I don't understand the law of thermodynamics. - Page 1
Forum section: electronics / beginners

author: Capernicus
author_rank: Frequent Contributor
timestamp: March 04, 2022, 02:19:40 pm
content: So I've now hit a new brick wall of not understanding things.Is the law of thermodynamics meaning at one level of psi for air, there is one level of temperature?The mistaken explanation for that is that there is more molecules hitting the side of the air vessel so there is more energy being put into friction generating heat. So the more pressure, the more often it hits the side of the air vessel, so the more friction then the more heat at once.This cant possibly be it I know, because then you could cook a steak on a 4,000 psi air vessel indefinitely, which would break the laws of thermodynamics right?I read this other thing that a coke can when it heats up it gains pressure on the inside, and if u pop a hot coke can it sprays everywhere.So Ive got it completely wrong, can someone please explain this to me?

author: TimFox
author_rank: Super Contributor
timestamp: March 04, 2022, 02:52:17 pm
content: Google will give you many replies on the "three laws of thermodynamics".The law you seem to be asking about, however, is the Ideal Gas Law (q.v.), not one of the three.For a given quantity of material, that law isPV=NRTwherePis the absolute pressure (not gauge pressure)Vis the volume of the container containing the gasNis the quantity of materialRis a constant that relates the others andTis the absolute temperature (above absolute zero).Boyle's Law is the simpler version, whereN, R,andTare held constant.https://courses.lumenlearning.com/boundless-chemistry/chapter/the-ideal-gas-law/

author: tszaboo
author_rank: Super Contributor
timestamp: March 04, 2022, 03:02:38 pm
content: First law: Energy doesn't get createdSecond law: You mix something hot and something cold, it will be warm.Third law: In very cold stuff, atoms don't move much.The laws don't really have anything to do with pressure. Look up adiabatic changes, and then apply the three laws.

author: aheid
author_rank: Regular Contributor
timestamp: March 04, 2022, 06:05:47 pm
content: Seems you are forgetting that the temperature is related to the kinetic energy of the molecules.If two molecules with different kinetic energy collide, their energies will "mix", so that the one with lower initial kinetic energy ends up with more and vice versa. This is the basis of heat conduction.Thus if the wall is kept cooler than the gas, the gas will on average lose kinetic energy, and hence temperature, over time. As per the ideal gas law already mentioned, a lower temperature means less pressure.The converse is also true, if the walls are kept hotter the pressure will rise. This is the basis for violent steam explosions for example.:https://en.m.wikipedia.org/wiki/Equipartition_theorem

