Dr Adequate says that the entropy cannot be expressed in layman's terms. As the author of a primer that I've posted here that derives the Second Law from first principles, I thought I'd take a crack at it.
There are many pithy sayings regarding the Second Law and entropy, but the problem with them is that they wind up being used in places where they make no sense. The reductions are accurate, but can only be understood within the concept of thermodynamics, not other areas. So if we're going to explain it in layman's terms, we have to start with making the point that entropy is a statement about energy. It isn't about "information" or "order" or "disorder."
In short, entropy is the energy in the system that cannot be used for work. Thermodynamics works by moving energy around from one place to another. But because this transfer is a physical process, this means that this energy will go into the physical transfer mechanism as well as to the delivery place. It's a bit like trying to carry water from one place to another: Some of it is always going to stick to the vessel. If I take a cup of water and pour it out, there will always be a few drops sticking to the cup. I can shake the cup and bang it, but there is always going to be something left over. That water that wets down the vessel is like entropy: It isn't being transferred from one place to another but is lost as you transfer all the rest of the water.
As you move the same energy around and around the system, you eventually come to a point where there isn't any energy left that can be used: It's all been bled away as entropy. But that assumes a completely closed system. If we had more water entering the system, we can afford to lose some of it sticking to the sides of the vessel.
This is why concepts of "information" or "disorder" make no sense in their regular senses: It isn't a question of "ordering" the energy. It's that it is no longer physically available to do any work. When we reduce the concept of the Second Law and entropy to the phrase, "The universe tends towards disorder," what we mean is that the energy distribution of the universe is tending toward a uniformity. You see, energy transfer depends upon variations in energy states. If everything is the same, then there can be no reaction.
Back to the water concept: If you have a cube of ice and a pot of boiling water, there will be energy flow between the two if we put them into a system: However, that energy flow only happens because of the disparity in their energy states. The heat flows from the boiling water into the ice and eventually, the water hits equilibrium. When everything hits the same temperature, no more energy flow takes place. All the heat is now unavailable to do any work because there isn't anywhere for it go. That doesn't mean there isn't any energy: It's just that there is no disparity between any two places in the system and thus, there can be no transfer of any energy of any kind.
Eventually, all the energy in the universe will be equally dispersed throughout the universe and no reactions of any significance can take place. That's what we mean when we say "disorder": There is no pattern to anything because everything is dispersed.
The better metaphors for entropy are the ones that I put through in my primer:
There are no perfect engines.
OR
There are no perfect refrigerators.
In the first sense, this has to do with the water example I gave above: Some of the water always sticks to the vessel. You cannot convert heat entirely into work. Some of the energy gets used on the engine itself, rather than being used to drive the engine. In the case of heat, that means some of the heat is used to heat the engine rather than being used to make work the engine facilitates. You can make the engine more efficient such that more energy is converted into work, but there will always be some heat lost to the engine itself and thus, there is no way to make it perfect.
In the second sense, this is a more practical idea: A refrigerator takes heat from a low-temperature area and moves it to a high-temperature area. But the only way it can do this is by doing some work to move it. Heat simply doesn't flow from low to high. When you put a piece of ice into boiling water, the ice doesn't become colder and dump the heat into the boiling water to make it hotter. Instead, the heat flow from hot to cold. And yet, a refrigerator is possible because of the work involved. You can make the refrigerator more efficient such that not as much work is required to move the heat, but there will always be some work required and thus, there is no way to make it perfect.
Entropy is simply energy unavailable to do work.
Rrhain
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