What is entropy in thermodynamics?

Entropy is fundamentally defined as a measure of disorder or randomness in a system. In thermodynamics, it quantifies the degree to which energy is distributed within a system and how much of that energy is unavailable to do work. High entropy indicates a high degree of disorder, where the energy in a system is spread out more evenly among the available microstates. Conversely, low entropy signifies a more ordered state, where energy is concentrated.

This understanding links closely to the Second Law of Thermodynamics, which states that the total entropy of an isolated system can never decrease over time; it can only stay the same or increase. Therefore, processes in nature tend to move towards states of higher entropy, causing systems to evolve towards disorder.

The other choices do not accurately capture the concept of entropy. Energy stored in a system is associated with enthalpy rather than entropy. A constant value for all closed systems does not reflect the dynamic nature of entropy, as it can change based on the state of the system. Additionally, temperature variation is related to thermal energy rather than being a direct measure of entropy itself. Thus, the first choice is the most fitting definition of entropy in the context of thermodynamics.