如果你用压缩气罐 或者说 压缩气体除尘剂
If you’ve ever used a can of compressed air , also called a gas duster,
to, I don’t know, clean crumbs out of your computer keyboard,
you’re probably aware that after a little while,
the air coming out of the can
and even the can itself, get really, really cold.
Like, cold enough they put frostbite warnings on the can!
It’s tempting to think that compressed air cans get cold
because when the gas comes out of the can it expands and thus cools off.
But that’s not exactly right.
Whether an expanding gas gets hotter or colder, and much hotter or colder it gets,
depends on the exact manner in which the gas expands.
And if we apply the relevant equation for “ normal ” gas expansion,
we predict that the gas inside the compressed air can should drop
from room temperature to around 100 degrees celsius below zero
which is, um,a way colder than what comes out of a compressed air can.
So the gas can’t be expanding in the “normal way” gases expand.
And here’s why: that would be like cutting the top off the can
and letting the gas expand freely in all directions.
But the gas is actually being squeezed out through a tiny valve.
This difference is key;
The gas passing through a valve isn’t simply expanding
It’s also being pushed through by the rest of the gas behind it!
And that compression from behind gives the gas enough heat energy
to essentially counteract the cooling from expansion.
But not exactly.
Most gases at room temperature do
get slightly colderwhen passing through a valve.
A good demo of this is to let the air out of a bike tire:
The valve gets colder , but not crazy cold.
Similarly, the gas leaving a can of compressed air
cools a little bit passing through the nozzle.
But this can’t be the only contributorto the cooling.
I mean, the can itself cools off
by significantly more than can be explained by expansion through a valve.
And it’s not like it’s even being sprayed by the air coming out.
No, the real cooling power is hinted at
by the warning labels
telling you to not to shake cans of compressed air or spray them upside down.
If you DO shake one, you’ll realize right away that it’s not just gas inside.
There’s liquid in there, too!
Liquid like 1,1-difluoroethane,
which is a gas at normal temperatures and pressures,
but a liquid once you pressurize it to around 6 times atmospheric pressure.
And it’s the essential component of thesecompressed air cans.
Inside the can,
1,1-二氟乙烷部分呈气态 部分呈液态 二者持衡
1,1-difluoroethane exists as both a liquid and a gas in equilibrium.
Just enough for the liquid boils off
to maintain six atmospheres of pressure in the top of the can,
a pressure high enough that reststays liquid.
Because it’s at six times atmospheric pressure,
when you open the valve the difluoroethane rushes out in a steady stream,
blowing away dusts and crumbs.
But this then means that the inside of the can is no longer pressurized enough
to keep the liquid from boiling.
so more of it boils off until the gas reaches six atmospheres
of pressure again and a new equilibrium is reached
with slightly less liquid in the can.
This is how the can is able to
keep blowing a stream of consistent strength even when mostly empty.
But more importantly to our temperature conundrum,
changes from liquid phase to gas phase require a TON of energy,
and that energy has to come from somewhere.
Just like how the evaporation of sweat removes energy from your skin, cooling you off,
压缩气罐内汽化的分子 或者说 沸腾的分子
Inside a can of compressed air, the molecules that vaporize, aka boil,
are the thing that steals energy from the liquid and cools it off.
Spraying out 10 % of the contents will cool the entire remainder of the can
by around 20 degrees celsius!
If it seems counterintuitive that a boiling substance cools itself off,
look no further than the humble pressure cooker.
Water normally boils above 100 degrees celsius.
but by sealing in steam, the pressure rises,
enabling the water in the pot to remain a liquid well beyond water’s normal boiling point,
just like the difluoroethane in a can of compressed air.
And releasing water vapor out of the nozzle of a pressure cooker
lowers the pressure inside, allowing a bit more water
to boil off as steam and lowering the temperature of the remaining water,
just like the difluoroethane in a compressed air can.
And if you keep letting off steam,
eventually the water will cool all the way back down to its regular boiling point of 100 degrees,
just like how if you keep spraying a can of compressed air,
the difluoroethane inside will cool all the way back down
to its regular boiling point of negative 25 degrees.
A can of compressed air is quite literally
a 1,1-difluoroethane pressure cooker.
and just like you shouldn’t shake your pressure cooker or turn it upside down,
unless you want to spray superheated water everywhere,
cans of compressed air don’t work very well sideways or upside down.
Instead of spraying out gas, you’ll spray out the liquid
that was only being kept liquified by the high pressure inside the can.
So it immediately vaporizes and drastically cools down
whatever it’s contacting.
Though difluoroethane can dissolve in water and is poisonous,
so definitely don’t use this ice for anything food-related.
In conclusion, the cause for the coldness of cans of compressed air
can be clarified by comprehending the consequent clue.
They aren’t actually cans of compressed air.
They’re cans of pressure-liquified 1,1-difluoroethane,
and lowering the pressure inside by spraying them
allows more liquid to boil off, coolingwhat remains.
The physics of regular stuff is super fun to learn about.
I mean, black holes and quantum mechanics are cool, too,
但这些并不像我们日常使用的物品那样 真实可感 息息相关
but they’re not quite as tangible or relatable as the things we interact with on a regular basis.
And if you, too, want to dive deeper into the physics of everyday objects,
look no further than Brilliant, this video’s sponsor.
Brilliant has a whole course on the physics of everyday objects,
像是冰箱 水塔 自行车这些
including fridges and water towers and bikes.
And Brilliant also has fun, short daily challenges and puzzles to learn about stuff
学习均值回归 流体 热力学 之类的
like regression to the mean and fluids and thermodynamics
without the huge time commitment it would take
to learn enough about Joule-Thompson expansion through a valve
to make a whole youtube video about it.
Brilliant continues to be an incredible supporter of MinutePhysics
and they are offering 20% off of a premium subscription to the first 200 of you
who go to brilliant.org/minutephysics,
which gives you full access to all of Brilliant’s courses,
puzzles and daily challenges.
再重复一遍 通过brilliant.org/minutephysics 订阅高级版可享8折优惠
Again,that’s brilliant.org/minutephysicsfor 20% off a premium subscription,
and to let Brilliant know you came from here.