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为什么压缩气罐越用越冷?

Why Do Compressed Air Cans Get Cold?

分钟物理
Minute Physics
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,
来清理键盘上的点心酥皮啥的
to, I don’t know, clean crumbs out of your computer keyboard,
过了一会你就可能会发现
you’re probably aware that after a little while,
罐子喷出的气体
the air coming out of the can
甚至于瓶身 都变得非常非常冷
and even the can itself, get really, really cold.
冷到厂家要在瓶身贴上冻伤警告!
Like, cold enough they put frostbite warnings on the can!
人们很容易认为压缩气罐变冷
It’s tempting to think that compressed air cans get cold
是由于气体排出时的膨胀 导致气体自身冷却
because when the gas comes out of the can it expands and thus cools off.
但这并不是很准确
But that’s not exactly right.
膨胀的气体是变冷还是变热 变得多冷或是多热
Whether an expanding gas gets hotter or colder, and much hotter or colder it gets,
这取决于气体的膨胀方式
depends on the exact manner in which the gas expands.
若使用标准气体扩散的相关方程
And if we apply the relevant equation for “ normal ” gas expansion,
可以预测 压缩罐中的气体
we predict that the gas inside the compressed air can should drop
会从室温下降至零下100度摄氏度左右
from room temperature to around 100 degrees celsius below zero
这远比实际从压缩罐中出来的气体冷
which is, um,a way colder than what comes out of a compressed air can.
所以实际上气体不会以“标准气体方程”膨胀
So the gas can’t be expanding in the “normal way” gases expand.
原因在于 那就相当于把瓶盖去切掉后
And here’s why: that would be like cutting the top off the can
让气体朝各个方向自由膨胀一样
and letting the gas expand freely in all directions.
但实际上气体是从一个小阀门中喷出来的
But the gas is actually being squeezed out through a tiny valve.
这就是造成差异的关键
This difference is key;
阀门喷出的气体并不是只是膨胀
The gas passing through a valve isn’t simply expanding
它还是被后方其余的空气给挤压出去的!
It’s also being pushed through by the rest of the gas behind it!
而后方的挤压给了气体足够的热能
And that compression from behind gives the gas enough heat energy
大致抵消了扩散时造成的冷却
to essentially counteract the cooling from expansion.
但并不完全是这样的
But not exactly.
室温下 大多数气体从气阀喷出后
Most gases at room temperature do
都会变得比室温低一些
get slightly colderwhen passing through a valve.
一个典型例子就是给自行车轮胎放气
A good demo of this is to let the air out of a bike tire:
气门的确变冷了 但并不是特别冷
The valve gets colder , but not crazy cold.
同样的 压缩罐中喷出的气体
Similarly, the gas leaving a can of compressed air
流经喷嘴时会略微冷却
cools a little bit passing through the nozzle.
但这并不是气体冷却的唯一因素
But this can’t be the only contributorto the cooling.
毕竟 这罐子的冷却
I mean, the can itself cools off
远比阀门扩散造成的的冷却程度高
by significantly more than can be explained by expansion through a valve.
所以倒不像是被后方气体挤压了这种情况
And it’s not like it’s even being sprayed by the air coming out.
真正冷却的原因
No, the real cooling power is hinted at
在罐体的警告部分就暗示了
by the warning labels
上面写着 不要摇晃压缩罐或者颠倒着使用
telling you to not to shake cans of compressed air or spray them upside down.
如果你真摇了 会发现里面不只有气体
If you DO shake one, you’ll realize right away that it’s not just gas inside.
里面还有液体!
There’s liquid in there, too!
比如说1,1-二氟乙烷
Liquid like 1,1-difluoroethane,
它在常温常压下呈气态
which is a gas at normal temperatures and pressures,
但在六倍大气压下压缩后会呈现液态
but a liquid once you pressurize it to around 6 times atmospheric pressure.
它是压缩气罐中物体的主要存在形式之一
And it’s the essential component of thesecompressed air cans.
在压缩罐里
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.
刚好使液体在罐内不至沸腾
Just enough for the liquid boils off
从而在罐顶维持六倍大气压
to maintain six atmospheres of pressure in the top of the can,
这个气压又足以使剩余部分维持液态
a pressure high enough that reststays liquid.
正因罐内是六倍的大气压
Because it’s at six times atmospheric pressure,
当你打开阀门的时候 二氟乙烷会稳定涌出
when you open the valve the difluoroethane rushes out in a steady stream,
并清理干净灰尘和碎屑
blowing away dusts and crumbs.
但这也意味着之后压缩罐内部的压力
But this then means that the inside of the can is no longer pressurized enough
将使液体沸腾
to keep the liquid from boiling.
只有更多二氟乙烷不再沸腾 才能使内部恢复到六倍大气压
so more of it boils off until the gas reaches six atmospheres
从而达到新的平衡
of pressure again and a new equilibrium is reached
此时罐中液体略微减少
with slightly less liquid in the can.
这也正是为何
This is how the can is able to
近乎空了的压缩罐仍喷出稳定的气流
keep blowing a stream of consistent strength even when mostly empty.
不过回到我们的温度问题 更重要的是
But more importantly to our temperature conundrum,
从液相变到气相需要非常多的能量
changes from liquid phase to gas phase require a TON of energy,
