According to the First Law of Thermodynamics, what remains constant in an isolated system?

In an isolated system, the First Law of Thermodynamics states that the total energy of the system is conserved. This law is often summarized by the equation ΔU = Q - W, where ΔU represents the change in internal energy, Q is the heat added to the system, and W is the work done by the system. In an isolated system, there is no heat exchange with the surroundings (Q = 0) and no work done (W = 0), meaning that the internal energy (U) remains constant.

This constancy of internal energy indicates that in an isolated system, energy cannot be created or destroyed; it can only change forms. Thus, the correct answer highlights that internal energy is the key quantity that remains unchanged in such systems.

Other choices—pressure, volume, and temperature—can fluctuate in an isolated system depending on the physical changes occurring within it. For example, if a gas expands or contracts, pressure and volume can change, and temperature can also vary due to energy redistribution. However, the internal energy is the primary factor that remains constant when no external energy interactions happen.