Motive power

Motive power
A young "drawer" pulling a coal tub up a mine shaft, whose "effect" constitutes the "elevation of a weight to a certain height" (Sadi Carnot).

In thermodynamics, motive power is an agency, as water or steam, used to impart motion. Generally, motive power is defined as a natural agent, as water, steam, wind, electricity, etc., used to impart motion to machinery; a motor; a mover. The term may also define something, as a locomotive or a motor, which provides motive power to a system. In current use, motive power may be thought of as a synonym for either "work", i.e. force times distance, or "power", an effect producing motion, depending on the context of the discussion.



In 1679, physicist Denis Papin conceived the idea of using steam to power a piston and cylinder engine, by watching a steam release valve of a bone-digester rhythmically move up and down. In 1698, based on Papin’s designs, mechanical designer Thomas Savery built the first engine. The first scientific treatise on the energetics of engines was the 1824 paper: Reflections on the Motive Power of Fire written by French physicist Sadi Carnot.

As an example, the Newcomen engine of 1711 was able to replace a team of 500 horses that had “powered” a wheel to pump water out of a mine, i.e. to “move” buckets of water vertically out of a mine. Hence, we have the precursory model to the term motive power. Based on this model, in 1832, Carnot defined work as “weight lifted through a height”, being the very same definition used to this day.

1824 definition

Carnot states, in the footnotes to his famous 1824 publication, “We use here the expression motive power to express the useful effect that a motor is capable of producing. This effect can always be likened to the elevation of a weight to a certain height. It has, as we know, as a measure, the product of the weight multiplied by the height to which it is raised.”

In this manner, Carnot is actually referring to "motive power" in the same manner we currently define "work". If we were to include a unit of time in Carnot's definition, we would then have the modern-day definition for power:

P=\frac{W}{t} = \frac{(mg)h}{t}\

Thus Carnot's definition of motive power is not consistent with the modern physics definition of "power", nor the modern usage of the term.

1834 definition

In 1834, the French mining engineer Emile Clapeyron refers to Carnot’s motive power as “mechanical action”. As an example, during the expansion stroke of a piston engine he states that: “the gas will have developed a quantity of mechanical action during its expansion given by the integral of the product of the pressure times the differential of the volume.” Clapeyron then goes on to use graphical methods to show how this "mechanical action", i.e. work in modern terms, could be calculated.

See also

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Look at other dictionaries:

  • Motive power — Motive Mo tive, a. Causing motion; having power to move, or tending to move; as, a motive argument; motive power. Motive faculty. Bp. Wilkins. [1913 Webster] {Motive power} (Mach.), a natural agent, as water, steam, wind, electricity, etc., used… …   The Collaborative International Dictionary of English

  • motive power — n. 1. any power, as steam, electricity, etc., used to impart motion; any source of mechanical energy 2. an impelling force …   English World dictionary

  • motive power — ► NOUN ▪ the energy used to drive machinery …   English terms dictionary

  • Motive Power — For the train manufacturer known as Motive Power Industries, see Wabtec. Motive Power is a bi monthly railway related magazine that focuses on diesel locomotives in Australia. The content includes photographs of locomotives trains, news about… …   Wikipedia

  • motive power — noun The power that enables something to move; also used in a figurative sense. , Using electricity as motive power for railroads will do away with fuel trains, tenders, coal handling, water, and all that. Syn: locomotion, motivity …   Wiktionary

  • motive power — noun Date: 1775 1. an agency (as water or steam) used to impart motion especially to machinery 2. something (as a locomotive or a motor) that provides motive power to a system …   New Collegiate Dictionary

  • motive power — noun the power or ability to move • Syn: ↑locomotion, ↑motivity • Derivationally related forms: ↑motive (for: ↑motivity), ↑locomotive (for: ↑ …   Useful english dictionary

  • motive power — Synonyms and related words: activator, animator, appliance, arouser, bunt, butt, clout, compulsion, convenience, drive, driving force, energizer, engine, enginery, facility, fixture, human dynamo, impellent, impelling force, impetus, impulse,… …   Moby Thesaurus

  • motive power — 1. any power used to impart motion; any source of mechanical energy. 2. Railroads. locomotives or other vehicles that supply tractive power. [1615 25] * * * …   Universalium

  • motive power — /ˈmoʊtɪv paʊə/ (say mohtiv powuh) noun 1. any power used to impart motion. 2. a source of mechanical energy. 3. Railways locomotives, etc., which supply tractive power …   Australian-English dictionary

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