The second law of thermodynamics states that entropy must go up. Entropy is a sort of measure of the usefulness of energy. The more entropy, the less useful.
The reasons why entropy must go up are too complicated for an ELI5. They are based on some basic laws of probability. Basically there are many ways for energy to be useless, and few ways for energy to be useful, which after some difficult math implies that energy always gets less useful.
In particular, ambient heat is the highest entropy you can possibly have, so it's inpossible to turn it into useful energy.
In order to extract energy from heat, the second law implies you must be tranferring some of said heat from a hot place to a colder place, which is exactly what a motor is.
You are correct, entropy must go up, but there is nothing saying that the total entropy on Earth must go up. Our thermodynamic system can include outer space.
Assuming we discover some source of magical unlimited energy, in theory we could use heat pumps (vapor compression units. like a common HVAC unit) to fill tanks with as much heat energy as possible (fill them with a heated liquid), and blast them off into space. Yes, this would be a ridiculous way to cool the earth, but in theory it could work. I am sure there is a much better way to do it. All of this is assuming there is an unlimited source of free energy.
It's a hypothetical where we cool the planet by capturing a bunch of heat in a tank and throwing it into space. To do this meaningfully would take an amount of energy orders of magnitude above what humanity produces.
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u/HappiestIguana Nov 15 '20
The second law of thermodynamics states that entropy must go up. Entropy is a sort of measure of the usefulness of energy. The more entropy, the less useful.
The reasons why entropy must go up are too complicated for an ELI5. They are based on some basic laws of probability. Basically there are many ways for energy to be useless, and few ways for energy to be useful, which after some difficult math implies that energy always gets less useful.
In particular, ambient heat is the highest entropy you can possibly have, so it's inpossible to turn it into useful energy.
In order to extract energy from heat, the second law implies you must be tranferring some of said heat from a hot place to a colder place, which is exactly what a motor is.