The conservation of energy is a fundamental principle in physics that states that energy cannot be created or destroyed, only transformed from one form to another. This means that the total amount of energy in an isolated system remains constant, even as it changes form. For example, energy can be transformed from kinetic energy to potential energy as an object moves higher, or from chemical energy to thermal energy in a combustion reaction. The conservation of energy helps to explain why energy balance is important in physical and biological systems.
Entropy is a measure of the amount of thermal energy in a system that is unavailable for doing useful work. It is a measure of disorder or randomness in a system. According to the second law of thermodynamics, entropy always increases over time in a closed system, meaning that the energy in the system becomes less useful for doing work and is spread out evenly. This increase in entropy represents a loss of energy efficiency and is a fundamental principle that affects many natural processes, from the workings of engines to the evolution of the universe.