而这能量必然来自某处
and that energy has to come from somewhere.
正如汗液的蒸发转移了你皮肤上的热量 让你很凉快
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,
会带走液体内部的能量并将其冷却
are the thing that steals energy from the liquid and cools it off.
超级冷!
Significantly!
喷出十分之一 就可将罐中剩余的物质降温
Spraying out 10 % of the contents will cool the entire remainder of the can
约二十摄氏度
by around 20 degrees celsius!
“沸腾会导致降温”听起来可能有些奇怪
If it seems counterintuitive that a boiling substance cools itself off,
那就联想一下高压锅吧
look no further than the humble pressure cooker.
水一般在一百摄氏度以上沸腾
Water normally boils above 100 degrees celsius.
但将蒸汽密封 气压上升
but by sealing in steam, the pressure rises,
会使锅中的水在超过沸点之后仍保持液态
enabling the water in the pot to remain a liquid well beyond water’s normal boiling point,
正如压缩罐中的二氟乙烷
just like the difluoroethane in a can of compressed air.
从高压锅阀中放出水蒸气
And releasing water vapor out of the nozzle of a pressure cooker
会降低锅内的气压 让部分水汽化
lowers the pressure inside, allowing a bit more water
不再沸腾 并将其余的水冷却
to boil off as steam and lowering the temperature of the remaining water,
正如压缩罐中的二氟乙烷
just like the difluoroethane in a compressed air can.
如果持续放气
And if you keep letting off steam,
最终水会冷却至常压下的一百摄氏度沸点
eventually the water will cool all the way back down to its regular boiling point of 100 degrees,
正如你一直用压缩气罐喷气
just like how if you keep spraying a can of compressed air,
罐中的二氟乙烷会一直冷却
the difluoroethane inside will cool all the way back down
至其常压下零下二十五摄氏度的沸点
to its regular boiling point of negative 25 degrees.
压缩气罐正如
A can of compressed air is quite literally
装满了1,1-二氟乙烷高压锅
a 1,1-difluoroethane pressure cooker.
你不会乱摇你的高压锅或把它翻个个儿用
and just like you shouldn’t shake your pressure cooker or turn it upside down,
除非你是想让滚烫的热水喷得到处都是
unless you want to spray superheated water everywhere,
压缩气罐斜着或倒着喷 都不大好用
cans of compressed air don’t work very well sideways or upside down.
那样喷出的不是气体 而是液体
Instead of spraying out gas, you’ll spray out the liquid
而液体只有在罐内的高压下才能维持液态
that was only being kept liquified by the high pressure inside the can.
所以它会立刻汽化 剧烈降温
So it immediately vaporizes and drastically cools down
无论附着到什么物体上
whatever it’s contacting.
都冻成冰!
INSTANT ICE!
但是二氟乙烷可溶于水且有剧毒
Though difluoroethane can dissolve in water and is poisonous,
所以千万不要用这种冰冷冻食物
so definitely don’t use this ice for anything food-related.
总之 压缩气罐冷却的缘由
In conclusion, the cause for the coldness of cans of compressed air
可通过以上解释得出结论
can be clarified by comprehending the consequent clue.
它们并非是一罐罐被压缩的气体
They aren’t actually cans of compressed air.
而是在一定压强下液化的1,1-二氟乙烷
They’re cans of pressure-liquified 1,1-difluoroethane,
向外喷出时 会降低罐内的气压
and lowering the pressure inside by spraying them
使得更多的液体不再沸腾 并将其余的液体冷却
allows more liquid to boil off, coolingwhat remains.
日常事物中蕴含的物理知识是如此有趣
The physics of regular stuff is super fun to learn about.
当然 黑洞啊和量子力学啊也很好
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.
如果你也想了解更多关于日常物品中的物理知识
And if you, too, want to dive deeper into the physics of everyday objects,
就来关注Brilliant网站吧 它是本视频的赞助方
look no further than Brilliant, this video’s sponsor.
Brilliant上有关于日常事物中的物理的全部课程
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.
其中也有寓教于乐 简短有趣的每日挑战和猜谜
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
无需太多时间
without the huge time commitment it would take
就能让你更深入地学到足以制作一个
to learn enough about Joule-Thompson expansion through a valve
有关焦耳汤姆逊膨胀内容的YouTube视频
to make a whole youtube video about it.
Brilliant一直是《分钟物理》的可靠的支持者
Brilliant continues to be an incredible supporter of MinutePhysics
前200名通过网址订阅高级版可享8折优惠
and they are offering 20% off of a premium subscription to the first 200 of you
仅限通过brilliant.org/minutephysics订阅的用户
who go to brilliant.org/minutephysics,
高级版订阅可以学习全部的Brilliant课程
which gives you full access to all of Brilliant’s courses,
还有猜谜和每日挑战
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,
订阅时让网站知晓 你是从这支视频得到消息的
and to let Brilliant know you came from here.

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视频概述

键盘清理,相机除尘,精密仪器整修,都离不开压缩气罐,但是为什么用着用着越来越冻手了呢?为什么罐体上会建议我们“点喷”使用呢?究竟是什么原理呢?

听录译者

收集自网络

翻译译者

晨晨

审核员

审核员BY

视频来源

https://www.youtube.com/watch?v=kjraelDMrFQ

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