## Wednesday, 21 September 2016

### "What happens to the temperature when an ideal gas is compressed?"

From Wiki:

A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume. Compression of a gas naturally increases its temperature.

A lot of people seem to be stuck on that and blankly refuse to accept what is almost a truism. For a gas, pressure and temperature are almost the same thing. Increase/decrease either and you increase/decrease the other (until it levels out at the temperature of its surroundings, obis).

Dinero refuses to grasp this point:

"The main point is - There is no physical law that equates constant high pressure to constant high temperature. If you think there is then state it. As I said in an earlier comment Scuba diving tanks that are full with compressed air are actually not warm to the touch."

The metal skin of scuba tanks is a good heat conductor/poor heat insulator, so the overall temperature of the tank and the compressed gas (liquid?) inside it is the same as the surrounding air. Duh. But what happens when you reduce the pressure on that gas by releasing the valve? It cools down. You notice this when you are refilling a cigarette lighter with compressed gas, there will be ice round the opening where the moisture in the air has frozen. This is the basic "how a fridge works" model, see below.

So allow me to present related matters illustrating this single phenomenon, which is quite independent of sunshine or insulating effects of clouds and other minor greenhouse gases, can we please focus on the topic in hand...

1. From here:

In Lectures of physics, vol 1 by Feynman, it is written:

"Suppose that the piston moves inward, so that the atoms are slowly compressed into a smaller space. What happens when an atom hits the moving piston? Evidently it picks up speed from the collision. [...] So the atoms are "hotter" when they come away from the piston than they were before they struck it. Therefore all the atoms which are in the vessel will have picked up speed. This means that when we compress a gas slowly, the temperature of the gas increases."

Obviously, if the gas under pressure is not in a perfectly heat-insulated container, it will cool down again/warm its surroundings. Which is why Boyle had to wait for his compressed gases to cool down again before observing that pressure is proportional to volume; the immediate reading showed a higher pressure/temperature. Taking the atmosphere as a whole, it is in fact a self-contained heat-insulated container which contains itself. Gravity does the 'work' holding it in and the vacuum outside is to all intents and purposes a perfect heat insulator*.

The reverse is also true, if the volume of a sealed container with gas in it is increased, the gas will cool (until it is warmed up again by the non-insulating container).

2. See for example how fridges work:

A vapor compression cycle is used in most household refrigerators, refrigerator–freezers and freezers.

In this cycle, a circulating refrigerant such as R134a enters a compressor as low-pressure vapor at or slightly below the temperature of the refrigerator interior. The vapor is compressed and exits the compressor as high-pressure superheated vapor.

The superheated vapor travels under pressure through coils or tubes that make up the condenser; the coils or tubes are passively cooled by exposure to air in the room. The condenser cools the vapor, which liquefies. As the refrigerant leaves the condenser, it is still under pressure but is now only slightly above room temperature.

This liquid refrigerant is forced through a metering or throttling device, also known as an expansion valve (essentially a pin-hole sized constriction in the tubing) to an area of much lower pressure. The sudden decrease in pressure results in explosive-like flash evaporation of a portion (typically about half) of the liquid. The latent heat absorbed by this flash evaporation is drawn mostly from adjacent still-liquid refrigerant, a phenomenon known as auto-refrigeration.

Clearly, there is an added kicker here, the latent heat absorbed when the compressed gas (liquid) boils/evaporates again, but the principle stands. Heat coming out of the back of the fridge is equal and opposite to the fall in temperature inside the fridge (ignoring the extra bit of heat generated by friction).

3. Or let us take a jaunt into space and see what scientists say about the atmosphere of Saturn, which is so far out that the warming effect of the Sun is negligible:

Saturn's temperature and pressure increase from the exterior of the planet toward its center, changing the makeup of the clouds. The upper layers of clouds are made up of ammonia ice. Traveling toward the core, clouds of water ice form, with bands of ammonium hydrosulfide ice intermixed. The lower layers of Saturn see higher temperatures and pressures. Water droplets are found here, mixed with ammonia.

