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Engineering LibreTexts

Gas Constant

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  • Introduction

    Gas_Constant.pngThe gas constant (also called the universal gas constant, molar gas constant or ideal gas constant) is a physical constant, denoted as R, which appears in many fundamental equations in physics, engineering and other sciences, such as the ideal gas law and other equations of state. Currently, the most accurate value of R is 8.3144621.[1]

    It is equivalent to the Boltzmann constant (kB) times Avogadro's constant (NA):


    And it appears in the ideal gas law as follows:


    P is the gas absolute pressure

    V is the volume of the gas

    n is the number of moles of gas

    R is the gas constant

    T is the gas absolute temperature


    The "specific gas constant"

    Some fields of engineering, notably aeronautics and astronautics, utilize what is called the "specific gas constant" of a gas or a mixture of gases. It is denoted as Rs and is defined as the universal gas constant divided by the molecular weight (M) of the specific gas or gas mixture:[2] [3] The adjacent table lists some example values of Rs for a number of gases.[4]


    For an ideal gas, the specific gas constant is related to the specific heats the gas:


    where cp is the specific heat for a constant pressure and cv is the specific heat for a constant volume.

    Unfortunately, many authors in the technical literature often use R as the specific gas constant without denoting it as such or stating that it is the specific gas constant. This can and does lead to confusion for many readers.


    1. ^ Molar gas constant Obtained on July 25, 2011 from the website of the National Institute of Standards and Technology (NIST), a United States federal agency.
    2. ^ John D. Anderson (2006), Hypersonic and High-Temperature Gas Dynamics, 2nd Edition, American Institute of Aeronatics and Astronautics (AIAA), ISBN 1-56347-780-7
    3. ^ Michael J. Moran, Howard N. Shapiro, Daise D. Boettner and Margaret B. Bailey (2010), ''Fundamentals of Engineering Thermodynamics'', 7th Edition, John Wiley and Sons, ISBN 0-470-495590-1
    4. ^ Note that the units of the specific gas constant differ from those of the universal gas constant.


    • Milton Beychok