This same 'information' is then 'recovered' when the resulting liberated internal energy can be put to use. It adds 'information' to each particle it 'sorts' to either the left or the right. In other words, what does the demon do to the system in a thermodynamic sense? In Maxwell's Demon, this entropy isn't appearing from nowhere we can't start with nothing and end up with something. In this case the total internal energy is Heat, which is proportional to temperature.Įntropy on its own is not a measure of energy or mass, in fact mass itself is better related to Heat - changes in Heat correspond to changes in energy (and therefore to changes in mass). This means that a decrease in entropy, increased the amount of available energy in the form of heat.Ĭonsider then, if the temperature distribution of the particles were larger - the same decrease in entropy would make available a larger proportion of the internal energy, as there would be a larger gradient possible.Ĭlearly, entropy relates to available internal energy. This is despite the heat energy existing before in the same quantity. The entropy - due to the re-arrangement, the internal energy (Heat) has been partitioned creating a gradient where there previously was none. The total energy has not changed (ignoring how the selection process works), the internal energy has not changed (particles are in different places but with their initial speeds), so what has changed? Maxwell's Demon, is a door / gatekeeper etc, that sifts particles such that these two sets end up apart from one another - say, the 'faster' particles on the left and the 'slower' particles on the right. This means there is a set of particles at a lower-than-average speed and a set of particles at higher-than-average speed, but these particles are not separated so no work can be extracted - both sets of particles are occupying the same volume (the whole box of gas) so no significant heat gradient can be found to extract work from. In a 'normal' situation the speeds of particles in a box of gas aren't going to be equal, but they'll approach a distribution that tends toward the average temperature. That is, if you cannot construct a heat gradient, no work can be done.įor something like Maxwell's Demon, you need to consider what happens to the heat content. The difference is important, since the Work done by a thermodynamic system is proportional to a change in Heat content. I don't know if it's right or not, but it helps me to think of Heat as 'thermal momentum' analogous to temperature being 'thermal speed'. But, Heat also relates to the quantity of particles - a large, warm box has more heat than a small box of the same temperature because there's more mass at that temperature. The Heat of a system, is the total thermal energy - a warm box of gas has more heat than a cold one. Increasing the temperature of an isolated system will increase the speeds of the particles, but does not of itself affect the entropy per se.Ĭontrast this with Heat, which most times is what is meant by talking about the internal energy of a system. Temperature refers only to the particles' speeds. There are a few concepts to go through regarding entropy in thermodynamics - it's a tricky subject easy to conflate with probabilistic interpretations.
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