Editing History of physics (section)

== 19th century ==

===Mechanics===

In 1821, [[William Rowan Hamilton|William Hamilton]] began his analysis of Hamilton's characteristic function. In 1835, he stated [[Hamiltonian mechanics|Hamilton's canonical equations of motion]].

In 1813, [[Peter Ewart]] supported the idea of the conservation of energy in his paper ''On the measure of moving force''. In 1829, [[Gaspard-Gustave Coriolis|Gaspard Coriolis]] introduced the terms of [[Work (physics)|work]] (force times distance) and [[kinetic energy]] with the meanings they have today. In 1841, [[Julius Robert von Mayer]], an [[amateur]] scientist, wrote a paper on the conservation of energy, although his lack of academic training led to its rejection. In 1847, [[Hermann von Helmholtz]] formally stated the law of conservation of energy.

===Electromagnetism===

[[File:Faraday-Millikan-Gale-1913.jpg|thumb|upright|left|[[Michael Faraday]] (1791–1867)]]

In 1800, [[Alessandro Volta]] invented the electric battery (known as the [[voltaic pile]]) and thus improved the way electric currents could also be studied. A year later, [[Thomas Young (scientist)|Thomas Young]] demonstrated the wave nature of light&nbsp;– which received strong experimental support from the work of [[Augustin-Jean Fresnel]]&nbsp;– and the principle of interference. In 1820, [[Hans Christian Ørsted]] found that a current-carrying conductor gives rise to a magnetic force surrounding it, and within a week after Ørsted's discovery reached France, [[André-Marie Ampère]] discovered that two parallel electric currents will exert forces on each other. In 1821, [[Michael Faraday]] built an electricity-powered motor, while [[Georg Ohm]] stated his law of electrical resistance in 1826, expressing the relationship between voltage, current, and resistance in an electric circuit.

In 1831, Faraday (and independently [[Joseph Henry]]) discovered the reverse effect, the production of an electric potential or current through magnetism&nbsp;– known as [[Faraday's law of induction|electromagnetic induction]]; these two discoveries are the basis of the electric motor and the electric generator, respectively.

In 1873, [[James Clerk Maxwell]] published ''[[A Treatise on Electricity and Magnetism]]'', which described the transmission of energy in wave form through a "luminiferous ether", and suggested that light was such a wave.  This was confirmed in 1888 when Helmholtz student [[Heinrich Hertz]] generated and detected electromagnetic radiation in the laboratory.
<ref>{{Harvtxt|Buchwald|1985}}</ref>
<ref>{{Harvtxt|JungnickelMcCormmach|1986}}</ref>
<ref>{{Harvtxt|Hunt|1991}}</ref>
<ref>{{Harvtxt|Buchwald|1994}}</ref>

===Laws of thermodynamics===
{{further|History of thermodynamics}}

[[File:Baron Kelvin 1906.jpg|thumb|upright|{{nowrap|[[William Thomson, 1st Baron Kelvin|William Thomson (Lord Kelvin)]]<br>(1824–1907)}}]]

In the 19th century, the connection between heat and mechanical energy was established quantitatively by [[Julius Robert von Mayer]] and [[James Prescott Joule]], who measured the mechanical equivalent of heat in the 1840s. In 1849, Joule published results from his series of experiments (including the paddlewheel experiment) which show that heat is a form of energy, a fact that was accepted in the 1850s. The relation between heat and energy was important for the development of steam engines, and in 1824 the experimental and theoretical work of [[Nicolas Léonard Sadi Carnot|Sadi Carnot]] was published. Carnot captured some of the ideas of thermodynamics in his discussion of the efficiency of an idealized engine. Sadi Carnot's work provided a basis for the formulation of the [[first law of thermodynamics]]&nbsp;– a restatement of the [[law of conservation of energy]]&nbsp;– which was stated around 1850 by [[William Thomson, 1st Baron Kelvin|William Thomson]], later known as Lord Kelvin, and [[Rudolf Clausius]]. Lord Kelvin, who had extended the concept of absolute zero from gases to all substances in 1848, drew upon the engineering theory of [[Lazare Carnot]], Sadi Carnot, and [[Émile Clapeyron]] as well as the experimentation of James Prescott Joule on the interchangeability of mechanical, chemical, thermal, and electrical forms of work to formulate the first law.
[[File:Rudolf Clausius 01.jpg|thumb|upright|{{nowrap|[[Rudolf Clausius]] (1822–1888)}}]]
Kelvin and Clausius also stated the [[second law of thermodynamics]], which was originally formulated in terms of the fact that heat does not spontaneously flow from a colder body to a warmer one. Other formulations followed quickly (for example, the second law was expounded in Thomson and [[Peter Guthrie Tait]]'s influential work ''Treatise on Natural Philosophy'') and Kelvin in particular understood some of the law's general implications. The second Law – the idea that gases consist of molecules in motion – had been discussed in some detail by Daniel Bernoulli in 1738, but had fallen out of favor, and was revived by Clausius in 1857. In 1850, [[Hippolyte Fizeau]] and [[Léon Foucault]] measured the [[speed of light]] in water and found that it is slower than in air, in support of the wave model of light. In 1852, Joule and Thomson demonstrated that a rapidly expanding gas cools, later named the [[Joule–Thomson effect]] or Joule–Kelvin effect. [[Hermann von Helmholtz]] put forward the idea of the [[heat death of the universe]] in 1854, the same year that Clausius established the importance of ''dQ/T'' ([[Clausius's theorem]]) (though he did not yet name the quantity).

