Xall'ant! (Short form of 'accelerant', also meaning excellent, also referring to the acceleration of the gases, also by explaining the joke I no longer made it funny anymore)
@@margin100px yes haha. I would've been 15 when I made that comment. And to this day the notion that "expansion is a cooling process" has stuck with me. Was literally thinking about it yesterday when thinking about how compressed air cans get cold with use
I think these demonstrations are wrong. The cooler temperatures are likely due to more ambient air entrained in the flow rather than the air or steam expansion
No, ambient/surrounding air do not have any affect in it, it is just when air/steam expands it gets cool, it gets cool because the excitement in molecule decreases.
Hi Prof. Hewett: OK, I have a question for you. Why is popcorn hot? With all that expansion going on in the popping process, should't it be really cold? Thanks.
John Burgeson the reason things cool when they expand is because they are pushing the surrounding air away. Giving the molecules in the air some of its energy. When popcorn pops it does push the air, and does lose some energy. So in theory it does cool down. Problem is that the expansion is so small, and the popcorn is so hot, that you don’t notice the difference.
John Burgeson actually I think,we hit popcorn to expand it....but, while releasing air form mouth,at the first..we are doing with open mouth making a huge passage for the air...so,no suppression and expansion takes place..but in second case..we are whiffing making very short passage for the air to come out..after the ending of lip holes..air is expanded..and cool.Here..no hitting is taken place..only releasing point is suppressed or expanded......but in case of popcorn heating takes place
@@somefuckstolemynick take a insulated device in which fluid expands in that case also cooling effect is absorbed example throttling device reason is joule thomson effect
The cooling in these demonstrations is not the result of expansion, rather accelerated mixing with ambient temperature. Cooling of a real gas via expansion is described by the Joule-Thompson effect, and is not significant in the pressures demonstrated here. A pressure cooker operates around 1 bar. Air expanding from 1 bar to atmospheric results in around 1 degree Fahrenheit change. If the Joule-Thompson effect was significant in this demonstration, he would be able to touch the metal nozzle that was in direct contact with the expanding gas. The idea that you can demonstrate the Joule-Thompson effect with your lungs is even more absurd, considering you can only generate about 2 psi with your lungs.
For Joule-Thompson effect to take place, the initial temperature should be below the inversion temperature of the gas. Interestingly, high pressure hydrogen, helium etc. heat up due to expansion from room temperatures:-)
The cooling takes place due to EXPANSION, according to the ideal gas equation PV/T = constant. The Joule-Thompson part is really insignificant here. Also the mixing with air can not make it colder than outside air. If the prof. puts his palms little higher in the steam, he could have felt it to be colder than outside air:-)
Here's an energy perspective on it (assume water flowing in a simple pipe with a constriction): As water is incompressible, and the space it has to flow through becomes smaller, it has to speed up (otherwise the pipe would, well, explode). Now, as the water speeds up, its kinetic energy increases. Since there is nothing adding energy to the system, and energy cannot be created or destroyed, you have to ask "where does this extra kinetic energy come from?". Answer: It is "stolen" from the pressure (which is a form of "random" kinetic energy). To answer how it is actually transformed you have to look at the system on a particle basis. This would really need a longer explanation, but consider something like a pinball machine, with openings both above and below the machine with thousands of balls pouring in and bouncing around. As the balls enter to pinball machine it would strike other balls get flung around randomly, bouncing against the walls and other balls. Every time the balls strike the walls and other balls, this is what is giving rise to the pressure in the system. From here, consider what it would take for a ball to leave the machine again. Its direction has to be more or less lined up with the bottom hole. Meaning that only balls that "lost" their sideways momentum (to other balls bouncing the opposite way, remember that total momentum is always conserved as well) would continue down out of the machine. These balls will now have most of their momentum forward, and less to the sides, meaning they would bounce into the walls less often. This in turn will be noticed by an outside observer as lower pressure. This is my understanding of it at least. I'm more sure of the energy-explanation than of the particle explanation. Anyone who has a better understanding feel free to correct me! :)
@@ts9749 yeah I'm well aware that it's technically not incompressible. In pretty much all situations (e.g. this one) the compression is negligible and does not affect the results. Hence I neglected it.
