What does the first law of thermodynamics state?

• A. Energy cannot be created or destroyed; it can only be transferred or transformed.
• B. Energy increases in isolated systems.
• C. Entropy of a closed system always decreases.
• D. Matter and energy are interchangeable with no losses.

Answer: (A)

The main idea is that energy is conserved. In any process, the total energy of a closed system stays the same, though energy can shift from one form to another or be transferred as heat or work. A handy way to think about it is that you can transform electrical energy into light and thermal energy, or heat can flow and do work, but the sum of all forms of energy in the system doesn’t change. The common formula ΔU = Q − W captures this, where adding heat or allowing the system to do work changes its internal energy by the appropriate amount.

Why this fits best: energy cannot be created or destroyed, only moved or transformed, which is exactly what this law states. The idea that energy increases in isolated systems contradicts conservation—the total energy remains constant. Entropy decreasing in a closed system goes against the second law of thermodynamics, which says entropy tends to increase. The notion that matter and energy are interchangeable with no losses overstates the relationship and ignores real losses in processes; mass-energy equivalence exists, but energy is conserved and real losses occur in practical transformations.