What must happen as the thermal energy of a system changes?

The correct answer highlights a fundamental principle of thermodynamics: when the thermal energy of a system changes, it is often accompanied by an exchange of energy in the form of work or heat. This principle is rooted in the first law of thermodynamics, which states that energy cannot be created or destroyed, only transformed from one form to another. Therefore, if thermal energy increases or decreases, there must be a corresponding transfer of work done on or by the system or a heat exchange with the surroundings.

In this context, if the thermal energy of a system increases, it may mean that the system has absorbed energy as heat or that work has been done on it. Conversely, if the thermal energy decreases, the system could be doing work on its surroundings or releasing heat. This dynamism emphasizes the interconnectedness of thermal energy, work, and heat flow within physical systems.

The other options do not accurately reflect the relationship between thermal energy changes and energy dynamics in a system. For instance, stating that thermal energy must remain constant contradicts the nature of energy transfer and transformation. Similarly, claiming that heat transfer can only occur in one direction oversimplifies the complexities of thermal dynamics, as heat can flow both ways depending on temperature differences. Lastly, the assertion that energy