Which principle best describes the first law of thermodynamics for a closed system?

The first law of thermodynamics for a closed system is fundamentally about the conservation of energy, which can be articulated through the net energy that crosses the system boundary. This principle states that the energy added to the system, whether through heat transfer or work done on the system, results in a change in the internal energy of that system.

When considering a closed system, the mass remains constant, and no matter crosses the boundaries; rather, it is energy that crosses. Therefore, the law can be mathematically expressed as the change in internal energy being equal to the net energy entering the system. Any energy input into the system must manifest as a corresponding change in energy state—this encapsulation of energy balance is what characterizes the first law.

The other choices, while touching upon relevant thermodynamic concepts, do not directly express this foundational principle of energy conservation as effectively as the correct option does. For instance, statements regarding mass conservation in a control volume or the tendency toward disorder (entropy) do not capture the key aspect of energy change in relation to energy transfer as emphasized in the first law.