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.