Perpetual motion violates either the first law of thermodynamics, the second law of thermodynamics, or both.The first law of thermodynamics is essentially a statement of conservation of energy. The second law can be phrased in several different ways, the most intuitive of which is that heat flows spontaneously from hotter to colder places; the most well known statement is that entropy tends to increase, or at the least stay the same; another statement is that no heat engine (an engine which produces work while moving heat between two places) can be more efficient than a Carnot heat engine. As a special case of this, any machine operating in a closed cycle cannot only transform thermal energy to work in a region of constant temperature.
Machines which are claimed not to violate either of the two laws of thermodynamics but rather are claimed to generate energy from unconventional sources are sometimes referred to as perpetual motion machines, although they are generally reported as not meeting the standard criteria for the name. By way of example, it is quite possible to design a clock or other low-power machine to run on the differences in barometric pressure or temperature between night and day. Such a machine has a source of energy, albeit one from which it is quite impractical to produce power in quantity.
It is customary to classify perpetual motion machines according to which law of thermodynamics it attempts to violate:
1. A perpetual motion machine of the first kind produces energy from nothing, giving the user unlimited 'free' energy. It thus violates the law of conservation of energy.
2. A perpetual motion machine of the second kind is a machine which spontaneously converts thermal energy into mechanical work. When the thermal energy is equivalent to the work done, this does not violate the law of conservation of energy. However it does violate the more subtle second law of thermodynamics . Note that such a machine is different from real heat engines (such as car engines), which always involve a transfer of heat from a hotter reservoir to a colder one, the latter being warmed up in the process. The signature of a perpetual motion machine of the second kind is that there is only one single heat reservoir involved, which is being spontaneously cooled without involving a transfer of heat to a cooler reservoir. This conversion of heat into useful work, without any side effect, is impossible by the second law of thermodynamics. What may prove more useful is to explain the existence of hot reservoirs to begin with. A hot reservoir inside an internal combustion engine is created by a spark igniting fumes which contain stores of chemical energy. The temperature of the fumes increases above that of the surroundings. This is not a perpetual motion machine since the ability to raise the temperature above that of the surroundings depends on finite chemical reactions always less than the total heat energy and mass-energy contained within the system. Since there are far more states in which heat distribution is closer to thermodynamic equilibrium than states in which heat is concentrated in small regions, heat will tend to smooth out over time to lower power densities of increasingly unusable forms.
3. A more obscure category is a perpetual motion machine of the third kind, usually (but not always) defined as one that completely eliminates friction and other dissipative forces, to maintain motion forever (due to its mass inertia). Third in this case refers solely to place in the above classification scheme, not the third law of thermodynamics. Although it is impossible to make such a machine,[4][5] as dissipation can never be 100% eliminated in a mechanical system, it is nevertheless possible to get very close to this ideal (see examples in the Low Friction section). Unfortunately, even if such a machine could be built, it would not serve as an endless source of energy, since any energy-extracting mechanism would also serve as a dissipative force. If we could build a frictionless flywheel, it would eventually slow down & stop completely if its kinetic energy were tapped for useful work, and we would get no more energy out than the amount that was initially put in to spin up the flywheel.
Use of the term "impossible" and perpetual motion
Like all scientific theories, the laws of physics are incomplete. "A world that was simple enough to be fully known would be too simple to contain conscious observers that might know it."[6] Outside of pure mathematics, stating that things are absolutely impossible is more a hallmark of pseudoscience than of true science. Nevertheless, the term is properly used to reflect those things that cannot be true without a significant rewrite of nearly all known scientific laws.[7]
The conservation laws are particularly robust. Noether's theorem states that any conservation law can be derived from a corresponding continuous symmetry.[8] In other words, so long