Which statement about energy transformations and the second law is correct?

• A. In energy transformations, energy is completely conserved with no losses.
• B. Entropy decreases during all processes.
• C. Most energy is converted into work.
• D. A portion of input energy is converted to work; the remainder is degraded as heat.

Answer: (D)

Energy transformations follow the second law: you can’t convert all input energy into useful work. Some energy always becomes heat that is dispersed into the surroundings, increasing the overall disorder. The total energy is conserved, but the useful portion is never equal to the input because of these inevitable losses as heat. So only a portion of the input energy is converted to work; the remainder is degraded as heat. The other statements ignore these real losses or contradict how entropy behaves in real processes.