An
isobaric process is a
thermodynamic process in which the pressure stays constant; . 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,
E is internal energy, and
Q is heat. Pressurevolume work (
by the system) is defined as

but since pressure is constant, this means that
 .
Applying the
ideal gas law, equation (2) becomes

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

then substituting equations (3) and (4) into equation (1) produces:

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

and if the gas involved in the isobaric process is monatomic then
C_{V} = (3/2)R and
C_{P} = (5/2)R.
An isobaric process is shown on a PV 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 equation (2) into equation (1) yields

The quantity
E + 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 succintly described as
 .
See also:
isochoric process,
isothermal process,
adiabatic process,
cyclic process.