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 04, 2022, 06:28:27 pm
content: Can you quote the particular phrasing you're reacting to, and explain more directly how you are mis/understanding it?Temperature is average energy. The gas is hitting the walls sure, but the walls are jiggling in response, and this is how they exchange heat energy. There is no "friction", just the exchange of jiggles.A microscopic explanation of friction might be illustrative (...or even more confusing). Consider that no surface is perfectly flat, nor crystallographically aligned. (And if they were, they would stick together rather well!) Contact is made at only very few points (asperities). At almost all of the facing surfaces, there's no contact, nothing more than wind shear in the airspace between them; only at the peaks that touch, is there friction or wear. And you can very easily imagine, then, the forces at those contact points, being quite extreme -- on par with the ultimate strength of the material itself, or if you prefer an atomic perspective, the binding energy per particle -- cause extremely intense jiggling as they touch, bond, slip and break, and as material is smooshed and broken and worn away. That jiggling spreads out through the material (phonons) and thus the temperature rises.So in friction, it's not at all an equilibrium situation, and it's also a nonreciprocal situation: effectively zero states (configurations of jiggles) will ever come together in such a way as to push the surfaces opposite the direction of applied shear force, let alone restore material to any damaged peaks. Which would be a very strange situation, after all!Another interesting thought experiment is, suppose you have a piston of gas consisting of just a handful of molecules, and suppose you move the piston away supersonically -- so that no molecules hit it during the move. Is this...weird? Well, the molecules still have to travel a longer distance, so the pressure is lower (i.e. the average rebound force of molecules bouncing off the piston face), but their velocities are the same (assuming an ideal container that doesn't exchange heat energy with the gas, i.e., it doesn't jiggle at all), so the temperature hasn't dropped as we would expect it to (adiabatic expansion). I'm not sure if this can be read as an energy input due to sonic boom, but if nothing else it illustrates there is something missing: for sonic motion, molecules are in contact with the piston as it moves, and therefore they rebound less (i.e. at the difference of velocities). Even if the piston movesatthe speed of sound, the half of the population moving faster than that, will hit it and mostly stop in their tracks, leaving a rarefied (but not empty) and cold wake behind the piston. The original volume will then expand into that space, and mix with this lower energy population. Note that there will be considerable acoustic energy in this case as well (waves reflecting back and forth), which will eventually be absorbed by the gas's viscosity, and thus that acoustic energy converted to heat.Note that thermodynamics requires two assumptions:1. The two states being compared, are both after infinite time, i.e., the system is static, in equilibrium. Or various special quasi-static cases, depending on how we bend the rules (and how well we are able to do so, without flagrantly violating them).2. There is a continuous path between states.Continuity is a deeply important property of physical systems, and the mathematical tools we use to describe them. Moreso the latter than the former, probably; we use continuity to describe these ultimately lumpy systems (made of atoms, literally!) because it's effective, and a hell of a lot easier than trying to track 10^26 particles -- something we won't be able to compute for a long time indeed, if it should be directly computable within the universe at all, even. If we violate those assumptions (say by a step change in volume), we shouldn't expect the rules to apply either.Acoustic energy is interesting, because it can store energy, but not forever; at infinite time, it too gets mixed in to the overall system. This mirrors the old electrical saw of two capacitors annihilating in charge: charge must be conserved, but how is half the energy difference lost if there is no resistance? Well, of course we've modeled the system incorrectly if it has no inductance or resistance; in particular, no real system has perfectly zero resistance at AC*. So at infinity, even if the Q factor of the RLC circuit is very high, it will not be infinite, and so the system comes to rest at the level predicted by charge conservation. Thermodynamics is a conservative law as well, so the resolution to this case is the same: we count the dynamic energy as well. (Adding or removing it, depending on whether we're talking total system energy, or of some limited part. Considering just the capacitors, it's lost. Put it in an ideal box, and its temperature rises, conserving the total.)*Superconducting resonators have Qs over 10^7, better than some quartz crystals -- but as far as I know, nothing's been demonstrated with hugely more than that. Anyway, we'd probably struggle to measure anything in the 10^9, 10^12, etc. range. Superconductors are only probably ideal at DC; loss can be observed, but it's not real clear how, AFAIK. Could a superconducting current be maintained at absolute zero, in absence of any external influence? Dunno.Tim

author: jpanhalt
author_rank: Super Contributor
timestamp: March 04, 2022, 06:39:53 pm
content: You may be looking for simply Boyle's and Charles' laws combined to this common equation for behavior of an ideal gas without any loss or gain of heat (i.e., adiabatic):

author: TimFox
author_rank: Super Contributor
timestamp: March 04, 2022, 07:35:35 pm
content: As I stated in the first reply, these equations are simplifications of the Ideal Gas Law.Simply put, "ideal gases" are those that are at a temperature well above their boiling point.For helium, that is well above 4 K; for air or nitrogen, that is well above 78 K; for steam that is well above 373 K.For useful refrigerants (such as the Freons), they are usually used at temperatures that involve condensation, and are therefore not ideal in this sense.

author: eugene
author_rank: Frequent Contributor
timestamp: March 04, 2022, 07:50:59 pm
content: Quote from: Capernicus on March 04, 2022, 02:19:40 pmThe Ideal Gas Law describes what happens when you take a fixed quantity of gas and change the volume, pressure, or temperature, then the other two will change in response.So, if you take some air (starting at atmospheric pressure and room temperature) and squeeze it down to 1/20th its original volume, the absolute temperature goes up by a factor of 20.Interestingly, if you then squirt some diesel fuel into the compressed air, it will ignite and raise the temperature even more. Applying the Ideal Gas Law one more time, the pressure will also increase even more. If all of this takes place inside a cylinder with a piston, then the increased pressure will push the piston out of the cylinder. If you connect the piston to the wheels of a vehicle, then you can use it to drive down the road.QuoteAlthough Coke isn't an ideal gas, the CO2 dissolved in it works in the same way. Start with a fixed quantity (12 oz) at atmospheric pressure and room temperature. If you raise the temperature without allowing the volume to change, then the pressure goes up.

