What does the first law of thermodynamics state?

• A. Energy cannot be created or destroyed, only transformed
• B. Energy can be created from nothing
• C. Energy is not conserved in most processes
• D. Energy changes randomly

Answer: (A)

Energy is conserved in every process: you can transform energy from one form to another, but the total amount stays the same. In practical terms, heat added to a system and the work done by or on the system account for changes in the system’s internal energy, and the overall energy balance is captured by ΔU = Q − W. If you put heat into the system, its internal energy rises; if it does work on the surroundings, its internal energy falls. The other ideas imply energy can appear from nothing, isn’t always conserved, or changes in a random way, which conflicts with the idea that energy is preserved overall.