Which statement best expresses the First Law of Thermodynamics?

• A. Energy can be created and destroyed.
• B. Heat energy can neither be created nor destroyed, it can only be changed in its form.
• C. Heat energy flows from cold to hot.
• D. The total energy of a system remains constant.

Answer: (B)

Energy cannot be created or destroyed; it can only be transformed from one form to another and transferred as heat or work. The statement expresses that idea by saying heat energy is not produced or annihilated on its own, but can change its form as energy moves through a system. In real systems, energy can flow in as heat, flow out as work, or change into internal energy, kinetic energy, or potential energy, yet the total amount of energy remains the same when you account for all forms and the surroundings. For example, burning fuel releases chemical energy as heat and light, some of which can drive motion; the overall energy balance is preserved even though the form changes. This is the essence of the First Law: energy conservation with transformation.