===Statistical mechanics (a fundamentally new approach to science)===
[[File:James Clerk Maxwell.png|thumb|upright|[[James Clerk Maxwell]] (1831–1879)]]
{{further|History of statistical mechanics}}
In 1860, [[James Clerk Maxwell]] worked out the mathematics of the distribution of velocities of the molecules of a gas, known today as the [[Maxwell-Boltzmann distribution]].

The atomic theory of matter had been proposed again in the early 19th century by the chemist [[John Dalton]] and became one of the hypotheses of the kinetic-molecular theory of gases developed by Clausius and James Clerk Maxwell to explain the laws of thermodynamics.
[[File:Boltzmann2.jpg|thumb|upright|[[Ludwig Boltzmann]] (1844–1906)]]

The kinetic theory in turn led to a revolutionary approach to science, the [[statistical mechanics]] of [[Ludwig Boltzmann]] (1844–1906) and [[Josiah Willard Gibbs]] (1839–1903), which studies the statistics of microstates of a system and uses statistics to determine the state of a physical system. Interrelating the statistical likelihood of certain states of organization of these particles with the energy of those states, Clausius reinterpreted the dissipation of energy to be the statistical tendency of molecular configurations to pass toward increasingly likely, increasingly disorganized states (coining the term "[[entropy]]" to describe the disorganization of a state). The statistical versus absolute interpretations of the second law of thermodynamics set up a dispute that would last for several decades (producing arguments such as "[[Maxwell's demon]]"), and that would not be held to be definitively resolved until the behavior of atoms was firmly established in the early 20th century.<ref>{{Harvtxt|Smith|Wise|1989}}</ref><ref>{{Harvtxt|Smith|1998}}</ref> In 1902, [[James Jeans]] found the length scale required for gravitational perturbations to grow in a static nearly homogeneous medium.

===Other developments===

In 1822, botanist [[Robert Brown (Scottish botanist from Montrose)|Robert Brown]] discovered [[Brownian motion]]: pollen grains in water undergoing movement resulting from their bombardment by the fast-moving atoms or molecules in the liquid.

In 1834, [[Carl Gustav Jakob Jacobi|Carl Jacobi]] discovered his uniformly rotating self-gravitating ellipsoids (the [[Jacobi ellipsoid]]).

In 1834, [[John Scott Russell|John Russell]] observed a nondecaying solitary water wave ([[soliton]]) in the [[Union Canal (Scotland)|Union Canal]] near [[Edinburgh]], Scotland, and used a water tank to study the dependence of solitary water wave velocities on wave amplitude and water depth. In 1835, [[Gaspard-Gustave Coriolis|Gaspard Coriolis]] examined theoretically the mechanical efficiency of waterwheels, and deduced the [[Coriolis effect]]. In 1842, [[Christian Doppler]] proposed the [[Doppler effect]].

In 1851, [[Léon Foucault]] showed the Earth's rotation with a huge [[pendulum]] ([[Foucault pendulum]]).

There were important advances in [[continuum mechanics]] in the first half of the century, namely formulation of [[elastic modulus|laws of elasticity]] for solids and discovery of [[Navier–Stokes equations]] for fluids.

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