according to the ideal gas equation PV=nRT meaning that volume and temperature are directly proportional. So shouldn't expansion (increase in volume) lead to increase in temperature (heating) instead of cooling? im very confused lol
Your argument assumes P=const. However, the truth is even more complicated, your equation is for ideal gas, but this effect works only for real gas! (Google Joule Kelvin effect)
@@vedator_sk oh also I realised that as the gas expands it will indeed heat up and do so by taking heat from the surrounding, that's how the surrounding will get cooled down haha why didn't I realise this sooner
farooq shaik no, the cooling comes from the expanding gas losing energy as it pushes the air away. Think of it as when one car hits another car, it will impart some of its energy into the other car and thus slow down. Heat is just another form of motion. If you google “Stirling engine” you will see an example of an expanding gas cooling down. But here the expanding gas doesn’t even come into contact with the surrounding air! The expanding gas pushes on a cylinder instead, and the energy goes into that cylinder (whereas in the video we see here, the expanding gas is pushing on atmospheric air). An expanding gas would still (at least initially) cool down, even if it expanded into a gas of the same (or even hotter!) temperature!
this video is rife with inaccuracies. for starters when you blow air it feels cooler than when you breathe out because the air is faster so it mixes more with ambient air, and the expansion demonstrated here is not adiabatic so any cooling would not be explained by the expansion alone.
Didn't you know that in America "expansion" starts with "X"? Sort of like "soldering" somehow loses the "L" and " Aluminium" somehow loses that pesky second "U" and "I". You know your teacher is a dummy when they can't even spell a simple word.
Man this guy has really Xpanded my knowledge of Fysics
highly underrated comment
Xall'ant! (Short form of 'accelerant', also meaning excellent, also referring to the acceleration of the gases, also by explaining the joke I no longer made it funny anymore)
Hey your comment is 8 year old
Are you alive ??
@@margin100px yes haha. I would've been 15 when I made that comment. And to this day the notion that "expansion is a cooling process" has stuck with me. Was literally thinking about it yesterday when thinking about how compressed air cans get cold with use
@@JamesGibsonVideo Oh you just read that 😁😁
Extremely important concept in the oil and gas industry
Do you work in the oil and gas industry? How do you use this concept exactly? Im curious
Perfect demonstration available on UA-cam
I really love this guy.... He is amazing.... I wish my physic teacher was like him :(
Joule-Thompson Effect?
Yes exactly...
1st law of thermodynamics
So why does helium heat up when it expands? Joule Thomson effect is tricky.
Holy crap this man literally brought pressure cooker into class room to show how to cook cold gas.
I think these demonstrations are wrong. The cooler temperatures are likely due to more ambient air entrained in the flow rather than the air or steam expansion
more ambient air mixing also more cooling from ambient air due to the high velocity of hot gas flowing
No, ambient/surrounding air do not have any affect in it, it is just when air/steam expands it gets cool, it gets cool because the excitement in molecule decreases.
Very helpful and easy to understand.
Does the steam cool more rapidly the faster it travels/ the more compression is behind it?
Absolutely remarkable
wow.. this guy is awesome,, nice video..
it is indeed remarkable sir
Hi Prof. Hewett: OK, I have a question for you. Why is popcorn hot? With all that expansion going on in the popping process, should't it be really cold? Thanks.
John Burgeson the reason things cool when they expand is because they are pushing the surrounding air away. Giving the molecules in the air some of its energy.
When popcorn pops it does push the air, and does lose some energy. So in theory it does cool down. Problem is that the expansion is so small, and the popcorn is so hot, that you don’t notice the difference.
That's pretty much the explanation I have been looking for (not for the popcorn question, but cooling on expansion), thanks!
John Burgeson actually I think,we hit popcorn to expand it....but, while releasing air form mouth,at the first..we are doing with open mouth making a huge passage for the air...so,no suppression and expansion takes place..but in second case..we are whiffing making very short passage for the air to come out..after the ending of lip holes..air is expanded..and cool.Here..no hitting is taken place..only releasing point is suppressed or expanded......but in case of popcorn heating takes place
@@somefuckstolemynick take a insulated device in which fluid expands in that case also cooling effect is absorbed example throttling device reason is joule thomson effect
@@somefuckstolemynick I have a doubt! If expansion is happening in closed chamber having vacuum initially, will it cool ?
Really amazing .. concept made easy..
Excellent example
The cooling in these demonstrations is not the result of expansion, rather accelerated mixing with ambient temperature. Cooling of a real gas via expansion is described by the Joule-Thompson effect, and is not significant in the pressures demonstrated here.
A pressure cooker operates around 1 bar. Air expanding from 1 bar to atmospheric results in around 1 degree Fahrenheit change. If the Joule-Thompson effect was significant in this demonstration, he would be able to touch the metal nozzle that was in direct contact with the expanding gas.
The idea that you can demonstrate the Joule-Thompson effect with your lungs is even more absurd, considering you can only generate about 2 psi with your lungs.
For Joule-Thompson effect to take place, the initial temperature should be below the inversion temperature of the gas. Interestingly, high pressure hydrogen, helium etc. heat up due to expansion from room temperatures:-)
The cooling takes place due to EXPANSION, according to the ideal gas equation PV/T = constant. The Joule-Thompson part is really insignificant here.
