From: CHRIS Z 21/05/00 0:20:24 Subject: Law of Thermodynamics post id: 72232 I, like most people who have studied science, have learned that perpetual motion is impossible because it violates the 2nd law. Ok, but can someone please explain why? From: Chaingun 21/05/00 0:29:01 Subject: re: Law of Thermodynamics post id: 72234 I think of the First Law as 'You can't win'(ie everything inexorably goes to maximum entropy) and the Second Law as 'you can't break even either'. Basically it says you can never get as much energy out of a closed system as you put into it - some is always wasted. This is why perpetual motion can't work - if it did, you would be getting as much out of the system as you put in. From: Dr. Ed G (Avatar) 21/05/00 1:03:25 Subject: re: Law of Thermodynamics post id: 72260 From a first principles standpoint it comes from statistical mechanics, which is pretty simple in its basis, but gets very complicated very quickly (and it usually comes as no suprise to students of statistical mechnics when they hear that Ludwig Boltzmann, arguably the principle progenitor of statistical mechanics, topped himself!). It's basically just a way of treating the probabilities of the manner in which particles distribute themselves and their energy throughout a system. One of the most important results, which is kinda obvious, from this is that (without external influence or energy) heat never flows from cold to hot, or even between objects at the same temperature. The more general upshot of this is that you can get free energy from a system without lowering the energy of that system. Another one is that if you put a cold object next to a hot object, the hot object will cool down and the cold object will heat up - against kinda obvious. The upshot of this is that in a Universe that is at a fairly uniform temperature of about 3 degrees above absolute zero, anything at a temperature above this will eventually cool down. Both these things mean you can never make a perpetual motion machine. The first means that any interaction of the workings of that machine with any object or system (apart from it) that has a lower energy will slow the machine down. The second means that it is statistically impossible to stop the interaction from the first from happening. Think about a free rotating bicycle wheel in the frictionless environment of deep space. There have been many questions on this forum as to whether or not this is perpetual motion. If you were to define "perpetual" as for a week, or a month, or a year, or so, then you could say yes this is perpetual motion. However, this is not a correct definition of perpetual! Perpetual mean unceasingly, for ever and ever and ever and ever, in perpetuity!!! In this case, not matter how lucky the flying wheel is, eventually it will hit something - whether it be a subatomic or an asteroid - which will interact with it and inevitably slow it down. This demonstrates the nature of statistical mechanics - you can't think about an object or particle in complete isolation as not object in the Universe exists in complete isolation (in fact it it did then technically it would cease to be part of this Universe) - and when you include the probability of an object encountering something that will slow it down (in the case of a perpetual motion machine), you end up with the laws of thermodynamics. Interestingly, no-one ever claims claims to be able to create a "perpetual stationary machine" which is just as impossible as a perpetual motion machine - eventually something will influence it to speed it up and therefore stop it being stationary. :-) Soupie twist, Ed G. From: DV (Avatar) 21/05/00 1:07:50 Subject: re: Law of Thermodynamics post id: 72262 The more general upshot of this is that you can get free energy from a system without lowering the energy of that system. I'm guessing there is a "not" left out. From: ATTO(ver) 21/05/00 1:18:58 Subject: re: Law of Thermodynamics post id: 72266 hi Dr ED Another one is that if you put a cold object next to a hot object, the hot object will cool down and the cold object will heat up - against kinda obvious. The upshot of this is that in a Universe that is at a fairly uniform temperature of about 3 degrees above absolute zero, anything at a temperature above this will eventually cool down. If the universe is cooling what is it next to to loose this heat...or isnt that what you ment???? now im really confuzzled Trev(TAO) From: Dr. Ed G (Avatar) 21/05/00 1:39:37 Subject: re: Law of Thermodynamics post id: 72272 DV - ummm, yes, thanks for that! ATTO(ver) - sorry, I meant the hot things in the Universe are losing energy to the inky blackness of space (mostly by radiation), so while he Universe expands eventually everything in the Universe will head pretty much towards a constant and low temperature, and if it continues to expand this temperature will steadily decrease... if it stops expanding and starts to contract, however, it will then start to heat up , though none of this increase will be useable for energy to do useful work, and everything will be at pretty much the same temperature (hence no net heat flows). From: rAT(vOTe) 21/05/00 1:49:39 Subject: re: Law of Thermodynamics post id: 72273 ok so the longer the universe expands the cooler it will become yes??? what is the temp of absolute zero???.....if that makes sense....and also how can there be a temp that is the absolute coldest there is with nothing lower???? cause if the universe keeps expanding wont the temp eventually get to and then below this temp??.....or is it sort of like a piece of string halved and halved and halved and halved.