According to the First Law of Thermodynamics for a closed system, what is true?

The First Law of Thermodynamics states that energy cannot be created or destroyed in an isolated system, but it can change forms. In a closed system, the law specifically indicates that the net energy entering or leaving the system is equal to the change in internal energy of the system. Therefore, the correct answer highlights this principle: the total change in energy of the system is a result of the energy that crosses its boundary.

This concept encompasses processes like heating, work done on the system, and energy leaving the system as work or heat. It reflects the conservation of energy, where the work done on the system and heat added to it will increase its internal energy, whereas work done by the system and heat lost will decrease its internal energy.

The other choices do not accurately represent this principle. For instance, the idea that the net energy crossing the system boundary equals the change in mass does not align with the law; mass may change in an open system but does not directly relate to energy in this context. The assertion that total internal energy is constant misinterprets the law; internal energy can change based on the energy exchange. Lastly, stating that energy cannot be transformed contradicts the foundational concept of energy conversion that is central to the First Law.