening.
And that is the fact that inductors have stored energy
and that energy has to go somewhere when you turn off
the electricity.
Inductors protest.
It's not just the #Resist stores that protest.
I had a lot of fun trying to really understand
how to explain induction.
I hope you enjoyed at least the pyrotechnics.
Thank you so much to MIT Technical Services
Group for showing me these demos and all your help.
Thank you guys for watching.
And happy physics-ing
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