What is entropy in thermodynamics?

Entropy is a fundamental concept in thermodynamics that quantifies the amount of energy in a physical system that is not available to do work. It can also be understood as a measure of disorder or randomness in a system. When energy transformations occur, such as in heat engines or chemical reactions, some energy is inevitably lost to entropy, which increases as the system moves towards equilibrium.

The correct answer highlights that entropy represents wasted energy or energy that disperses and cannot be harnessed for useful work. This is critical in understanding the second law of thermodynamics, which states that the total entropy of an isolated system can never decrease over time, meaning that processes evolve towards higher entropy and lower usable energy.

The other options presented do not accurately define entropy. While temperature is related to thermal energy and affects entropy, it is not a measure of entropy itself. Measuring the efficiency of a thermodynamic process pertains more broadly to how much work can be extracted from energy inputs rather than measuring disorder. The total energy of a closed system is a different concept as well—entropy specifically deals with the unusable portion of this energy rather than merely its quantity.