The center of Jupiter is more than 11 times deeper than Earth's center and the pressure may be 50 million to 100 million times that on Earth's surface! The tremendous pressure at the center of planets causes the temperatures there to be surprisingly high. At their cores, Jupiter and Saturn are much hotter than the surface of the Sun!

Strange things happen to matter under these extraordinary temperatures and pressures. Hydrogen, along with helium, is the main ingredient of Jupiter's and Saturn's atmospheres. Deep in their atmospheres, the hydrogen turns into a liquid. Deeper still, the liquid hydrogen turns into a metal!

We can pretty much rule out the Sun as a source of this heat, and their atmospheres are mainly H and He with practically no traditional greenhouse gases. Leaving us with one surviving explanation of the three as to why they are so hot in the middle.

What you have to remember is that we think of as the 'surface' of the earth is in fact the bottom of our (relatively thin) atmosphere (where the pressure induced warming is strongest), and the 'surface' of gas giants is the top of their (very thick) atmospheres.

PaulC156 keeps digging in the comments:

"So gravity exerts a force on matter which thus transfers energy of motion / kinetic energy which in turn can be measured by increases in pressure, density and temperature. Nothing controversial until you go from the general and ideal situation of a collection of molecules in space to the specific Earth bound circumstance of an atmosphere over a heated surface."

Jolly good, some agreement, so what is heating the centres of gas giants? Are they not just collections of molecules in space?

"That latter phenomenon [from inbound ultra violet from the sun transforming to out bound infra red] is given a token nod in the form of ‘it may have a small impact of a couple of degrees’. This is just hand waving."

No it is not given a token nod, the whole phenomenon of clouds reflecting the Sun's rays back up or back down when it's cloudy, and CO2 and CH4 turning long wave into short wave radiation and reflecting some of it back down is incontrovertibly true - but it is a completely separate phenomenon. The same as the Sun heating things up in the first place. They are not three alternative explanations for the same thing and we ought not waste time arguing over which is 'correct', they are three quite independent factors which are all have an effect.

It is like accelerating in a car when you are going downhill, there is no point having an argument over whether it is accelerating purely because of gravity or purely because you've pressed the accelerator, as both are having the same effect to some degree, the interesting bit is splitting up the total acceleration into the part due to gravity and the part due to pressing the accelerator.

So I might as well point out that PaulC156 (not his real name!) is only giving the basic Gas Laws a token nod.

Bayard also refuses to accept that the extreme case of what happens in the middle of gas giants is repeated on a small scale in the Earth's atmosphere:

"Mark you still haven't made the necessary distinction between movement (of mass) and static states. To all intents and purposed the Earth's atmosphere is static…"

To all intents and purposes, the H and He which make up 99% of the volume of Jovian Planets (a fancy name for gas giants) are static. It floats up, it sinks down etc.

4. Or even further afield and ask how stars form:

Gravity pulls the dust and gas together.

As the gas falls together, it gets hot. A star forms when it is hot enough for nuclear reactions to start. This releases energy, and keeps the star hot.

Where does heat come from? From gravity compressing the hydrogen atoms/molecules. Luckily, the earth does not get anywhere hot enough to trigger nuclear reactions!
--------------------------------------

Big hole in your analysis I think:
'the vacuum outside is to all intents and purposes a perfect heat insulator'
The vacuum provides no insulation for the radiant heat, which is why nights are colder than days.
Greenhouse gases on the other hand do provide insulation for radiant heat...

1. Yes, fair point, heat radiates from the earth equal and opposite to what comes in from the Sun. The net effect is zero. But let us rule the Sun out of this equation. There was no Sun shining on the earliest clouds of hydrogen, but nonetheless, they heated up (see also Saturn, Jupiter, above), the heat did not radiate out into space or else they would never have ignited. Deny that if you will. I am talking about a specific phenomenon that is independent of heat from the Sun.