>.>.>.>.>.>.>.>.>.>.........you never quite get down to nothing and on the other side if it starts to contract has any one worked out how hot it would get or is there also an absolute high for temp???? Trev(TAO) From: Dr. Ed G (Avatar) 21/05/00 3:53:09 Subject: re: Law of Thermodynamics post id: 72277 ok so the longer the universe expands the cooler it will become yes??? Correct. what is the temp of absolute zero???.....if that makes sense....and also how can there be a temp that is the absolute coldest there is with nothing lower???? Good question!!! That's one of the reasons its so difficult (impossible, in fact) to get to absolute zero. You can reduce the temperature of one part of a system by extracting energy from it... but thermodynamically to do so, more heat/energy has to flow from another part of the system to a third part of the system (this is why a refridgerator radiates more heat than it takes from whatever's inside it). cause if the universe keeps expanding wont the temp eventually get to and then below this temp??.....or is it sort of like a piece of string halved and halved and halved and halved.>.>.>.>.>.>.>.>.>.>.........you never quite get down to nothing Spot on! and on the other side if it starts to contract has any one worked out how hot it would get or is there also an absolute high for temp???? Difficult to say. Certainly in the last stages of the 'big crunch', it is unlikely that any of the laws of physics that we know or understand would work in a manner that we know or understand... :-/ From: rAT(vOTe) 21/05/00 4:06:57 Subject: re: Law of Thermodynamics post id: 72278 wow you still here too Dr ED thanks for the info..tis all a little clearer now one more thing while ive got you here....ok so all the heat that was/is here will always be here just spread out through a greater area....so no more heat can get in right???? If yes what about when i rub my hands together real quick like and they get hot..friction....but for them to get hot, heat has to come from somewere yes(and if im am making heat then i must be getting it from somewere so that means that lets say a little spot on a planet going around a distant star might be losing some of its heat( can heat travell this fast???)..where does this heat come from, and then thus go......what im trying to say is, is there a way we could affect this "loss" of heat(is it a loss of heat as the universe cools down or just a redistrabution of heat over a greater area??) hmm after reading this i just realised that is not really that much clearer at all.....how do you guys get your brains to soak up this stuff?? Trev(TAO) From: Dr. Ed G (Avatar) 21/05/00 4:45:27 Subject: re: Law of Thermodynamics post id: 72281 Well, you can convert heat to different forms of energy, but eventually it'll all get converted back to heat. So, as far as we expected, there's only a finite amount of energy in the Universe, and once it's all been converted to heat that's where the story ends (that is, if the big crunch doesn't start an entirely new story). When you rub your hands together you're converting kinetic energy (the motion of your hands) to heat. You get the kinetic energy by converting adenosine triphosphate (ATP) to adenosine diphosphate (ADP) [well, that's not the whole story but it was as much as I learnt at school] - i.e. converting the electrostatic potential energy stored in chemical bonds (chemical energy) to kinetic energy. The chemical energy is derived from converting one form of chemical energy from he food you eat to another form of chemical energy (largely in the form of ATP molecules) that your muscles use. At some stage, the chemical energy in your food got there by converting radiant energy from the Sun to the chemical energy in sugars via the chlorophyll in plants. And of course the radiant energy of the Sun comes it trying to cool down. The only reason it doesn't actually cool down is the continual conversion of hydrogen into helium by nuclear fusion - but when the hydrogen runs out it most certainly will. So effectively the process of warming up you hands is simply a thermodynamic heat flow (albeit a convoluted one) from the Sun (hot) to your hands (cold), and then of course into the inky blackness of space (colder still). You're right about the volume thing. Heat tends to spread out, in the same way as gas molecules do, to fill as much space as it has access to. With no extra input of heat/energy this results in things cooling down. how do you guys get your brains to soak up this stuff?? Nothing better to do... and it keeps us off the streets! :-) [can you imagine what the world would be like if you had all of us science geek mutants actually out on the streets? :-) ] Soupie twist, Ed G. This forum is un-moderated. 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