RW1

The equation you're refering to is pretty simple really. The conservation of energy means the amount of energy going into plus the amount of energy coming out of some (control) volume in various forms will equal the change in the amount of energy stored inside the volume. Energy is recognized going in or out in various forms, enthalpy (H), work (W) and heat (Q). What is in the volume is internal energy (U). There are other forms of energy that can affect the control volume in a thermal system such as potential and kinetic energy changes of the bulk fluid but they don't typically contribute any significant amount to the system and are often neglected. Any time energy is generated within your control volume, that energy is recognized as heat going in (Qin). For example, an explosive detonating, a fuel/air mixture burning, or even an electric heating element which is inside the volume. Of course, if there's a molecular change from hydrogen and oxygen to water for example, one also takes that into account. This type of analysis is very common for chemical engineers. Mechanical engineers like myself don't usually deal with processes involving chemical change within a control volume.

Yes, Jacques is correct. Heat from the sun can be used to create a high temperature heat sink that is hotter than the environment. Anyone that's fried ants with a magnifying glass can attest to that. A thermodynamic cycle can then be created that takes the energy from the sun at high temperature, converts some of it to work energy, and rejects the rest of the energy to the lower temperature environment. As Jacques points out, as the temperature difference between the two heat sinks drops, the efficiency (or the amount of work that can be pulled out of the system) "drops to zero when there is no temperature difference."

Just to clarify, air or other gasses expanding across a valve often get cold because of the way molecules rearrange themselves as they expand irreversibly. Note that the expansion of a gas across a valve is an irreversible expansion. It is also isenthalpic (enthalpy of the gas doesn't change). As it expands it can get colder, though some gasses such as helium actually get warmer on expanding like this. It is the reduction in temperature of the gas that makes the valve cold and when the valve gets cold, water from the atmosphere condenses on it.

The simple explanation: The first law of thermodynamics is the conservation of energy. Just as mass can not be created nor destroyed, energy can not be created or destroyed.* So the first law is like a bank ledger book that says all the energy we have can only be moved about into different forms such as internal energy, heat or work. In the case of a high pressure tank blowing air out and the pressure decaying, energy in the form of internal energy (which is a function of the gas' temperature) is converted to work energy and lost as it escapes out the tank valve. It actually doesn't do any work per se since it is irreversible and that energy goes into the environment. So the internal energy of the gas inside the tank goes into the environment and the result is that the gas gets cold. More rigerous explanation: The first law can be rewritten as follows for a control volume: dU = Hin - Hout + Win - Wout + Qin - Qout Note that potential and kinetic energy of the fluid has been neglected here which is common because those energies are typically negligable. For the case of a high pressure gas tank blowing that gas to atmosphere, there is no work done, nothing going into the tank, and we shall initially assume no heat flow. The first law can then be rewritten: dU = Hout Note that Hout = Uout + PV So the energy that is escaping as each bit of mass escapes is equal to the pressure times the specific volume. Thus, the internal energy (dU) of the gas inside the tank decreases by the amount PV with every chunk of mass that escapes. Note also that PV changes continuously as the pressure decays, becoming smaller and smaller as pressure drops. Another interesting way of looking at this is to break up the gas inside the tank into very small control volumes so that there are hundreds or millions of them inside the tank. For every control volume (CV) that remains inside the tank, we find that control volume expanding as other control volumes leave. The CV's that remain inside the tank don't "know" the others left, they only know they are expanding. If they have no heat flow (adiabatic expansion) then we will also realize they are expanding reversibly and isentropically. The entropy is constant, but the energy per unit mass drops as they expand. The conclusion of this is that the gas that remains in the tank acts as if it is expanding reversibly and adiabatically along a line of constant entropy. Note that this analysis of the gas inside the tank will match the first law calculation where dU = Hout. Once the tank begins to cool, the assumption there is no heat transfer becomes invalid because obviously there will be heat transfer from the metal tank to the gas and once the metal tank gets cold there will be heat transfer from the environment to the metal tank and then to the gas inside. So the surrounding air begins to cool, albeit only very slightly, as the tank cools. With any significant volume of ambient gas, the temperature change will be negligable. *Note there is the exception of energy being equivalent to matter so we can generalize this and say that matter and energy can not be created nor destroyed but we don't need to consider this when considering typical thermal engines here on earth. It is sufficient to recognize that energy is neither created nor destroyed.

