Which statement correctly describes the First Law of Thermodynamics?

Which statement correctly describes the First Law of Thermodynamics?

• A. Energy cannot be created or destroyed; it can only be transformed.
• B. Entropy must increase.
• C. At absolute zero, entropy is maximum.
• D. Heat is the only form of energy.

Answer: (A)

Energy cannot be created or destroyed; it can be transformed from one form to another. This is the First Law of Thermodynamics, the principle of conservation of energy. In any process, the total energy of a system remains the same when you account for all forms of energy and the energy that crosses the system boundary as heat or does work. For example, electrical energy powering a device may become light, sound, and thermal energy, but the total amount of energy is preserved. A convenient way to remember it is that the change in a system’s internal energy equals the heat added to the system plus the work done on the system (ΔU = q + w, with the sign convention depending on the chosen frame). The other statements involve different thermodynamic ideas: entropy increasing reflects the Second Law, not energy conservation; entropy at absolute zero is not maximum (for a perfect crystal it would be zero); and heat is not the only form of energy—energy exists in many forms, and heat is a mode of energy transfer rather than a standalone form of energy itself.

Energy cannot be created or destroyed; it can be transformed from one form to another. This is the First Law of Thermodynamics, the principle of conservation of energy. In any process, the total energy of a system remains the same when you account for all forms of energy and the energy that crosses the system boundary as heat or does work. For example, electrical energy powering a device may become light, sound, and thermal energy, but the total amount of energy is preserved. A convenient way to remember it is that the change in a system’s internal energy equals the heat added to the system plus the work done on the system (ΔU = q + w, with the sign convention depending on the chosen frame). The other statements involve different thermodynamic ideas: entropy increasing reflects the Second Law, not energy conservation; entropy at absolute zero is not maximum (for a perfect crystal it would be zero); and heat is not the only form of energy—energy exists in many forms, and heat is a mode of energy transfer rather than a standalone form of energy itself.