In thermodynamics

Thermodynamics

Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...

, motive power is an agency, as water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 or steam

Steam

Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...

, used to impart motion

Motion (physics)

In physics, motion is a change in position of an object with respect to time. Change in action is the result of an unbalanced force. Motion is typically described in terms of velocity, acceleration, displacement and time . An object's velocity cannot change unless it is acted upon by a force, as...

. Generally, motive power is defined as a natural agent, as water, steam, wind, electricity

Electricity

Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

, etc., used to impart motion to machinery; a motor

Engine

An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...

; a mover. The term may also define something, as a locomotive

Locomotive

A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...

 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

Power (physics)

In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...

", an effect producing motion, depending on the context of the discussion.

History

In 1679, physicist Denis Papin

Denis Papin

Denis Papin was a French physicist, mathematician and inventor, best known for his pioneering invention of the steam digester, the forerunner of the steam engine and of the pressure cooker.-Life in France:...

 conceived the idea of using steam to power a piston and cylinder engine

Steam engine

A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...

, 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

Thomas Savery

Thomas Savery was an English inventor, born at Shilstone, a manor house near Modbury, Devon, England.-Career:Savery became a military engineer, rising to the rank of Captain by 1702, and spent his free time performing experiments in mechanics...

 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

Nicolas Léonard Sadi Carnot

Nicolas Léonard Sadi Carnot was a French military engineer who, in his 1824 Reflections on the Motive Power of Fire, gave the first successful theoretical account of heat engines, now known as the Carnot cycle, thereby laying the foundations of the second law of thermodynamics...

.

As an example, the Newcomen engine

Newcomen steam engine

The atmospheric engine invented by Thomas Newcomen in 1712, today referred to as a Newcomen steam engine , was the first practical device to harness the power of steam to produce mechanical work. Newcomen engines were used throughout Britain and Europe, principally to pump water out of mines,...

 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

Mechanical work

In physics, work is a scalar quantity that can be described as the product of a force times the distance through which it acts, and it is called the work of the force. Only the component of a force in the direction of the movement of its point of application does work...

". If we were to include a unit of time in Carnot's definition, we would then have the modern-day definition for power:


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.