Editing Thermodynamics (section)

===First law===
[[File:Kurzzeitfotografie sektkorken 06-19-s02 2017-09-03 01 hinnerk-ruemenapf exif.jpg|upright|thumb|Opening a bottle of [[sparkling wine]] ([[high-speed photography]]). The sudden drop of pressure causes a huge drop of temperature. The moisture in the air freezes, creating a smoke of tiny ice crystals.<ref>{{cite web |url=https://www.chemistryviews.org/details/ezine/889289/Sparkling_Wine_Champagne__Co__Part_2/ |publisher=Chemistry Europe (chemistryviews.org)  |work=Sparkling Wine, Champagne & Co |title=Sparkling Wine, Champagne & Co - Part 2 |date=17 December 2010 |access-date=17 April 2023}}</ref><ref>Klaus Roth: ''Sekt, Champagner & Co. So prickelnd kann Chemie sein'' in ''Chemie unserer Zeit'' 8. Dezember 2009: Vol. 43, Issue 6, S. 418-432 [[doi:10.1002/ciuz.200900520]]</ref><ref>Klaus Roth: ''Chemische Köstlichkeiten'', Wiley-VCH Verlag GmbH & Co. KGaA, 2010, ISBN 978-3527327522, S. 47</ref>]]
The [[first law of thermodynamics]] states: ''In a process without transfer of matter, the change in [[internal energy]],'' <math>\Delta U</math>'', of a [[thermodynamic system]] is equal to the energy gained as heat,'' <math>Q</math>'', less the thermodynamic work,'' <math>W</math>'', done by the system on its surroundings.''<ref>Bailyn, M. (1994). ''A Survey of Thermodynamics'', American Institute of Physics, AIP Press, Woodbury NY, {{ISBN|0883187973}}, p. 79.</ref><ref group=nb>The sign convention (Q is heat supplied ''to'' the system as, W is work done ''by'' the system) is that of [[Rudolf Clausius]]. The opposite sign convention is customary in chemical thermodynamics.</ref>

:<math>\Delta U = Q - W</math>.

where <math>\Delta U</math> denotes the change in the internal energy of a [[Thermodynamic system#Closed system|closed system]] (for which heat or work through the system boundary are possible, but matter transfer is not possible), <math>Q</math> denotes the quantity of energy supplied ''to'' the system as heat, and <math>W</math> denotes the amount of thermodynamic work done ''by'' the system ''on'' its surroundings. An equivalent statement is that [[perpetual motion machines]] of the first kind are impossible; work <math>W</math> done by a system on its surrounding requires that the system's internal energy <math>U</math> decrease or be consumed, so that the amount of internal energy lost by that work must be resupplied as heat <math>Q</math> by an external energy source or as work by an external machine acting on the system (so that <math>U</math> is recovered) to make the system work continuously.

For processes that include transfer of matter, a further statement is needed: ''With due account of the respective fiducial reference states of the systems, when two systems, which may be of different chemical compositions, initially separated only by an impermeable wall, and otherwise isolated, are combined into a new system by the thermodynamic operation of removal of the wall, then''

:<math>U_0 = U_1 + U_2</math>,

''where'' {{math|''U''<sub>0</sub>}} ''denotes the internal energy of the combined system, and'' {{math|''U''<sub>1</sub>}} ''and'' {{math|''U''<sub>2</sub>}} ''denote the internal energies of the respective separated systems.''

Adapted for thermodynamics, this law is an expression of the principle of [[conservation of energy]], which states that energy can be transformed (changed from one form to another), but cannot be created or destroyed.<ref>Callen, H.B. (1960/1985).''Thermodynamics and an Introduction to Thermostatistics'', second edition, John Wiley & Sons, Hoboken NY, {{ISBN|9780471862567}}, pp. 11–13.</ref>

Internal energy is a principal property of the [[thermodynamic state]], while heat and work are modes of energy transfer by which a process may change this state. A change of internal energy of a system may be achieved by any combination of heat added or removed and work performed on or by the system. As a [[State function|function of state]], the internal energy does not depend on the manner, or on the path through intermediate steps, by which the system arrived at its state.