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Old 19th August 2019, 03:08 PM   #41
JimOfAllTrades
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Originally Posted by Robin View Post
I forget the point the guy on the radio was making.
I've heard the same basic story a couple of times in "you have to think outside the box" type presentations.
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Old 19th August 2019, 03:54 PM   #42
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Originally Posted by BillC View Post
According to U.S. Standard Atmosphere 1976, atmospheric density varies from 1.225g/cm3 at 0m altitude to 1.112g/cm3 at 1000m, so I would expect the density at the top of the 1m cube box to be 1.225 - (1.225-1.112)/1000 = 1.224887 g/cm3, assuming it entrapped the same mass of air as an imaginary cube of the same size at sea level.

Not quite that simple because the rate of change of pressure is not linear. That is a higher rate of change at lower elevation and lesser at higher elevation.
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Old 19th August 2019, 04:27 PM   #43
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Originally Posted by Thor 2 View Post
Not quite that simple because the rate of change of pressure is not linear. That is a higher rate of change at lower elevation and lesser at higher elevation.
Maybe think about it in case you make a fool of yourself like I did
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Old 19th August 2019, 04:46 PM   #44
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Originally Posted by theprestige View Post
Opening and closing the valves costs energy. The real question is whether the box-pump (BP) produces more energy than is necessary to operate its own mechanisms. Running the BP at a lower altitude would tend to increase the pressure gradient and therefore the energy output. But it would also mean your valves would have to work harder against the pressure differentials involved. Same with a larger box, I think.
But if what I had said had been correct, then you could seal the box except for a small hole at the top and then plug the hole and the centre of mass of the air inside would move upwards.

And the same size hole and valve would work on a box of any size.

That would definitely be free energy. So, for that reason and all the other reasons given I was wrong.

I should have thought it through first.

(EDIT: and the sealing and unsealing valves could both be at equilibrium pressure, meaning that a minimal force would be required to open and close them independent of the mass of air that is being moved. So I opened the door to a number of absurdities).
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Old 19th August 2019, 04:48 PM   #45
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Originally Posted by Myriad View Post
Accelerations of less than a full G can produce noticeable (as well as measurable) changes within a closed container of gas as well.

One example you might have experienced is carrying helium party balloons in a car or other vehicle. When the vehicle accelerates, the balloons move forward (despite inertia), because the acceleration increases the air pressure toward the rear of the vehicle.

That's a most interesting observation.

Whenever I observe something like this I try to figure out why, and also think about how the observed phenomena can be put to use in some way. I think many useful inventions have their roots in chance observations. The word serendipity comes to mind.
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Old 19th August 2019, 09:13 PM   #46
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Originally Posted by Thor 2 View Post
Not quite that simple because the rate of change of pressure is not linear. That is a higher rate of change at lower elevation and lesser at higher elevation.
In any case, more or less linear from 0 to 1000 metres so the density difference is more or less as he states.
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Old 19th August 2019, 10:10 PM   #47
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Originally Posted by Robin View Post
In any case, more or less linear from 0 to 1000 metres so the density difference is more or less as he states.
All these problems could be avoided if we replace the gas by incompressible liquid. All the principles will remain the same, except the gradient will become linear. The solution will also become more intuitive: I don't think anybody doubts there is a pressure gradient in a closed container of water...
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Old 19th August 2019, 11:05 PM   #48
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Originally Posted by curious cat View Post
All these problems could be avoided if we replace the gas by incompressible liquid. All the principles will remain the same, except the gradient will become linear. The solution will also become more intuitive: I don't think anybody doubts there is a pressure gradient in a closed container of water...
I don't think anyone doubted there was a pressure gradient in a closed container of air either.

I initially thought that sealing it in a container would give it a different pressure gradient from the surrounding air.
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Old 20th August 2019, 02:34 PM   #49
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Originally Posted by Robin View Post
In any case, more or less linear from 0 to 1000 metres so the density difference is more or less as he states.

More or less?

Density of air varies by about 13% over 1000 Metres.
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Old 20th August 2019, 03:10 PM   #50
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Originally Posted by Thor 2 View Post
More or less?



Density of air varies by about 13% over 1000 Metres.
Why do you think that means it is not more or less linear from 1 to 1,000 metres?
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Old 20th August 2019, 03:31 PM   #51
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Originally Posted by Robin View Post
Why do you think that means it is not more or less linear from 1 to 1,000 metres?

Really? You can't see that the difference in pressure from 0 -1 metre is 13% more than the difference in pressure between 999 - 1000 metre?
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Old 20th August 2019, 03:38 PM   #52
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Originally Posted by Thor 2 View Post
Really? You can't see that the difference in pressure from 0 -1 metre is 13% more than the difference in pressure between 999 - 1000 metre?
Um, that's not what you said.

You said density of air varies by about 13% over 1000 metres. If you meant that the rate of change of density with altitude is 13% more at 0m than 1000m then okay, but it wasn't what you wrote.
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Old 20th August 2019, 03:53 PM   #53
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Originally Posted by Jack by the hedge View Post
Um, that's not what you said.

