What does the concept of entropy mean?

Question

What does the concept of entropy mean?

In: 96

Answer 1

Others have pointed out that it is a measure of the order/disorder in a system. But I think that is not quite getting to the point.

The point is that it is a way to know how much work can be done in a closed system.

First, it is easy to misunderstand “closed system”. This is a concept of an isolated hunk of stuff where nothing comes in, and nothing goes out. Well, there is no such thing other than (arguably) the entire universe.

For example, the Earth is sitting in space, not touching anything, but is not closed because energy comes from the sun as radiation particles. And the Earth radiates lower heat energy, and has particles stripped away.

So this is more of a thought experiment, and in practice, a way to find inputs to a system when those inputs may not be obvious.

So… As a thought experiment: if you make a closed system out of the earth, then the plant life will all die from lack of light, then everything else will die in a chain reaction. That is not the interesting bit. Over time, the crust of the Earth would heat up and melt. Why? In Earth’s current state, the core is the hottest due to several processes, and the crust is cool because it is able to radiate heat to space. In a “closed system Earth”, Earth can no longer radiate out heat, so the entire Earth becomes the same temperature over time. And that temperature is high enough to melt the crust.

This is more interesting than it might seem at first. Most power generation has to do with temperature differences. The simple example is a steam engine. It is the concentrated heat on one side that provides the energy. In our “closed system Earth”, one the heat has equalized, there is much less opportunity to create such engines. The Earth would be in a higher entropy state. Less work can be accomplished.

In the real world, we can literally dump water into a deep hole, have earth heat the water to steam, and use the steam to run a generator. This is a lower entropy state. More work can be accomplished.