Also the mixing with air can not make it colder than outside air. If the prof. puts his palms little higher in the steam, he could have felt it to be colder than outside air:-)
Teacher: Why does a gas get cool when it xpands?
...because, as a gas Xpands, it gets cool.
Comments: This man is a genius!
The challenging question, is why does a constriction in the path of airflow cause it to expand, rather than compress?
The part behind the restriction is under compression. The gas is able to expand on the other side.
Here's an energy perspective on it (assume water flowing in a simple pipe with a constriction):
As water is incompressible, and the space it has to flow through becomes smaller, it has to speed up (otherwise the pipe would, well, explode).
Now, as the water speeds up, its kinetic energy increases. Since there is nothing adding energy to the system, and energy cannot be created or destroyed, you have to ask "where does this extra kinetic energy come from?". Answer: It is "stolen" from the pressure (which is a form of "random" kinetic energy).
To answer how it is actually transformed you have to look at the system on a particle basis. This would really need a longer explanation, but consider something like a pinball machine, with openings both above and below the machine with thousands of balls pouring in and bouncing around.
As the balls enter to pinball machine it would strike other balls get flung around randomly, bouncing against the walls and other balls. Every time the balls strike the walls and other balls, this is what is giving rise to the pressure in the system.
From here, consider what it would take for a ball to leave the machine again. Its direction has to be more or less lined up with the bottom hole. Meaning that only balls that "lost" their sideways momentum (to other balls bouncing the opposite way, remember that total momentum is always conserved as well) would continue down out of the machine. These balls will now have most of their momentum forward, and less to the sides, meaning they would bounce into the walls less often. This in turn will be noticed by an outside observer as lower pressure.
This is my understanding of it at least. I'm more sure of the energy-explanation than of the particle explanation.
Anyone who has a better understanding feel free to correct me! :)
@@somefuckstolemynick I know this was from two years ago, but water is not incompressible. Just saying.
@@ts9749 yeah I'm well aware that it's technically not incompressible. In pretty much all situations (e.g. this one) the compression is negligible and does not affect the results. Hence I neglected it.
beautiful
according to the ideal gas equation PV=nRT meaning that volume and temperature are directly proportional. So shouldn't expansion (increase in volume) lead to increase in temperature (heating) instead of cooling? im very confused lol
Your argument assumes P=const. However, the truth is even more complicated, your equation is for ideal gas, but this effect works only for real gas! (Google Joule Kelvin effect)
@@vedator_sk oh also I realised that as the gas expands it will indeed heat up and do so by taking heat from the surrounding, that's how the surrounding will get cooled down haha why didn't I realise this sooner
Joule Thomson EFFWCT
Expand begins with X?????
Deberían activar los subtítulos en español, para que la gente lo pidiera entender.
Excellent
Open the upper plate of cooker then real expansion takes place lol
I think the atmospheric air is cooling the steam down ?
farooq shaik no, the cooling comes from the expanding gas losing energy as it pushes the air away. Think of it as when one car hits another car, it will impart some of its energy into the other car and thus slow down. Heat is just another form of motion.
If you google “Stirling engine” you will see an example of an expanding gas cooling down. But here the expanding gas doesn’t even come into contact with the surrounding air! The expanding gas pushes on a cylinder instead, and the energy goes into that cylinder (whereas in the video we see here, the expanding gas is pushing on atmospheric air).
An expanding gas would still (at least initially) cool down, even if it expanded into a gas of the same (or even hotter!) temperature!
No
Wow!
Super sir
Very poor explanation. Where the energy goes? I have a lot other questions.
🤯🤯🤯Simply wow
Air also expands when its heated 😵
Damnn this guys like Eisenhiem the Illusionist!
amazing.....
Woahhh 😃
u should read his books
This doesn't explain WHY expanding gas cools.
this video is rife with inaccuracies. for starters when you blow air it feels cooler than when you breathe out because the air is faster so it mixes more with ambient air, and the expansion demonstrated here is not adiabatic so any cooling would not be explained by the expansion alone.
he is the guy
Starts with an X!
Didn't you know that in America "expansion" starts with "X"? Sort of like "soldering" somehow loses the "L" and " Aluminium" somehow loses that pesky second "U" and "I".
You know your teacher is a dummy when they can't even spell a simple word.
Let him cook
Fire Marshall Bill Xplains fisix
Mayadon ka tabi sir...
“Begins with an X..Expand”...i just face palmed
Must not have been too tiresome, since your hand was already in front of your face
Awful !... What a explanation ☺️
terrible. cool and cold is for dummies. screws people up.
Very helpful and easy to understand.
beautiful