2. The vacuum provides the same (lack of) insulation in daytime and night time. As far as I am aware, the reason it is colder at night is because the Sun is not shining on that part of the earth. The relative difference between temperature "where the Sun is shining" and "where it isn't shining" is a separate topic (and easily explained) to "why is it warmer at ground level than at higher altitudes (for a given surface temperature)".

3. Of course greenhouse gases i.e. clouds of H2O vapour reflect radiant heat. Everybody can notice that when it is cloudy at night it is surprisingly warm. That is quite a separate topic to "why is it warmer at ground level than at higher altitudes (for a given surface temperature)". Clouds at low elevation (fog) are warmer than clouds higher up.

mombers said...

Big hole in your analysis I think:
'the vacuum outside is to all intents and purposes a perfect heat insulator'
The vacuum provides no insulation for the radiant heat, which is why nights are colder than days.
Greenhouse gases on the other hand do provide insulation for radiant heat...

Dinero said...
This comment has been removed by the author.

M, I have added some extra explanations.

D, I never said anything of the sort. I wouldn't describe fridges or heat pumps as primordial.

paulc156 said...

So gravity exerts a force on matter which thus transfers energy of motion / kinetic energy which in turn can be measured by increases in pressure, density and temperature.
Nothing controversial until you go from the general and ideal situation of a collection of molecules in space to the specific Earth bound circumstance of an atmosphere over a heated surface. One where the direct heat from radiation can’t be downplayed and where the heat thus generated is reflected back from the surface in the form of infra red which is itself readily absorbed by GHG’s which in turn radiate randomly some of their heat back onto the surface whilst some is absorbed by other non GHG’s in the mix and some simply travels out of the atmosphere as infra red photons.

That latter phenomenon [from inbound ultra violet transforming to out bound infra red from the sun] is given a token nod in the form of ‘it may have a small impact of a couple of degrees’
This is just hand waving. Numerous studies have been conducted which support the empirical observations that co2 especially, is a super absorber of infra red energy at the very same frequencies [part of the spectrum] at which the Earth’s surface is ‘observed’ to radiate. The studies suggest much greater impacts on any hypothetical co2 free Earth.

A significant part of this infra red is re-emitted back to the surface where it is re-absorbed. One of the feedback mechanisms that co2 is responsible for is cloud formation.
Carbon dioxide doesn’t condensate out at temperatures anywhere near surface temperatures so acts as a catalyst for water vapour to form clouds, because water condensates more readily at warmer temps. Hence removing it entirely would not only act on the surface temperature directly by allowing all emitted infra red to pass straight through to space but by removing such feedback mechanisms as described would be expected to facilitate far lower temperatures due to much reduced cloud formation/water vapour. Subsequent increases in albedo are further second round feedbacks logically expected to follow.

Regards Marks gravity-pressure explanation for lower temperatures with altitude. Two principle reasons for lower temperatures at higher altitudes given an atmosphere of similar density to our own are indeed the effects of gravity/pressure and convection. As warm surface air rises it expands and cools. Once you allow for a composition of gases that include even tiny amounts of GHG’s the actual process of convection is driven by the warming that occurs in the lower atmosphere. Remove it and the convection itself is dramatically reduced. So co2 in particular is responsible for a large degree of surface warming initially but also cloud formation/water vapour which itself is a major warming gas as well as the convection process by which air cools rapidly as it rises.

Dinero said...

The post has a lot of points, what in summary, is your thesis regarding atmospheric air pressure and temperature.

Dinero said...

> Mark
You have explained how fridges work and how gasses heat up during compression and then cool again , but what is your thesis regarding atmospheric air pressure and atmospheric temperature.

Graeme said...

Mark

I know this is only a model but the measurements that do exist for atmospheric temperatures suggest that the embedded graph is not too wide of the mark:

https://en.wikipedia.org/wiki/U.S._Standard_Atmosphere

so in the troposphere pressure does seem to align with teperature, however, things get out of hand in the stratosphere. How do you account for that.

Graeme said...

Here is another account, which starts off with the ideal gas laws and then shows what needs to be added to tie up to reality.