author: Capernicus
author_rank: Frequent Contributor
timestamp: March 05, 2022, 08:36:49 am
content: Quote from: eugene on March 04, 2022, 07:50:59 pmSo what is the absolute temperature of air at 14 psi?Thanks everyone for trying to help, but I dont understand yet. I'm getting a slightly better idea what's going on. I'm planning out a machine atm, and its heating something up in a 200psi atmosphere (just electrical resistively), does this mean its actually going to be hotter than it would be at 14psi?And I'm wondering now can I do away with the heater and just use the compressed air as the heating source???So... when I'm cooking a steak, the steak is actually pulling the heat away from the hot plate as its cooking? - thats wierd, I've never thought of it this way before.If I spray and evaporate water on a compressed air vessel - does this drop the pressure inside so it can fit more air in with the same mechanical energy?

author: Capernicus
author_rank: Frequent Contributor
timestamp: March 05, 2022, 09:37:36 am
content: Ok I think I understand.So if I pump something up to 1000psi, and then i suck the heat out of it to be the same as outside the cylinder (temperature).So then the pressure goes down to 400 or whatever, because now its all colder, even tho the amount of packed air molecules is increased absolutely.But the thing is it doesnt heat up again, because its matching the outside temperature, its ~400 psi now (or whatever it ends up being) at ambient room temperature - unless i pump more in?- OR - is there an absolute temperature quantity there due to how many air molecules are packed together, that keeps it above ambient constantly? (but pigs are flying there, so Im doubting it at this stage. It means u stick it under a sterling engine and it goes forever.)

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 05, 2022, 11:04:43 am
content: I think the thing you are missing is that it takes work to compress that gas. That work increases the temperature of the gas. But since the gas is now hotter than the surroundings it loses heat (that lost heat is what is cooking your steak). But since you are not replacing the heat the temperature drops. If nothing else changes the pressure drops with it, though not all the way back to the original pressure. If you connect your tank to a high pressure source more gas flows in. The details get messy because that may represent an additional increment of heat, but eventually the system stabilizes at the ambient temperature and with a final quantity of gas given by Boyle's law (in the ideal case). Boyle's Law is actually a very good approximation to reality for common gases at the temperatures and pressures usually seen when everything is at equilibrium.

author: Capernicus
author_rank: Frequent Contributor
timestamp: March 05, 2022, 11:09:50 am
content: yes thanks I understand now!!!

author: TimFox
author_rank: Super Contributor
timestamp: March 05, 2022, 02:49:03 pm
content: Note that the pressure, temperature, and volume are independent variables, but relate to each other and the quantity of gas by the Ideal Gas Law.In a balloon, for example, the volume can vary, and the temperature of a gas in a steel pressure vessel usually reaches equilibrium with the external (ambient) temperature.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 05, 2022, 03:31:42 pm
content: kinetic energy is used in the process of pressurisation- once the vessel is pressurised the energy is stored as potential energy. Of course, once pressurised, the vessel has to cool down to the ambient temperature so there are losses.

author: Capernicus
author_rank: Frequent Contributor
timestamp: March 05, 2022, 04:31:32 pm
content: the loss can go in your favour tho, cause what if u needed it cold and the extra pumping energy was ok to waste.

author: TimFox
author_rank: Super Contributor
timestamp: March 05, 2022, 04:32:27 pm
content: Could you re-phrase that?

author: gcewing
author_rank: Regular Contributor
timestamp: March 06, 2022, 08:33:07 am
content: Quote from: Capernicus on March 05, 2022, 08:36:49 amThe question is underspecified. It depends on the density of the air.

