Point of Interest (POI)
Definition
The POI is found at an (x,y) co-ordinate.
Where it occurs
It is defined by (p,V) on the pressure-volume curve and (T,s) on the temperature-entropy curve.
Special features
On our graphs, a small red cirle to indicates its position.
How to draw it
Locate the mouse at a point on the graph and left-click. The table will then report the corresponding mass, volume, pressure, temperature, specific internal energy,
specific enthalpy and specific entropy.
The theory
According to the state postulate the thermodynamic state of a simple system can be specified by any two independent properties plus mass. Specifying pressure, volume and mass allows us to compute all other thermodynamic properties.
The following applies to air when it is treated as an ideal gas and subject to the Ideal Gas Law . The thermodynamic temperature is
$$T=\frac{p V}{m R} $$
where \(R=0.287 kJ/kg\) is the specific gas constant. The specific internal energy and specific enthalpy are:
$$ u = c_v (T-273 K) $$
$$ h = c_p (T-273 K) $$
where 273K is a datum temperature, \(c_v =0.718 kJ/ kg K\) is the specific heat capacity at constant volume, and \(c_p = c_v + R = 1.005 kJ/ kg K\) is the
specific heat capacity at constant temperature.
The software calculates specific entropy using a datum temperature of 273 K and a datum pressure of 1.013 bar
$$ s = c_p ln (T/273) - R ln (p/1.013 bar) $$
The entropy equation follows rearrangement and integration of the Gibbs Equation.
(a.k.a. the Thermodynamic Identity).
Exercises
Move the POI to different parts of the graph. Note the changes to values in the table. In particular move the POI along horizonal lines and vertical lines.
Links
.. to follow.
On Thermodynamics relationships.