Talk:Adiabatic process

Accuracy Dispute & Chages, 2008-03-26

I made several changes to the article, and more may be needed. In order of appearance on the page:

1) Paragraph 3 previously stated:

"...A transformation of a thermodynamic system can be considered adiabatic when it is quick enough that no significant heat is transferred between the system and the outside. The adiabatic process can also be called quasi-static..."

The second sentence is confusing or simply incorrect; a quasi-static process is one in which all states can effectively be considered equilibrium states (see quasi-static), which seems to apply better to a case when the gas does come into thermal equilibrium with the outside of the system. Although a quasi-static assumption may be necessary for a reversible process (don't know this for sure), it is certainly not necessary for an adiabatic process (e.g., a rapid, irreversible gas expansion where inertial effects are important). I have removed it to avoid confusion.

2a) In the "Adiabatic Heating and Cooling" section, Paragraph 2 previously stated:

Adiabatic cooling occurs when the pressure of a substance is decreased, such as when it expands into a larger volume. An example of this is when the air is released from a pneumatic tire; the outlet air will be noticeably cooler than the tire.

The phrase "such as when it expands into a larger volume" suggests a gas flowing into a larger volume or reservoir, which is incorrect. The spirit of the sentence is that gas increasing in volume undergoes adiabatic cooling, but this is ambigious in the original wording. I have replaced it with "...the pressure of a substance is decreased as it does work on its surroundings," which is closer to the reality. However, this still doesn't mention a volume increase, which would be nice to have (i.e., an explanation of the fact that a gas increasing in volume and thus decreasing in pressure--or vice versa--does work on its environment, and thus cools).

2b) I removed the example of air being released from a pneumatic tire in the quote above, as the cooling is partially due to the Joule-Thomson effect, not entirely due to adiabatic expansion. Some cooling due to adiabatic expansion may be observed inside the tire itself (the volume of gas which remains in the tire at any given time is expanding and doing work on the exiting air), but this does not account for the reason air may feel cold as it just begins to exit the tire. Adiabatic expansion may be noticeable as a decrease in the temperature of the tire though how much so depends on the heat capacity and mass of the rubber relative to those of the gas.

A good analogy here is an aerosol can. The can cools in your hand as it is sprayed due to evaporation of the fluid inside (similar to adiabatic expansion), and the stream coming out of the nozzle is cooled by some combination of adiabatic expansion and the Joule-Thomson effect.

It would be nice if someone could find a good "everyday life" example for this section that does not include a dissipative/irreversible process, though... the best I can think of right now is a vacuum pump (when gas is expanded by a piston inside a cylinder), but I don't think many people will be able to relate to this, and it's not immediately apparent that the gas does net work on the piston as is required for adiabatic cooling (it seems to be the other way around at first glance). Perhaps a pneumatic piston motor would work well, where expanding high-pressure gas does work on a piston, and should come out colder than it went in?

3) In the "Ideal Gas" section, Paragraph 1 previously stated:

"The mathematical equation for an ideal fluid undergoing an adiabatic process is"

This entire section is only valid for a reversible (isentropic) process, and does not apply to, say, a mass passing through a throttle (even though this is an adiabatic process). I've updated the section heading, introductory sentence, and the beginning of the second paragraph ("For adiabatic processes, it is also true that") to reflect this. However, more changes may be needed throughout the section, and it would be nice to have an explanation of why this applies only in the reversible case... in short, it's because the work on a gas volume does not equal PdV any more, but some things may be said about inertial or viscous effects, quasi-statics (relating to pressure changes and reversibility, not thermal interactions with the system boundary), etc.

Thanks in advance for any contributions to the article!

Cheers, --Masegado (talk) 20:53, 26 March 2008 (UTC)

Continental and local scale

can you help me with a continental or global scal in the atmosphere and a local or regional scale

Continental or Global scale would be the Hadley cell and a local or regional scale would be sea and land breezes. Hope this helps!

Perhaps articles like Joule-Thomson effect should be considered as relevant to the topic. --Saperaud 08:04, 22 December 2005 (UTC)

Edit, 2006/01/30

I edited the section on adiabatic heating/cooling. The "coolness" felt when pursing one's lips and blowing on one's own skin is due almost entirely to enhanced convective heat transfer; the actual air temperature drop is so small (due to the tiny pressure drop involved) that if one blows on a thermometer in this manner, there will be a negligible temperature change.

I instead offered the different example of deflating a tire, where the pressure drop is on the order of several hundred KPa (several dosen psi); I have seen frost form on valve stems because of this.

Also added the more extreme heating example of a motorized air compressor operating at higher pressures than a bicycle pump.

Joe Frickin Friday 17:54, 30 January 2006 (UTC)

Very similar effect occurs when looking at a Methonol Cannon, you can observe this phenomenon very easily

added 2006-01-04

in 'Derivation of formula' section equation (3) there is dE = something - shouldn't it be dU ?

Symbol for the ratio of specific heats

I fully realize that the Greek letter gamma (${\gamma }$) is used by physicists and others as the symbol representing the ratio of specific heat at constant pressure to the specific heat at constant volume. However, I would like to point out that thousands of engineers worldwide use a lowercase k to represent that ratio. I strongly believe that this article should mention that fact. mbeychok 00:10, 1 March 2006 (UTC)

Adiabat?

So, what exactly is an adiabat? The article doesn't make this clear. Jonabbey 04:31, 7 June 2006 (UTC)

I belive it is a line on a graph that indicates 0 heat flow along that line, like an isotherm is a line denoting constant temperature.

Correct. An adiabat is a curve on the pV graph where Q=0. An adiabatic process is one that follows one of these curves. An adibat is always steeper than an isotherm. 75.5.254.164 00:51, 11 December 2006 (UTC)

No, an adiabat is a curve with constant entropy (i.e. its an "isentrope"). Q (or δQ) is not a state function, so there are many curves through the same point that could be adiabatic, but only one that is isentropic. PAR 02:58, 11 December 2006 (UTC)

Derivation of TV^gamma-1

The derivation of TV^gamma-1 is a lot longer than the one in my physics text book (Young & Friedman 11th ed.) and doesn't explain what Cv is. I'd fix this myself, but my mathML sucks. 75.5.254.164 00:51, 11 December 2006 (UTC)

We are doing a project on the adiabatic process and we need a lot of help. Can anyone help us? 63.172.1.2 13:42, 28 February 2007 (UTC)Paco and Taco

Something seems wrong to me: Equation (1) states that dU = - δW, whereas first equation below equation (4) states that δW = - p dV = α p dV + α V dp = dU, i.e. δW = dU. Giving that the end of the derivation is correct, there should be a confusion between the physical definition of work (work done by the system) and the chemical one (work done one the system). Equation (1) is valid for physical work definition. Question: is equation (2) valid for physical work or chemical work ? Parey (talk) 08:07, 25 January 2008 (UTC)

Adiabatic Internal combustion Engine

Fountain Powerboats of Washington NC claims to have created an adiabatic internal combustion engine based upon a marinized Chevrolet engine. Though the project has not proven publicly it is really an adiabatic cycle engine it has demonstrated extreme power production. Anyone care to propose how this would work?

Jsmithnc 20:25, 13 August 2007 (UTC)

Discrete Formula

It looks like something is wrong with the final form of the discrete formula derived - if you just plug in the ideal gas law for t2 and t1 and cancel things, you get alpha=-1 always. This can't be right - alpha depends on the degrees of freedom for the particular molecules (ndof/2), and could take on many values. —The preceding unsigned comment was added by 67.161.105.146 (talk) 07:15, August 21, 2007 (UTC)