You said density of air varies by about 13% over 1000 metres. If you meant that the rate of change of density with altitude is 13% more at 0m than 1000m then okay, but it wasn't what you wrote.

I assumed readers would realise that the density is directly related to pressure difference per unit length or hight. Was this a big ask?
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Old 20th August 2019, 03:54 PM   #54
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Originally Posted by Thor 2 View Post
Really? You can't see that the difference in pressure from 0 -1 metre is 13% more than the difference in pressure between 999 - 1000 metre?
Where are you reading that from?

He references density and links some data from the engineering toolbox website. Are we looking at the same data?

According to the data I am seeing, the densities differ from a linear approximation by less than 1% from 0 to 3000 metres.
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Old 20th August 2019, 03:56 PM   #55
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Originally Posted by Thor 2 View Post
I assumed readers would realise that the density is directly related to pressure difference per unit length or hight. Was this a big ask?
Again, we appear to be looking at different data. I am looking at the table "U.S. Standard Atmosphere Air Properties - SI Units" in the link provided.
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Old 20th August 2019, 04:14 PM   #56
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I am happy to be corrected on this. Here is the table that was linked:

and here is a graph of the whole range which is, as Thor 2 correctly stated, non-linear:

and here is a graph of the range from -1000 metres to 4000 metres which is, as I said, more or less linear:

So it seems to me that the pressure drop over 1 metre would be, to a reasonable approximation, of the order suggested by BillC
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The non-theoretical character of metaphysics would not be in itself a defect; all arts have this non-theoretical character without thereby losing their high value for personal as well as for social life. The danger lies in the deceptive character of metaphysics; it gives the illusion of knowledge without actually giving any knowledge. This is the reason why we reject it. - Rudolf Carnap "Philosophy and Logical Syntax"
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Old 20th August 2019, 04:55 PM   #57
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As far as I can see you would have to go to 6 significant figures to see the difference that the extra 13% would make to the estimate.
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The non-theoretical character of metaphysics would not be in itself a defect; all arts have this non-theoretical character without thereby losing their high value for personal as well as for social life. The danger lies in the deceptive character of metaphysics; it gives the illusion of knowledge without actually giving any knowledge. This is the reason why we reject it. - Rudolf Carnap "Philosophy and Logical Syntax"

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Old 20th August 2019, 08:37 PM   #58
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Originally Posted by smartcooky View Post
Schrödinger's Gas Law?
That's only applicable if the gas in the box is mostly cat ions.
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Old 21st August 2019, 12:47 PM   #59
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It really depends on what is in the box.

Gonna give you somthing so you know what's on my mind:

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Old 21st August 2019, 01:05 PM   #60
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Originally Posted by Dr. Keith View Post
It really depends on what is in the box.

Gonna give you somthing so you know what's on my mind:

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Gwyneth Paltrow's head is my go-to.

In related news, I heard an interview with Andy Samberg, in which he said that collaborating with Justin Timberlake on that sketch took his craft as a music producer to a whole new level. Timberlake brought a wealth of experience and professionalism to the project. Samberg learned a lot about how to make good music well from the experience. I have always found it amusing that such a silly song would be so full useful career advice for the people working on it.
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Old 22nd August 2019, 05:22 PM   #61
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Originally Posted by theprestige View Post
Gwyneth Paltrow's head is my go-to.

In related news, I heard an interview with Andy Samberg, in which he said that collaborating with Justin Timberlake on that sketch took his craft as a music producer to a whole new level. Timberlake brought a wealth of experience and professionalism to the project. Samberg learned a lot about how to make good music well from the experience. I have always found it amusing that such a silly song would be so full useful career advice for the people working on it.
SNL seems to have a long history as a training ground for America's entertainment industry.
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The non-theoretical character of metaphysics would not be in itself a defect; all arts have this non-theoretical character without thereby losing their high value for personal as well as for social life. The danger lies in the deceptive character of metaphysics; it gives the illusion of knowledge without actually giving any knowledge. This is the reason why we reject it. - Rudolf Carnap "Philosophy and Logical Syntax"
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Old Yesterday, 12:45 PM   #62
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Originally Posted by theprestige View Post
Gwyneth Paltrow's head is my go-to.

In related news, I heard an interview with Andy Samberg, in which he said that collaborating with Justin Timberlake on that sketch took his craft as a music producer to a whole new level. Timberlake brought a wealth of experience and professionalism to the project. Samberg learned a lot about how to make good music well from the experience. I have always found it amusing that such a silly song would be so full useful career advice for the people working on it.
Originally Posted by Robin View Post
SNL seems to have a long history as a training ground for America's entertainment industry.
Yeah, Lorne Michaels is some sort of alchemist. He really brings together the right people over and over. Sure, there are misses. But that happens when you let people loose. I've seen him say that what hurt most were skits that rocked in dress rehearsal, but bombed live for some reason. There are just so many factors to getting it right.
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