Isobar
Definition
... a process at constant pressure, denoted by a horizontal line on a pV diagram and a curve on a Ts diagram.
Where it occurs.
In the theoretical Brayton Cycle, heat is added and rejected at constant pressure. In some versions of the theoretical Diesel Cycle, heat is added at constant pressure. The pressure changes along transport pipes are often far smaller than those across compressors and expanders in the same cycle. By the same logic, constant pressure heating/ cooling might be reasonably assumed in heat exchangers.
Special features
If a the pressure of a system is fixed, then any reversible addition of heat is equal to the change in the enthalpy of the system.
How to draw it
Locate the POI and press the isobar button.
The theory
At (T,s) for a given POI, the pressure follows from, rearrangement and integration of the Gibbs Equation.
$$ p = 1.013 \, exp( s/R) (T/273.)^{(\gamma/(\gamma-1)} $$
From the Ideal Gas Law, specific volume follows
$$ v = V/m = RT/p $$
The calculations of ideal gas internal energy and enthalpy are explained in conjunction with
the point of interest.
Exercises
Click on the graph to get a POI and plot an isobar through it. Click along the isochor to set new POIs and note how temperature and entropy vary.
Links
... to follow.