Ralph Musgrave said...

Assume there’s just one atom in a car cylinder which is at room temperature. The atom bounces up and down between the piston and the top of the cylinder. Atoms at room temperature move around at about 1,000mph. If the piston then moves upwards at 1mph to its “maximum compression” point, the atom will gain a good 100mph in speed.
How is that possible, given the paltry speed of the piston? Answer….

During the half second or so during which the piston is moving, the atom collides with it a hundred times or so, and it gains 1mph each time.

I’d appreciate nominations for a Nobel Prize for this amazing insight.

Din, I don't have any magic insight, i am just giving some real life examples of the effects. For some reason there is a hot dispute over the effects on/of the atmosphere… I am baffled why people think that the laws of physics are different on Earth.

G, no model is perfect, I am not trying to envisage one, I am just explaining Gas Laws and giving some examples.

The heat of the stratosphere is a quite distinct fourth phenomenon has little to do with the others and not of relevance here.

RM, next time they ring up and ask for my suggestions I will nominate you. It is probably as simple as that.

But in practice, pressure/temperature increases in reverse proportion to volume, so you'd better put some convincing numbers on it.

Dinero said...

I also have been reading up on the subject

The main point is - There is no physical law that equates constant high pressure to constant high temperature .

If you think there is then state it.

As I said in an earlier comment Scuba diving tanks that are full with compressed air are actually not warm to the touch.

"Obviously, if the gas under pressure is not in a perfectly heat-insulated container, it will cool down again/warm its surroundings. Which is why Boyle had to wait for his compressed gases to cool down again before observing that pressure is proportional to volume; the immediate reading showed a higher pressure/temperature"

James Higham said...

Fine. How does this relate to LVT though?

Dinero said...

"Which is why Boyle had to wait for his compressed gases to cool down again before observing that pressure is proportional to volume; the immediate reading showed a higher pressure/temperature"

So after they cooled down they were at high pressure and room temperature.

Demonstrating that there is no physical law that equates constant high pressure to constant high temperature.

Bayard said...

Mark you still haven't made the necessary distinction between movement (of mass) and static states.

To all intents and purposed the Earth's atmosphere is static. True, some hot air rises in thermals and some cold air comes down to replace it, but compared to the mass of the atmosphere, this is negligible. What we have is cold, low pressure air at high level and warm, high pressure air at low level. Although the low level air is at a higher pressure than the high level air, it has neither recently been compressed, nor is it being compressed. It has been that pressure for millenia.

The entirety of your argument concerning the four gas laws, Feynman and the fridge deals with movement, where the pressure of a gas is being changed, either by compressing it in a cylinder, or expanding it through a nozzle. This does not apply to atmospheres, where, although as the observer moves through the atmosphere, the pressure and temperature change, the actual gas that makes up the atmosphere doesn't move and so is neither compressed, nor heated nor cooled nor expanded.

In our static atmosphere, we have warmer air at low level and colder air at high level. This can be adequately explained by conduction and a limited amount of convection. The sun warms the surface of the Earth, the surface warms the low level air and that heats warms up the air above it and so on until there is no air left to warm. The volume of space around the Earth increases proportional to the cube of the distance from the Earth, and the gravitational attraction which holds the atmosphere to the Earth decreases by the square of the distance, so the further you go out the more thinly the heat energy is spread out therefore the heat density falls, therefore the temperature falls. In addition the gravitational attraction which holds the atmosphere to the Earth decreases by the square of the distance, so the density of the air falls and hence its pressure.
AFAICS, that adequately explains the hot low air/cold high air phenomenon, without bothering Charles, Gay-Lussac or Boyle or involving greenhouse gases. It also explains why the surface would be 30C or whatever cooler if there was no atmosphere at all.

It is obvious that the Earth and its atmosphere are heated by the sun, which heat must be radiated out at a equal rate to which it is received, or else the Earth would soon attain the temperature of molten rock. There is no other way the Earth can lose heat other than radiation, as the vacuum of space, as you point out, is a perfect insulator to conduction and convection. So whether the Earth slowly heats up or cools down depends on the balance of radiation coming in and radiation going out. No matter is actually moving, only energy.

