imported>Paul Wormer |
imported>Paul Wormer |
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| and its value is determined entirely by the temperature ''T'', pressure ''p'', and composition ''N''<sub>A</sub>, ''N''<sub>B</sub>, ... (molar amounts of substances A, B, ... ) of the system and not by its history. | | and its value is determined entirely by the temperature ''T'', pressure ''p'', and composition ''N''<sub>A</sub>, ''N''<sub>B</sub>, ... (molar amounts of substances A, B, ... ) of the system and not by its history. |
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| ==Internal energy==
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| Often one considers a system with thermal conducting walls, so that (small) amounts of [[heat]] Δ''Q'' can go through the wall in either direction: if Δ''Q'' > 0, heat enters the system and if Δ''Q'' < 0 heat leaves the system. Also one usually considers one manner
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| of performing (small) amount of work Δ''W'' by or on the system:
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| :<math>
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| \Delta W = p dV \,
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| </math>
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| If ''dV'' > 0 the volume of the system increases and work is performed ''by'' the system, if ''dV'' <0, work is performed ''on'' the system, hence the internal energy increase of the system by the work term obtains a minus sign:
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| :<math>
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| dU = \Delta Q -\Delta W \, \quad\quad\quad\ (1)
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| </math>
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| Notes:
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| *Other forms of work are possible, for instance [[magnetization]] Δ''W'' = '''H'''•d'''M''', inner product of magnetic field (a vector) '''H'''—not to be confused with enthalpy ''H''—with small change in molar magnetization ''d''<b>M</b> (also a vector).
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| * The symbol Δ indicates a small quantity (sometimes called an inexact differential), not necessarily a differential. The symbol ''d'' indicates a [[differential]] of a differentiable function
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| ::<math>
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| dF(x_0) \equiv \lim_{x\rightarrow 0} \frac{F(x_0+x)-F(x_0)}{x} x
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| </math>
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| * Remarkable about equation (1) is that the sum of two small quantities (inexact differentials)
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| gives a differential of a state function. This is why equation (1) is the first fundamental law (postulate) of thermodynamics
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| '''To be continued''' | | '''To be continued''' |
Revision as of 10:39, 21 June 2009
In thermodynamics, enthalpy is the sum of the internal energy U of a system and the product of pressure p and V of the system,
Enthalpy used to be called "heat content", which is why it is conventionally indicated by H.
The work term pV has dimension energy, in SI units joule, and H has the same dimension. Enthalpy is a state function, a property of the state of the thermodynamic system
and its value is determined entirely by the temperature T, pressure p, and composition NA, NB, ... (molar amounts of substances A, B, ... ) of the system and not by its history.
To be continued