Eli5 If putting something under pressure increases the temperature why is the bottom of the ocean so cold?
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So the bigger question here is what actually *is* heat?
Everything in the universe is made up of tiny little particles, whizzing around at various speeds and bumping into each other and transferring their energy. (Think of it like hitting a stationary pool ball with another pool ball; the first ball slows down a little bit, and the second ball starts moving.) The total energy added up of all these particles whizzing around and hitting each other is what we call the heat of an object.
There are two ways you can increase the heat of a given volume, then: you can either make the particles move faster (by adding more energy, like you would if you were putting something in a kettle), or you can fit more particles in it (by compressing it). It’s important to note that in this latter case, you’re not actually adding energy to any individual particle; they’re all still going at the same speed, but they’re bumping into each other more often by virtue of there being more things to bump into. It’s like running around an empty football field versus walking slowly on a crowded street. There’s more energy in the latter because there are more people moving, even though they’re moving more slowly individually.
So why doesn’t the ocean floor stay hot? Well, it’s because it’s not a *closed system*. When particles hit each other, they transfer energy away from the bottom of the ocean (where things are slightly more compressed) to other particles, which transfer it to other particles, and so on and so on until that energy is more evenly spread-out on average than you might expect. Remember, we didn’t *add* any energy to any individual water molecule; the sun’s warmth is too far away for that. All we did was put slightly more of them in the same volume, and over time the number of collisions evens things out.
It’s also important to note that it’s not really *pressure* that makes as much difference as *compression*. Water isn’t really easy to compress in the way that gases are; if you take a bottle of air down to the ocean floor, it’ll be crushed, but if you take a sealed bottle of water down it will basically look the same. The reason for this is that there isn’t all that much space between water molecules to start with, so it’s very difficult to squish them together in a way that adds *more* water molecules to a given volume. That’s why the effect is probably a lot more limited than you might be expecting to start with.
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