First Law of Thermodynamics

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First Law of Thermodynamics </dt>
A statement of the conservation of energy for thermodynamic systems (not necessarily in equilibrium). The fundamental form requires that the heat absorbed by the system serve either to raise the internal energy of the system or to do work on the environment:

'dq = du + dw '
where dq is the heat

added per unit mass; du is the increment of specific internal energy; and dw is the specific work done by the system on the environment. Although dq and dw are not perfect differentials, their difference, du, is always a perfect differential. Example of the application of this equation: in an adiabatic free expansion of gas into a vacuum, all three terms are zero. </dd>
For reversible processes the mechanical work is equal to the expansion against the pressure forces, i.e.,

dw = pdv
where p is the pressure and v is the specific

volume. For a perfect gas, the internal energy change is proportional to the temperature change,

du = cvdT
where cv is the specific heat

at constant volume and T is the Kelvin temperature. Therefore, the form of the first law usually used in meteorological applications is

dq = cvdT + pdv
Use of the equation of state yields

an alternative form,

dq = cpdT - dp
where cp is the specific

heat at constant pressure.
For open systems the variation of total rather than specific quantities is important:

dQ = dU + pdV - hdm
where Q is the total heat; U is the total

internal energy; V is the volume; m is the mass of the system; and h is the specific enthalpy.
If a system contains the possibility of nonmechanical work, such as work done against an electric field, this work must be included in the first law.
See second law of thermodynamics, third law of thermodynamics, energy equations. </dd>


This article is based on NASA's Dictionary of Technical Terms for Aerospace Use