What key concept does the second law of thermodynamics emphasize regarding energy transfer?

The second law of thermodynamics emphasizes that energy transfer can occur spontaneously, but it highlights that this process naturally progresses in a particular direction. This law introduces the concept of entropy, which is a measure of the disorder or randomness in a system. In a spontaneous process, the total entropy of an isolated system tends to increase over time, meaning energy will disperse or spread out.

For example, when heat flows from a hot object to a cold one, it illustrates spontaneous energy transfer resulting in an increase in entropy. In this context, spontaneous does not mean it occurs without energy input; rather, it follows a natural tendency toward equilibrium where energy is more evenly distributed.

The other choices do not align with the second law’s principles. Energy cannot be created, and the law does not support the idea that energy transfer is inherently reversible or that energy cannot be converted from one form to another; rather, it acknowledges that while energy can change forms, these transformations may not be efficient and can lead to increased entropy.