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Thermodynamic entropy is a measure of how organized or disorganized energy is present in a system of atoms or molecules. It is measured in joules of energy per unit kelvin. Entropy is an important part of the third law of thermodynamics.
Imagine that a group of molecules has ten units of energy. If the energy in those molecules is perfectly organized, then the molecules can do ten units of work. However, if the energy became less organized (that is, the entropy increased), the molecules might only be able to do six units of work, even though they still have ten units of energy in them.
When total entropy is reached, there is no more energy to spend. A good example of this is a cup of tea. Now suppose your cup of tea is hot. The tea has a lot of energy compared to the room the tea is in. Over time the heat in the tea will spread into the room. The tea will become colder. This is because the energy (heat) in the tea escapes into the surrounding area. Once the tea has gone cold, there is no more heat that can be spread. The tea has thus reached total entropy.
There are two types of "rooms" we recognize when we are talking about entropy: An open system and a closed system. The former means that energy (such as heat) can freely flow into and out of the room. A closed system means that the room is closed off from the outside; no energy can go in or out.
In the case of the tea, the room was a closed system; it had no means for energy to enter. But we can also make it an open system by placing a stove in the room. If we turn on the stove, we can use the heat from the stove to reheat the cup of tea. New usable energy has been brought into the environment. The entropy has thus decreased. The heat that went from the stove into the tea can then again escape into the room again until total entropy has been reached. This is what the second law of thermodynamics describes.
A real life example of an open system is the Earth. The surface gets a lot of energy from the Sun every day. This allows plants to grow and water to stay liquid. If we took away the Sun, plants would die and water would freeze because the surface of our planet would be too cold.