Second law of thermodynamics

It’s about entropy. The entropy is a measure of a system’s energy that is unavailable for work, or of the degree of a system’s disorder. The change in entropy is related to the change in energy, pressure, temperature, and in volume.

The second law of thermodynamics states that the disorder in the universe always increases.

R. Clausius (1822–88) stated the law in a way: “heat cannot be transferred from one body to a second body at a higher temperature without producing some other effect”.

For example: Imagine that your room is the universe. After cleaning your room, it always has a tendency to become messy again. As the disorder in the universe (your room) increases, the energy is transformed into less usable forms. Because of that, the efficiency of any process will always be less than 100%.

Heat machines and refrigerators are constructed to operate base on the second law of thermodynamics.

Third law of thermodynamics
First law of thermodynamics
Zeroth law of thermodynamics