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LEC 2-thermodynamics

# LEC 2-thermodynamics - Thermodynamics(Review LEC 2 Equation...

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Thermodynamics (Review) LEC 2 SHU Fall 2012 1 Equation of State Equation of state describes the relation between various properties of a substance. In this class, we only consider simple compressible systems , which means electrical, magnetic, surface tension, gravitational effects are neglected. In this system, only two independent properties are required to uniquely describe the system’s state. Ideal gas: model of gas at 0 pressure and infinite temperature (zero density). Ideal gas is a reasonable model for gas with temperature great than , or pressure less than . : critical temperature: depended on the components, all gas has a temperature, if the gas temperature is higher than that, no matter how much pressure applied on it, it won’t be liquefied. : critical pressure: the pressure needed to liquefy the gas at critical temperature. For air, , and The universal equation of state for ideal gas: (you probably learn this from you high school chemistry) where is pressure (in ), is volume (in ), is amount of gas in ( ), is universal gas constant, and is temperature (in ). However, we don’t like to use in engineering; instead, we like to use mass ( ). In that case, the equation can be reformatted into: where is the molecular mass of the gas (in kg/mol), m is the mass (in kg) and is the gas constant, which is different for different gas. Example: molecular weight for air is 29g/mol (0.029kg/mol), R for air is: Divide volume ( ) from each side of the state equation: This is the most often used gas state equation in aerodynamics.

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