How the heck does that work?!
Hey Crazies.
I’m adding a new kind of video into the mix where I explain how things work.
Today, we’re covering the Crookes Radiometer.
What’s a Crookes Radiometer?
Great question!
It was invented by an English chemist and physicist named William Crookes.
A chemist and a physicist?!
Wow, that doesn’t happen a lot these days.
Anyway, a Crookes radiometer looks something like this.
There are 4 vanes, each with a white side and a black side.
They’re enclosed in an air-tight glass bulb where most of the air has been sucked out.
When you shine light on it, the vanes spin.
Simple enough to demonstrate, but a huge pain to explain.
There are 3 different effects at play: pressure from light, also known as radiation
pressure, pressure from heated air, and air vortices.
Let’s look at those one at a time.
Option 1: Radiation Pressure!
When atoms run into other atoms, they exchange momentum.
If there are a lot of atoms, we call that pressure.
Radiation is just a catch-all word for particles that carry energy away.
Any type of light counts as radiation.
Photons have momentum too, which they can also give to atoms after hitting them.
If there are a lot of photons, we call that radiation pressure.
In the radiometer, the photons hit the white side twice as hard as the black side,
which gives us a net force in this direction.
There’s a slight problem though.
The momentum of a single photon is proportional to its energy and, therefore, its frequency.
That is a seriously tiny number!
When you consider in the dimensions of a radiometer vane; the type of, strength of, and distance
to your light source; and finally multiply by four; you get about 2.5 nano-newtons!
That’s itty bitty.
So, while radiation pressure makes solar-sails possible and balances the inward gravity of
a star, those things are big.
To get the tiny vanes in the radiometer to respond you’d have to vent all the air to
space and shut off gravity to minimize friction.
Physicslaaaaaaaaaaaaand!
I really need to figure out where that's coming from.
Anyway, I can do those things because I’m on a spaceship!
But you can’t and the company that makes these certainly can’t either.
We know there’s more going on here for multiple reasons.
If radiation pressure was it, it would spin this way, but a standard radiometer spins
the other way.
It doesn’t even need to be light.
Just getting it hot or cold will make it spin.
Keep in mind, scientists have been discussing this thing for over a century now.
It started in the late 1800s between Maxwell and Reynolds.
They kept publishing papers as rebuttals to each other, but then Maxwell died and the
Royal Society wouldn’t let Reynolds respond anymore.
After that, no progress was made for almost 40 years because we just didn’t have the
technology to verify anything.
Option 2: Pressure from heated air!
A standard Crookes radiometer does have air only a little bit, but it’s there.
That air gets heated near the vanes, but more so on the black side.
The temperature increase causes a pressure increase and that pressure pushes the vanes
forward.
Is it really that simple?
No.
This is the Science Asylum.
You think we’re going to stay that superficial?
Temperature increases and pressure increases don’t always correlate.
We need to consider something called the Mean Free Path.
That’s the average distance a molecule can travel before you expect to run into another
molecule.
If the air is really thin, that could be the size of the vanes in the radiometer.
But, since the air molecules in the radiometer are also hitting the vanes the pressure isn’t
evenly distributed across those vanes.
Ricocheting molecules can keep other molecules from hitting the vane.
That happens more near the center than the edge so the pressure across the vane looks
more like this.
It’s higher near the edges and less near the center.
However!
The more air molecules there are in the glass bulb, the smaller the mean free path.
That does mean the pressure can distribute across the vanes better but, too many, and
it’ll distribute throughout the whole radiometer better resulting in effectively zero pressure
difference on the vanes.
Option 3: Air Vortices!
Even when there isn’t a pressure difference moving the vanes along that temperature difference
is still there.
With more molecules around, that temperature difference pushes the air like this and, by
Newton’s third law, the vanes are pushed in the opposite direction.
The radiometer vanes still spin.
But as we mentioned before, the more air molecules there are in the glass bulb, the smaller the
mean free path.
If you put too many molecules in there, they hit each other so much their motion becomes
random again.
That’s why this doesn’t work at atmospheric pressure.
So which is it?
Option 2 or option 3?
Both!
Manufacturers of these radiometers have found a sweet-spot at about 1 pascal.
That’s about 10 millionths of an atmosphere, so it’s pretty low.
But it’s just the right number of air molecules, so that both a pressure difference and an
air flow are at play inside the radiometer.
Complicated!
So, crazies, what did you think?
Do you want to see more of this type of video?
Have you ever wondered how something worked?
Let us know in the comments.
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And until next time, remember, it’s OK to be a little crazy.
The featured comment comes from Rodney Smith who asked:
Does using a magnet make it weaker?
Yep.
The energy to do that work has to come from somewhere.
The result is that a truly permanent magnet is impossible.