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Small amount of smoke in vacuum chamber, what will happen?


Ghideon
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In another thread a forum member linked to a video that displays the behaviour of smoke in a vacuum chamber*. I had some off topic doubts about the setup and the displayed effect and I wish to discuss** in a fresh thread. Focus is on a scientific explanation of the generic case rather than possible errors, cheats etc in one specific movie.

Situation: We have a commercially available vacuum chamber*** operating as specified. The vacuum pump has created the best vacuum it can. Inside the chamber there are small items capable of generating smoke (matches, a piece of fuse etc) without requiring oxygen. In all cases the smoke seems to settle in the bottom of the chamber. The effect was explained something like there’s vacuum above the smoke. As long as there is just a little smoke, gravity just pulls it down. But if the smoke “falls down” in the vacuum to form a misty layer at the bottom of the container, shouldn’t remaining air do the same; “fall down” and form a transparent layer (before any smoke is added)? And if remaining air is in the bottom of the chamber, why is the pump connected to the top?

At this time I am not able to formulate an exact question, but hopefully some of the responses will help. Initially I think of something like “How do one model low pressure gas in earth gravity” or “What would a highly sensitive pressure gauge show if it is moved around in the container, any measurable variations?”.

But I also speculate about “smoke” in this case; is it an opaque gas or airborne particles? And does that have impact of the effect in the video? My initial guess was that smoke is particles and they fall to the bottom due to low pressure. But I don’t consider my guesswork to be science and I would appreciate some help.

*Video: https://youtu.be/Yb2YuC7UbwI

**from a mainstream scientific point this time  

*** By that I mean the setup is utilising “less than ideal vacuum”


 

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9 minutes ago, Ghideon said:

In another thread a forum member linked to a video that displays the behaviour of smoke in a vacuum chamber*. I had some off topic doubts about the setup and the displayed effect and I wish to discuss** in a fresh thread. Focus is on a scientific explanation of the generic case rather than possible errors, cheats etc in one specific movie.

Situation: We have a commercially available vacuum chamber*** operating as specified. The vacuum pump has created the best vacuum it can. Inside the chamber there are small items capable of generating smoke (matches, a piece of fuse etc) without requiring oxygen. In all cases the smoke seems to settle in the bottom of the chamber. The effect was explained something like there’s vacuum above the smoke. As long as there is just a little smoke, gravity just pulls it down. But if the smoke “falls down” in the vacuum to form a misty layer at the bottom of the container, shouldn’t remaining air do the same; “fall down” and form a transparent layer (before any smoke is added)? And if remaining air is in the bottom of the chamber, why is the pump connected to the top?

At this time I am not able to formulate an exact question, but hopefully some of the responses will help. Initially I think of something like “How do one model low pressure gas in earth gravity” or “What would a highly sensitive pressure gauge show if it is moved around in the container, any measurable variations?”.

But I also speculate about “smoke” in this case; is it an opaque gas or airborne particles? And does that have impact of the effect in the video? My initial guess was that smoke is particles and they fall to the bottom due to low pressure. But I don’t consider my guesswork to be science and I would appreciate some help.

*Video: https://youtu.be/Yb2YuC7UbwI

**from a mainstream scientific point this time  

*** By that I mean the setup is utilising “less than ideal vacuum”


 

In a vacuum, particles will settle according to their relative masses; heaviest first.

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10 minutes ago, Ghideon said:

My initial guess was that smoke is particles and they fall to the bottom due to low pressure. But I don’t consider my guesswork to be science and I would appreciate some help.

Yes indeed.

Smoke is a particulate disperse system (solid in gas), formerly called a colloid although that term is now more restricted.

So what happens when you remove the gas?

Edited by studiot
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4 minutes ago, studiot said:

So what happens when you remove the gas?

I have Ideal vacuum (more or less) and some dust on the bottom of the container. 

So just to clarify (maybe this is what I should have asked from the beginning): In a vacuum chamber, not ideal vacuum, is the pressure of the remaining air pretty much the same everywhere? There is not a "varying degree of vacuum" at different heights.

 

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2 hours ago, Ghideon said:

 

I have Ideal vacuum (more or less) and some dust on the bottom of the container. 

So just to clarify (maybe this is what I should have asked from the beginning): In a vacuum chamber, not ideal vacuum, is the pressure of the remaining air pretty much the same everywhere? There is not a "varying degree of vacuum" at different heights.

 

I looked at you referenced video and the author seems to have different aims in mind from your question.

Looking at my screenshot you can see black dots on the base of the jar where particles have fallen.

 

I am not sure if you understood my previous comment that smoke is a two phase disperse system and removing air takes away some if not all the second phase, leaving a one phase system, which is not dispersed.

Carbohydrate compounds produce water vapour and carbon dioxide when burned so there will have been an increase in gas present duringand after  burning, but wth a different chemical composition. The reading on the pressure gauge was not shown at this stage.

 

All in all a poorly designed experiment with no clear aim surprising outcome.

vacsmoke1.jpg

Edited by studiot
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4 hours ago, studiot said:

I looked at you referenced video and the author seems to have different aims in mind from your question.

Most likely yes, my questions arose from various discussions, not just from watching the video. In hindsight I probably could have posted the question here without referencing the video but I thought a reference was advisable. 

4 hours ago, studiot said:

All in all a poorly designed experiment with no clear aim surprising outcome.

I agree. That’s why I tried to lead the thread towards a more general case.

4 hours ago, studiot said:

I am not sure if you understood my previous comment that smoke is a two phase disperse system

At that time when you posted? No.
Now, after some additional reading, inspired by your post? Yes.
Many thanks, you have helped me improve my limited knowledge in this area! I think I can consider my original question answered. 
Thanks also to @StringJunky, your comment made me think about perfect vacuum vs partial vacuum and small particles vs large objects etc.

 

I’ll try to sum up some misunderstandings I had earlier should someone find it interesting. Those not interested in my personal version of lack of knowledge may stop reading. There’s also a risk that I fail to explain my fails and just look like I haven't improved my knowledge at all, time will tell...   :) 

1. I believed something like: “smoke is a mixture of opaque gases”. Fail, wrong. 
2. I believed something like: “The visible amount of smoke is proportional to the number of air molecules at that height from the bottom of the vacuum chamber”. Fail, wrong.
 

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