Isobaric Process

An isobaric process is a thermodynamic process in which the pressure stays constant: Δp = 0 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:

The yellow area represents the work done

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

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 the last two equations into the first equation produces:

.

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  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.