"the whole phenomenon of clouds reflecting the Sun's rays back up or back down when it's cloudy, and CO2 and CH4 turning long wave into short wave radiation and reflecting some of it back down(1) is incontrovertibly true - but it is a completely separate phenomenon. The same as the Sun heating things up in the first place. (2) They are not three alternative explanations for the same thing and we ought not waste time arguing over which is 'correct', they are three quite independent factors which are all have an effect."

No they are not. (1), the so-called greenhouse effect explains one phenomenon and (2) solar warming explains another. Your third effect doesn't explain anything because it only applies to movement and the atmosphere is static. Jupiter and Saturn have very hot, dense cores but so does the Earth. This heat is not caused by anything being compressed except to the extent that it survives from when the planets were formed. No compression is happening now and the high pressures are not producing heat. If you put a cool scuba tank into a perfectly insulated chamber it does not get any hotter regardless of the pressure inside it when it was first put into the chamber. So the pressure is not causing the heat. The heat has to be there in the first place.

Dinero said...

> Bayard

Good explanation concerning pressure and temperature except for this sentence.

"the further you go out the more thinly the heat energy is spread out therefore the heat density falls, therefore the temperature falls"

Thats not correct, that does not describe the temperature measured in gases. Temperature is not dependent on the concentration of the gas. As you said yourself Scuba tanks are not warm to the touch.

Where the atmosphere is very thin where it is an an insulator as is a vacuum and partial vacuum.

Temperature is not dependent on the concentration of the gas.

Scuba tanks are not warm to the touch as you said yourself.

The reason the upper atmosphere is colder is because the molecules there are moving less quickly because they have cooled from the expansion process or not heated up in the first place.

B, I refer you once again to the examples of the gas giants. There is no significant warming from the Sun, there are little or no greenhouses gases. So having eliminated those two contributory factors, what we are left with is the basic Gas Laws.

Don't play the trick of comparing Earth's rocky core with gas giants' atmospheres, that is a totally daft comparison. Why the centre of the Earth is so hot, but the centre of the Moon so cold (apparently) has got ABSOLUTELY NOTHING to do with the Gas Laws.

You can't just ignore actual facts as presented -and AFAIAA accepted by most scientists for centuries - try to look at all the examples and then slot the earth's atmosphere into the bigger picture between the extremes of fridges and gas giants, recognise the overall pattern, and then you will see what I mean.

He will laugh at this sort of thing:

"The sun warms the surface of the Earth, the surface warms the low level air and that heats warms up the air above it and so on until there is no air left to warm… AFAICS, that adequately explains the hot low air/cold high air phenomenon, without bothering Charles, Gay-Lussac or Boyle or involving greenhouse gases. It also explains why the surface would be 30C or whatever cooler if there was no atmosphere at all."

It is accepted (I hope and for the sake of this discussion) that without an atmosphere at all, the Earth's surface would have the same average temperature as the Moon's average temperature. Call it -15C.

If the lower atmosphere were on average -15C, getting colder further up, then this explanation must be correct.

But how can a surface that (would be) -15C heat up the air above it to +15C? Answer: it can't. it is the Gas Laws that cause this.

JH: "Fine. How does this relate to LVT though?"

I'm glad you asked, and a lot actually, in that deniers (science deniers and economics deniers aka Homeys alike) refuse to accept that there is an equilibrium

Molecules attract each other by gravity to the centre = pressure = heat. That heat is fixed where the pressure is and cannot escape.

Similarly, people are gregarious. Many say they hate towns and prefer the countryside, but in fact people are drawn towards other people. Which is why most people live in towns and not in the countryside. This pushes up productivity and hence wages and hence attracts more people and hence rental values etc.

The people who say "but the heat will dissipate" are ignoring the permanent influence of gravity.

