An isobaric process is a thermodynamic process in which the pressure stays constant: The term derives from the Greek isos, "equal," and barus, "heavy." The heat transferred to the system does work but also changes the internal energy of the system:
According to the first law of thermodynamics, where W is work done by the system, U is internal energy, and Q is heat. Pressure-volume work (by the system) is defined as: (Δ means change over the whole process, it doesn't mean differential)
but since pressure is constant, this means that
Applying the ideal gas law, this becomes
then substituting the last two equations into the first equation produces:
and if the gas involved in the isobaric process is monatomic then and .
An isobaric process is shown on a P-V diagram as a straight horizontal line, connecting the initial and final thermostatic states. If the process moves towards the right, then it is an expansion. If the process moves towards the left, then it is a compression.
An isochoric process is described by the equation . It would be convenient to have a similar equation for isobaric processes. Substituting the second equation into the first yields
The quantity U + p V is a state function so that it can be given a name. It is called enthalpy, and is denoted as H. Therefore an isobaric process can be more succinctly described as
Variable density viewpoint
A given quantity (mass M) of gas in a changing volume produces a change in density ρ. In this context the ideal gas law is written
- R(T ρ) = M P
where T is thermodynamic temperature above absolute zero. When R and M are taken as constant, then pressure P can stay constant as the density-tempertature quadrant (ρ,T ) undergoes a squeeze mapping. It is this context that explains Peter Olver's use of the term isobaric group when referring to the group of squeeze mappings on page 217 of his book Classical Invariant Theory (1999).
- Adiabatic process
- Cyclic process
- Isochoric process
- Isothermal process
- Polytropic process
- Isoenthalpic process
ca:Isolínia cs:Izobarický děj de:Isobare Zustandsänderung et:Samajoon fa:فرآیند همفشار hr:Izolinije it:Trasformazione isobara lt:Izobarinis procesas hu:Szintvonal nl:Isobaar proces no:Isobar prosess nn:Isobar prosess sl:Izobarna sprememba sr:Изохипсе fi:Isoviiva uk:Ізобаричний процес