What is entropy? Also, can anyone explain how it relates to matter, gravity, time, other physics terms?
In: Physics
What is energy? Energy is a difference in potential in a system that can be exploited to do work.
Imagine an airtank full of compressed air, inside of a sealed box, attached to a motor. Initially the air inside the tank is higher pressure than the air inside the box and so if a valve is opened, the air flows from the area of higher pressure, to lower pressure and work is performed. But eventually the air pressure in the box would be equal to the air pressure in the tank, and no more work could be performed.
You could say that the initial configuration of high pressure tank, low pressure box, is unstable or chaotic. The air wants to be equally pressurized and if allowed to, it would flow out until the pressure was equal everywhere. You could say it wants to move from a chaotic situation to an ordered one. This is what entropy is, the tendency for a closed system (like a sealed box, or the universe) to move from a high energy, chaotic state, to a low energy, ordered state.
This works for any closed system. Imagine a sealed globe with tiny animals living inside of it perfectly in balance with little plants and algae living inside of it as well. The animals live, eat, and breath exactly enough to balance out the air and food the plants provide, and the plants get just what they need from the animals in order to grow as well. This is a sealed system but it’s not a perpetual motion machine.
The plants and animals only stay balanced and alive if the plants get sunlight because they convert that sunlight to chemical energy, which the plants and the animals use for food. It only works because energy is continuously pumped into the system. If you truly sealed it, including sealed it away from sunlight, then it would slowly begin to die. Even if the plants could extract energy by eating the animals, like carnivorous plants, the amount of energy they receive from doing so would be less than the energy the animal used to grow up and become a meal. Every time one organism ate another, breathed in or out, etc, there would be less total energy in the system the same way that every time that air tank let air out into the box, there was less difference in pressure.
Eventually there would be no more energy in the sealed system, all the chemical energy would be used up, all the molecules in their lowest energy state. All the temperature would be the same. All the air pressure would be the same. All the potential kinetic energy of things moving or falling, would drop to 0 and the system would be perfectly ordered, and very dead.
There is a finite amount of things in the universe, a finite amount of potential energy in the universe, and that energy is constantly being converted into work, or into other forms of energy, and every time it does, there is less remaining energy in the universe. The number never goes up, only down. It will take hundreds of billions of years for the universe to use all of that energy up, but eventually everywhere in the universe will be the same temperature, the same pressure. There will be no more potential energy to be used. The universe will be very dark, very still, and very dead. This is entropy; the heat death of the universe.
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