I already did. Thanks. Air motors and heat pumps are real devices. The device he shows is neither of these, it is a perpetual motion machine because it violates the second law of thermodynamics. The web site is trying to get donations and thus is attempting to get money for a device which will never work even in principal. It is therefore a fraud and is attempting to relieve the unwary such as yourself from your hard earned $$$. I wouldn't expect you to understand this, you need an engineering or scientific background. Anyone with an understanding of how nature works will see this machine as an impossibility instantly. Any engineer can see the problem with this right away, but I would agree with you that the problem is totally unclear to a layman such as yourself. For engineers and engineering students - here is how I'd suggest looking at this: Put a control volume around the device and examine what is going in and out. He has enthalpy in at #8, heat in at #2, enthalpy out at #2 and work out at #2. For a steady state system such as this, dU = 0, and from the first law we can write his system as follows: dU = 0 = Hin + Qin - Hout - Wout From this we can go one of two ways by simply recognizing he's created a closed loop if we take the exhaust from #2 and put it into #8 with the addition or removal of heat: 1. Hout < Hin: In this case, we could add heat to the air after it exits and before it enters (Qin2) and thus the entire system becomes: dU = 0 = Qin - Wout + Qin2 or: Wout = Qin + Qin2 thus a violation per the Clausius statement (ie: all heat was converted to work) 2. Hout > Hin: Here we must remove heat from the air after it exits and before it enters (Qin2). dU = 0 = Qin - Wout - Qin2 or Wout = Qin - Qin2 This violation of the second law is not quite as obvious. What makes this a violation is the fact that the temperatures of the heat sinks for Qin and Qin2 are identical (ie: ambient temperature). In this case, Qin2 removes heat to the environment and Qin adds heat to the environment and the difference between the two is the amount of work done. Remembering that removing heat from the environment and convert it all to work is a violation of the second law of thermodynamics explains why this particular solution to the heat engine proposed by the con man at aircaraccess.com won't work. This is a violation of the Kevin-Planck Statement. See Wikipedia: http://en.wikibooks.org/wiki/Second_Law_%28Engineering_Thermodynamics%29#Statement_of_the_Second_Law_of_Thermodynamics Glad to hear it. Of course I have no 'hard feelings'. I'm not the person that needs to learn this stuff. I've been an engineer practicing thermodynamics for decades. Why would I feel my experience or knowledge is threatened? If you want to learn something, you need to ask, not tell people you've done something which you simply have no concept about. Regarding your heat engine, you really haven't explained it, but it seems to have the same fundamental problem. Your work seems to violate the second law. Energy can't be removed from the environment and converted to work without rejecting heat to a low temperature sink. That much is clear. What exactly your 'idea' is, is unclear. Please elaborate if you'd like a more thorough evaluation.

Nathan, The web site you're pointing to is an internet fraud. Donald Robertson is a con man of the worst sort. He claims an invention on this page which takes air, compresses it, and uses that compressed air to run an air motor which produces more power than is needed for compression. This is in clear violation of the second law of thermodynamics and if you look closely at his "invention" you'll find numerous violations of the first law of thermodynamics as well. For example, he shows no energy removal to reduce the temperature of the air once inside the cylinder, yet at the discharge he shows the air motor rejecting air at either a temperature which must be below ambient to produce more work than is needed to compress it, or he mistakenly shows heat entering the air prior to it being rejected by the air motor. It is unclear which he is proposing, but both of these processes are in violation of the first law. To be clear, the invention shown is a perpetual motion machine. That in itself is insufficient to make the claim that Donald Robertson is a con man/thief/internet fraud, etc. What is clearly internet fraud is the fact he asks for "donations" for his air car concept. Don't donate here. Or here.