Homeys who say "but there is plenty of cheap housing in low wage areas/sparse populations" are ignoring the desire of people to live in high wages and where there are lots of other people.

Dinero said...

Mark

the statement in the Jupiter link is wrong

"the tremendous pressure at the center of planets causes the temperatures there to be surprisingly high"

pressure does not cause temperature. That is wrong.

That is why in the your quote from Boyle he measured the high pressure of a gas that was at room temperature.

Din, in that case you will have to send thousands of letters to thousands of websites and academic book publishers worldwide, explaining to them that they are wrong.

I have explained Boyle's observations several times and you keep ignoring what actually happened first.

Bayard said...

"Thats not correct, that does not describe the temperature measured in gases. Temperature is not dependent on the concentration of the gas."

Ok, explain, the mechanism behind the cooling of a gas when it expands. If temperature is a measure of the heat density of a gas, then as it becomes less dense, it becomes cooler. If there is another explanation for this, I'd like to hear it.

"B, I refer you once again to the examples of the gas giants. There is no significant warming from the Sun, there are little or no greenhouses gases. So having eliminated those two contributory factors, what we are left with is the basic Gas Laws."

Look, there is no point in me arguing with you until you address the point of the lack of change. Atmospheres, both Earth's and the gas giants' are not undergoing compression therefore are not being heated up. Change is not happening. Yes the air cools down when it comes out from the scuba tank, but that is a change in pressure. To all intents and purposes atmospheres are steady states. Ok they are thin and cold at the outside and hot and dense at the bottom, but to get from thin and cold to hot and dense, all we are moving is the observer, we are not moving the gas. It is only when we move the gas that we raise the temperature.

"You can't just ignore actual facts as presented -and AFAIAA accepted by most scientists for centuries - try to look at all the examples and then slot the earth's atmosphere into the bigger picture between the extremes of fridges and gas giants, recognise the overall pattern, and then you will see what I mean."

Correlation is not causality. Just because an explanation fits all the facts presented does not mean it's the truth. You have brought up several phenomena, all of which appear to be explained by your theory, but are actually caused by some other mechanism. This is exactly what the Global Warmmongers do. They start with a theory, the Earth is getting warmer because we are burning lots of fossil fuels, and assemble facts and figures to support that theory, rather than looking at the facts and figures and finding a theory that explains not only those facts and figures but others too.

"If not, then I can't help you, chat to a friendly old school physics teacher instead. He will laugh at this sort of thing"

He will indeed, especially your complete disregard of the laws of thermodynamics.

Dinero said...

> Bayard

Fundamentally without other interactions with its surroundings gases don't cool when they expand , when a gas expands into a vacuum for instance it does not cool down , the temperature is the same for a small volume or large volume.

> Mark

Yes I remember have hearing refernces to "high pressures and temperatures" on popular science TV programs, but they must be Glib or just wrong.

Boyles observations of what happened first were an effect of change. The important point being the CHANGE in pressure as Bayard explains above. The heating was due to work being done on the gas.

We keep going round in circles here. The important observation is that compressed air cylinders are not warm to the touch and Bayard brought that up 54 comments ago, it was one of the first comments on the first thread.

Are you thinking about static high pressures or pressure changes.

Bayard said...

"Fundamentally without other interactions with its surroundings gases don't cool when they expand , when a gas expands into a vacuum for instance it does not cool down , the temperature is the same for a small volume or large volume."

However, it is demonstrable that a gas expanding through a nozzle cools down. Where is the energy in that gas going and what is moving it there?

Dinero said...

Because as the gas molecules expand outwards they collide with the surrounding air molecules and the gas molecules slow down. These collisions also cause the surrounding molecules to recoil away from from the expanding gas taking the kinetic energy transferred from the collision with them. That seems to be a good explanation.

Dinero said...

Here is a far better explanation. The surrounding air molecules are moving in all directions towards and away from the expanding gas , but the expanding gas molecules are predominantly moving towards the air molecules and so more of the collisions that occur will feature energy transfer from the gas molecule to the air molecule.