What is the first law of thermodynamics?

• A. Energy can be created and destroyed under certain conditions.
• B. Heat energy can neither be created nor destroyed, it can only be changed in its form.
• C. Heat energy flows from hot to cold spontaneously.
• D. Energy is conserved only in closed systems.

Answer: (B)

The main idea tested is that energy cannot be created or destroyed; it can only be transformed from one form to another, and the total energy in a system plus its surroundings remains constant. In thermodynamics, heat and work are the two ways energy crosses system boundaries, and the first law is often summarized by ΔU = Q − W, meaning the change in a system’s internal energy equals heat added minus work done by the system. Saying that heat energy can neither be created nor destroyed and can only change form aligns with that principle because it emphasizes energy conservation and the ability to convert heat into other forms of energy or into work. The other options invoke ideas from the second law or misstate the scope of conservation (energy is conserved overall, not only in closed systems), which do not describe the first law as accurately.