Which statement best reflects the First Law of Thermodynamics?

Which statement best reflects the First Law of Thermodynamics?

• A. Energy is created when heat is added.
• B. Energy is transferred only as heat.
• C. Energy increases in all closed systems.
• D. Energy is neither created nor destroyed.

Answer: (D)

The main idea being tested is energy conservation in a closed system: energy cannot be created or destroyed, it can only be transformed or transferred between bodies. The First Law formalizes this by saying the change in a system’s internal energy equals the heat added to the system plus the work done on the system (or, equivalently, the heat transfer and work accounting for energy entering or leaving). This means energy can show up as higher temperature, as phase changes, or as work done on or by the surroundings, but the total energy stays constant when you consider the system and its interactions. That’s why the statement stating energy is neither created nor destroyed is the best fit. It captures the essence that energy is conserved, regardless of how it changes form or moves around. The other ideas misstate how energy behaves. Energy isn’t created simply by adding heat—adding heat changes the form or amount of energy within the system, but energy isn’t new from nothing. Energy isn’t transferred only as heat—work is another way energy moves between systems. And energy doesn’t necessarily increase in all closed systems; the system’s internal energy can rise or fall depending on heat and work interactions, while the total energy accounting for surroundings remains constant.

The main idea being tested is energy conservation in a closed system: energy cannot be created or destroyed, it can only be transformed or transferred between bodies. The First Law formalizes this by saying the change in a system’s internal energy equals the heat added to the system plus the work done on the system (or, equivalently, the heat transfer and work accounting for energy entering or leaving). This means energy can show up as higher temperature, as phase changes, or as work done on or by the surroundings, but the total energy stays constant when you consider the system and its interactions.

That’s why the statement stating energy is neither created nor destroyed is the best fit. It captures the essence that energy is conserved, regardless of how it changes form or moves around.

The other ideas misstate how energy behaves. Energy isn’t created simply by adding heat—adding heat changes the form or amount of energy within the system, but energy isn’t new from nothing. Energy isn’t transferred only as heat—work is another way energy moves between systems. And energy doesn’t necessarily increase in all closed systems; the system’s internal energy can rise or fall depending on heat and work interactions, while the total energy accounting for surroundings remains constant.