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.
Or how about Jupiter:
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!
* Mombers adds:
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.
Wednesday, 21 September 2016
My latest blogpost: "What happens to the temperature when an ideal gas is compressed?"Tweet this! Posted by Mark Wadsworth at 10:20