Final answer:
Energy conversions are not completely efficient due to the loss of some energy as heat, as explained by the second law of thermodynamics. This heat loss occurs in various processes, such as lighting a bulb or during cellular metabolism.
Step-by-step explanation:
Energy conversions are never completely efficient because some energy is always lost in the form of heat to the environment. This concept is explained by the second law of thermodynamics, which states that all energy transfers and transformations are never completely efficient. For example, in a light bulb, electrical energy is converted into light energy, but some of it is lost as heat energy due to the resistance in the filament. Likewise, during cellular metabolic reactions, some energy is not converted into useful work but instead is released as heat energy, which is often beneficial for warm-blooded organisms to maintain body temperature.