Entropy is a measure of the disorder or randomness in a system. It is a concept in thermodynamics that describes the tendency of a system to move towards a state of equilibrium and maximum randomness.
Entropy is defined as the amount of heat energy in a system that is unavailable to do work. In other words, it is a measure of the amount of energy that is converted from a useful form (such as heat) to an unusable form (such as disorder).
Heat is used to define entropy because it is a more fundamental and measurable property in thermodynamics. Work, on the other hand, is a derived quantity and can vary depending on the path taken to achieve it. Heat is also a more intuitive concept and easier to measure in practical experiments.
Entropy is a universal concept that applies to all physical systems, including biological, chemical, and even economic systems. It explains why things tend to become more disordered over time, and why energy is constantly being lost or wasted in various processes.
According to the second law of thermodynamics, the total entropy of a closed system will always increase over time. This is known as the arrow of time and is a fundamental principle in understanding the behavior of physical systems. However, in certain cases, entropy can decrease in local regions, as long as the total entropy of the system continues to increase.