An isobaric process is a thermodynamic process in which the pressure stays constant: $\backslash Delta\; P\; =\; 0$. The heat transferred to the system does work but also changes the internal energy of the system:

$Q\; =\; \backslash Delta\; U\; +\; W\backslash ,$

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:

$W\; =\; P\; \backslash Delta\; V\backslash ,$

but since pressure is constant, this means that

$W\; =\; \backslash Delta\; (P\backslash ,V)$.

Applying the ideal gas law, this becomes

$W\; =\; n\backslash ,R\backslash ,\backslash Delta\; T$

assuming that the quantity of gas stays constant (e.g. no phase change during a chemical reaction). Since it is generally true that

$\backslash Delta\; U\; =\; n\backslash ,c\_V\backslash ,\backslash Delta\; T$

then substituting the last two equations into the first equation produces:

$Q\; =\; n\backslash ,c\_V\backslash ,\backslash Delta\; T\; +\; n\backslash ,R\backslash ,\backslash Delta\; T$

$=\; n\backslash ,(c\_V\; +\; R)\backslash ,\backslash Delta\; T$.

The quantity in parentheses is equivalent to the molar specific heat for constant pressure:

$c\_p\; =\; c\_V\; +\; R$

and if the gas involved in the isobaric process is monatomic then $c\_V\; =\; \backslash frac\{3\}\{2\}R$ and $c\_p\; =\; \backslash frac\{5\}\{2\}R$.

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.

Defining Enthalpy

---

An isochoric process is described by the equation $Q\; =\; \backslash Delta\; U$. It would be convenient to have a similar equation for isobaric processes. Substituting the second equation into the first yields

$Q\; =\; \backslash Delta\; U\; +\; \backslash Delta\; (P\backslash ,V)\; =\; \backslash Delta\; (U\; +\; P\backslash ,V)$

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 4000 isobaric process can be more succinctly described as

$Q\; =\; \backslash Delta\; H\; \backslash ,$.

See also

---

• Adiabatic process
• Cyclic process
• Isochoric process
• Isothermal process
• Polytropic process
• Isoenthalpic process

Thermodynamics | atmospheric thermodynamics

Izobarický děj | Isobare Zustandsänderung | Trasformazione isobara | 定圧過程 | Isobaar proces | Przemiana izobaryczna | Izobarna sprememba