So energy lost in one reaction is not necessarily unusable in another reaction.
Yes. But you must remember that we talk about a "system." That is, we assume that we have accounted for all players in the game: We have the pot of hot water, the ice cube, and some sort of "engine" that can do work as the heat flows from the hot water to the frozen water.
Eventually, all of those things will be the same temperature, there will be no more heat flow, and the engine grinds to a halt.
We call this system "closed."
Now, suppose we could add something to this system. Perhaps we heat some of the water so that it's even hotter or take some of the water and refreeze it. Then we can run the engine again, but notice that it required something outside the system to do this.
We call this system "open."
Note, a "closed" system can be quite complicated. For example, suppose the engine between the hot water and the ice cube is used to freeze some water. That'll certainly keep things going for a while but since we can never convert all the heat into work, eventually everything equalizes.
This is why the thermodynamic concept of entropy is dependent upon temperature. Mathematically, entropy is defined as energy ("Q") divided by temperature ("T"). Thus, the entropy of a reaction is related to the temperature at which the reaction takes place.
There's a connected concept: Enthalpy. It's sort of the opposite of entropy: Where entropy is the energy unavailable to do work, enthalpy is the energy that is available to do work. This is why some reactions result in a decrease of entropy. If the amount of energy used to run the reaction is sufficiently large such that the total energy of the system reduces, then we can reduce entropy, too. Remember that example I gave above where we use an engine to create ice? That's an example of energy being used to reduce entropy.
You take some coal and burn it, producing heat. That heat is used to heat up water into steam. The steam is used to turn a turbine. The turbine generates electricity. The electricity is used to power a refrigerator. The refrigerator is used to make ice.
Now, the water going from liquid to solid is a decrease in entropy. But the amount of energy that was used and processed in this system is much more than the entropic decrease.
Why would a layperson need to understand entropy?
To a certain extent, they don't. There is the joy of simply being an educated person, but it isn't like the typical person needs to be able to make a thermodynamic calculation in their daily lives.
Around here, however, it's important because one of the common complaints made by creationists is that evolution somehow violates the Second Law. They can never explain how, but they are certain that it does. It's because they don't understand what entropy is.
There are multiple failures in their claim connected to their misconception of entropy. The first is their assumption that a "more complex" organism (and note that what makes it "more complex" is never defined) represents less entropy than a "more primitive" one (again, notice the lack of definition as to what the means.)
Why? Why would this necessarily be the case? If we assume the concept of going from a single cell to a multicellular organism is what we mean by "more complex," it would seem that entropy has actually increased: There are so many more reactions taking place that the biological functions have plenty of opportunities to bleed off energy as entropy.
But even so, why would a "more complex" process be a problem? If the new process is more efficient, isn't that a more favored reaction? It can make use of the available resources better, supporting more organisms than one that is wasteful. This doesn't stop entropy from happening. It just happens more slowly.
And on top of that, there's this thing called the sun. If you take a look at the way life on this planet works, tracing back the energy, you find that most life traces back their energy to the sun. Suppose that refrigerator example from above was powered by solar cells rather than coal. Well, we've got a ton of sunlight available: That refrigerator is going to be working for quite some time, generating a lot of ice. The reason why this works is because the huge thermodynamic reaction of the sun and its entropic increase far outweighs the puny entropic decrease that we have here on this planet.
The idea that evolution is a violation of physics is laughable and shows a fundamental misunderstanding of both biology and physics.
Thank you for your submission to Science. Your paper was reviewed by a jury of seventh graders so that they could look for balance and to allow them to make up their own minds. We are sorry to say that they found your paper "bogus," specifically describing the section on the laboratory work "boring." We regret that we will be unable to publish your work at this time.
Minds are like parachutes. Just because you've lost yours doesn't mean you can use mine.
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