What is the efficiency of a machine defined as?

• A. Useful work divided by total energy expended
• B. Total energy expended divided by useful work
• C. Energy dissipated as heat
• D. Output energy only

Answer: (A)

Efficiency is a measure of how much of the energy you put into a machine actually becomes useful work. It is defined as the ratio of useful work (useful energy output) to the total energy expended (input energy). Since it’s a ratio of output to input, the result is a dimensionless number, often expressed as a percentage. In real systems, not all input energy becomes useful work, because some is lost as heat, friction, or other losses, so efficiency is typically less than 100%. For example, putting in 100 joules and getting 60 joules of useful work gives an efficiency of 60%. The other formulations either invert the ratio (which could produce values >1), refer only to losses, or refer only to output without considering the input, none of which define efficiency.