THOMAS NEWCOMEN (1663-1729)

1712 – England

‘Uses the property of condensing steam to create a partial vacuüm in a cylinder and therefore pull a piston. The system was highly inefficient but was used to pump water from mines’

Today, the credit for the steam engine is usually given to James Watt, while the name Thomas Newcomen remains shrouded in obscurity.

The design of his low-pressure steam engine involved heating water underneath a large piston that was encased in a cylinder.

Steam that was released as a result of the heating forced the piston upwards. A jet of water was then released from a tank above the piston. The sudden cooling of the steam made it condense, creating a partial vacuüm which atmospheric pressure then pushed down on, forcing the piston downwards again. The piston was attached to a two-headed lever, the other side of which was attached to a pump in the mineshaft. As it moved up and down, the lever moved likewise and a pumping motion was created in the shaft, which could be used to eject floodwater.

The first engine could remove about 120 gallons per minute, completing about twelve strokes in that time, and had the equivalent of about 5.5 horsepower. Even though the engine was still not particularly powerful, was hugely inefficient to run, and burnt huge amounts of coal, it would work reliably 24-hours a day.

The steam engine originally developed by Newcomen for work in the mines was quickly developed by engineers like JAMES WATT and RICHARD TREVITHICK (1771-1833) into the steam locomotive.

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NICOLAS SADI CARNOT (1796-1832)

1824 – France

‘The Carnot cycle is the most efficient cycle for operating a reversible heat engine’

It illustrates the principle that the efficiency of a heat engine depends on the temperature range through which it works.

The cycle has a four-stage reversible sequence:

adiabatic compression and isothermal expansion at high temperature; adiabatic expansion and isothermal compression at low temperature.

( ADIABATIC: – no heat flows into or out of a system; ISOTHERMAL: – at a constant temperature )

Carnot suggested that the puissance motrice (motive power, by which he meant work or energy) of a heat engine was derived from the fall of heat from a higher to a lower temperature.

Carnot was the first to grasp the principles that later became known as the second law of thermodynamics.

By the time of Carnot’s death it had become clear there was no such thing as a calorific fluid ; heat is a form of energy, one of many, and the sum of all forms of energy in an isolated system is conserved. This has come to be known as the first law of thermodynamics.

In the case of a steam engine, the heat taken in at the boiler is not equal to the heat removed at the condenser. The work done by the ideal engine is the difference between the two. The first modern experiment proving the first law of thermodynamics was performed by a student of John Dalton’s, JAMES JOULE.

 

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JAMES PRESCOTT JOULE (1818- 89)

1843 – England

‘A given amount of work produces a specific amount of heat’

4.18 joules of work is equivalent to one calorie of heat.

In 1798 COUNT RUMFORD suggested that mechanical work could be converted into heat. This idea was pursued by Joule who conducted thousands of experiments to determine how much heat could be obtained from a given amount of work.

Even in the nineteenth century, scientists did not fully understand the properties of heat. The common belief held that it was some form of transient fluid – retained and released by matter – called CALORIC. Gradually, the idea that it was another form of energy, expressed as the movement of molecules gained ground.
Heat is now regarded as a mode of transfer of energy – the transfer of energy by virtue of a temperature difference. Heat is the name of a process, not that of an entity.

Joule began his experiments by examining the relationship between electric current and resistance in the wire through which it passed, in terms of the amount of heat given off. This led to the formulation of Joule’s ideas in the 1840s, which mathematically determined the link.

Joule is remembered for his description of the conversion of electrical energy into heat; which states that the heat (Q) produced when an electric current (I) flows through a resistance (R) for a time (t) is given by Q=I2Rt

Its importance was that it undermined the concept of ‘caloric’ as it effectively determined that one form of energy was transforming itself into another – electrical energy to heat energy. Joule proved that heat could be produced from many different types of energy, including mechanical energy.

john collier portrait of james prescott joule (1200 x 1600)

JAMES JOULE

The apparatus pictured was used by James Joule to demonstrate equivalence of mechanical work and heat. He calculated the work done by the pull of gravity on the weight. That pull turned the paddle wheels, which mixed the water in the insulated container. The water was warmed by the mixing, showing that heat = work

Calorimeter used by Joule in his 1876 determination of the mechanical equivalent of heat.

Joule was the son of a brewer and all his experiments on the mechanical equivalent of heat depended upon his ability to measure extremely slight increases in temperature, using the sensitive thermometers available to him at the brewery. He formulated a value for the work required to produce a unit of heat. Performing an improved version of Count Rumford’s experiment, he used weights on a pulley to turn a paddle wheel immersed in water. The friction between the water and the paddle wheel caused the temperature of the water to rise slightly. The amount of work could be measured from the weights and the distance they fell, the heat produced could be measured by the rise in temperature.

Joule went on to study the role of heat and movement in gases and subsequently with WILLIAM THOMSON, who later became Lord Kelvin, described what became known as the ‘Joule-Thomson effect’ (1852-9). This demonstrated how most gases lose temperature on expansion due to work being done in pulling the molecules apart.

Thomson thought, as CARNOT had, that heat IN equals heat OUT during a steam engine’s cycle. Joule convinced him he was wrong.

The essential correctness of Carnot’s insight is that the work performed in a cycle divided by heat input depends only on the temperature of the source and that of the sink.

Synthesising Joule’s results with Carnot’s ideas, it became clear that a generic steam engine’s efficiency – work output divided by heat input – differed from one (100%) by an amount that could be expressed either as heat OUT at the sink divided by heat IN at the source, or alternatively as temperature of the sink divided by temperature of the source. Carnot’s insight that the efficiency of the engine depends on the temperature difference was correct. Temperature has to be measured using the right scale. The correct one had been hinted at by DALTON and GAY-LUSSAC’s experiments, in which true zero was -273degrees Celsius.

A perfect cyclical heat engine with a source at 100degrees Celsius and a sink at 7degrees has an efficiency of 1 – 280/373. The only way for the efficiency to equal 100% – for the machine to be a perfect transformer of heat into mechanical energy – is for the sink to be at absolute zero temperature.

Joule’s work helped in determining the first law of thermodynamics; the principle of the conservation of energy. This was a natural extension of his work on the ability of energy to transform from one type to another.

Joule contended that the natural world has a fixed amount of energy which is never added to nor destroyed, but which just changes form.

The SI unit of work and energy is named the joule (J)

link to James Joule - Manchester Museum of Science & Industry

Manchester Museum of Science & Industry

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