Which of the following best describes energy efficiency in a machine?

• A. The ratio of total energy input to useful work
• B. The ratio of useful work performed to total energy expended
• C. The ratio of heat to temperature
• D. The ratio of energy to mass

Answer: (B)

Energy efficiency is about how much of the energy you supply to a machine is actually used to do useful work. It is the ratio of useful work output to total energy input, usually shown as a percentage. This means if you put in a certain amount of energy, only a portion goes into the desired work, and the rest is lost as heat, sound, etc. The best description of this idea is the ratio of useful work performed to total energy expended. For example, if a machine consumes 100 J and does 60 J of useful work, its efficiency is 60%. The other options don’t describe efficiency: one would be the reciprocal of efficiency, one mixes heat with temperature, and one is energy per mass, which is energy density rather than how effectively energy is converted to work.