efficiency

Meaning: Efficiency is the fraction of the energy or work put into a device that emerges as useful output. The rest is usually dissipated as heat through friction, air resistance, or electrical resistance; energy is conserved. Real machines fall below 100%, and heat engines are limited further by the second law.

Equation: e = Wout/Win, where W is work or energy in joules (J) and e is a unitless ratio, often a percentage. For a heat engine, e = 1 − QC/QH, where QH is heat absorbed from the hot reservoir and QC is heat released to the cold reservoir, in J. The Carnot maximum is e = 1 − TC/TH, with T in kelvin (K).

On the MCAT: Items give a pulley, incline, or motor and ask for input work given friction. Heat engine items ask how e changes when TH rises or TC falls (it increases), or for W from QH and QC; on a P–V loop, net work is the enclosed area.

Common confusion: Using Celsius in the Carnot formula is a frequent error; kelvin is required. Efficiency is not power: a high-power machine can be inefficient. A simple machine can cut the force needed but not the work.