Chemistry 30 Diploma Practice Exam

What is the efficiency of a machine defined as?

Useful work divided by total energy expended Total energy expended divided by useful work Energy dissipated as heat Output energy only

Correct Answer:

Useful work divided by total energy expended

Explanation:

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.

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