author: TimFox
author_rank: Super Contributor
timestamp: March 06, 2022, 03:02:40 pm
content: The Ideal Gas Law (when applicable) answers that question, but in terms of the other variables that he did not specify quantitatively.If there is no container (balloon or air tank) to define the volume, then re-arrange the equation in terms ofN/V, the density of the gas (usually in moles/m3).By the way, if you need to know the actual air pressure in your vicinity, note that the Weather Bureau normalizes local readings to the value that would have been measured at mean sea level, so that readings from one station to another can be compared. At mean sea level, when there are no hurricanes about, average absolute air pressure is 760 mm Hg = 14.7 lb/in2To determine normal air pressure at your location, there are calculating toolshttps://www.mide.com/air-pressure-at-altitude-calculator

author: electro-56
author_rank: Contributor
timestamp: March 06, 2022, 10:52:41 pm
content: For years I have run into what seemed to be contradictions in this area. Recently I came across a blog post from someone called Matt Ferraro, who offers a credible explanation:https://mattferraro.dev/posts/joule-thomson

author: RJSV
author_rank: Super Contributor
timestamp: March 07, 2022, 01:22:48 am
content: Capernicus:I had started to reply, but then looks like you were in that, just then.So, instead of re-hashing that, here is a sort-of opposite stated effect:Suppose you do the opposite, of pumping up pressure, and 'waiting' a period of time, for the cyl to cool back down, equalizing temperature w everything:Suppose you put in pressured freon, seal it at room temperature, then wait, opposite actions from last hypothetical, ...Oh hell, can't think this right, but I'm thinking that the opposite actions, could make a situation, where change would make the freon 'suddenly' get really hot; but I'm now realizing can't necessarily state things opposite.What I'm thinking, is expand the freon, this it will absorb heat coming inward to the vessel, become room temperature, (or wherever), and THEN use that:Thinking (was), you let the ROOM PRESSURE rush in, that would create a 'huge surge', in temperature, BUTI GUESS, THATS WRONG...I think that mind excercise didn't follow the proper way of pV= next.Plus, non-ideal and all that...But was trying for symmetrical application, seeing as how an 'opened' can, of freon will cause a local freeze.Heat transfer is a most respected aspect, of applied science, for sure !

author: RJSV
author_rank: Super Contributor
timestamp: March 07, 2022, 01:34:47 am
content: ...a little more thought on this...(and way too much eye squinting)...I would, first, put a moveable cylinder head, between the freon chamber, and the air inlet recieving 'chamber' that is for 'pushing' and will pressurize that low pressure FREON, but I think, there isn't a bunch of 'HEAT CONTENT' there, to suddenly raise the temps...I think maybe that heat content, is different from the kind of 'heat deficit' effect, that CAN cause sudden freezing, when you open that can, of room temperature but pressurized freon.Very tough, to keep this correct...

author: Bassman59
author_rank: Super Contributor
timestamp: March 08, 2022, 03:01:01 am
content: The Three Laws of Thermodynamics.1. You can’t win.2. You can’t break even.3. You can’t even quit the game.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 08, 2022, 04:08:33 am
content: Quote from: RJHayward on March 07, 2022, 01:34:47 amThermodynamics can be difficult to follow in the simplest cases. When you pop the can of Freon you are dealing with phase changes which add a whole other level of complexity. Also in these non-equilibrium states there are real chances of departing from the ideal gas law.All of this complexity is why those who work the field do a lot of simulation and then perform measurements to see if their models were adequate to the problem at hand. While the simple approximations often work, the exceptions are even more common than in our home field of electronics.Weather modeling is one of the most common and widely available examples of how the simple models get you to some good rough answers, but not good enough so more and more complexity is added. And now using massive amounts of computing capacity they can do a pretty darn good job of predicting the weather a day or two in advance, and actually do a surprisingly credible job a week or so out.

author: timenutgoblin
author_rank: Regular Contributor
timestamp: March 08, 2022, 04:29:51 am
content: Quote from: Capernicus on March 05, 2022, 04:31:32 pmThat sounds like the principle of operation of a refrigerator or air conditioner.https://youtu.be/gVLhrLTF878https://youtu.be/7NwxMyqUyJw

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 08, 2022, 05:10:38 pm
content: Quote from: timenutgoblin on March 08, 2022, 04:29:51 amexactly... in cold winter countries its easy to collect woods to burn to get heat, so less engineering task there. but in hot countries, we cant simply do that to get cold air, so engineering has to find a hack to it... from basic gas law PV=nRT or PV/T, engineers used thermodynamics processes such as Isobaric, Isothermal or Isochoric processes to get ingenious cycles such as Carnot, Stirling or Brayton etc to formally explain or invent things like refrigeration or combustion engines...https://en.wikipedia.org/wiki/Thermodynamic_cycleto do energy transfer from heat...