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Isobaric Process
This model illustrates the isobaric process, i.e. the process of quasi-static expansion or compression of a gas under a constant pressure p. You have the opportunity to modify the pressure on the gas. Study the dependency graph V(T) of an isobaric process on your computer screen. The energy diagram displays the quantity of heat Q that is obtained by the gas, the work W that is done by the gas, and the change in its internal energy ΔU.
Note that in the process of isobaric expansion, the temperature of the gas rises as its internal energy increases, and the gas produces positive work. In the process of isobaric compression, the temperature and the internal energy of the gas decrease, and the work done by the gas is a negative number. During the expansion, the gas absorbs heat. During compression, it gives off heat to adjacent bodies.

Consider the process of isobaric compression or expansion in an ideal gas. Choose the magnitude of the external pressure. The state of the gas is presented on the P-V diagram.
Isobaric processes that pertain to the gas can be represented on the (V,T)-diagram by a set of straight lines V~T that are determined by the pressure values.

For one mole of an ideal gas

V = const = R

, where R = 8.31 J/(mole K) is the universal gas constant.
The work produced by a gas in an isobaric process is
W P(V2 - V1) = PΔV
The first law of thermodynamics for an isobaric process states that
Q = U(T2) - U(T1) + P(V2 - V1) = ΔU + PΔV,
where U(T1) and U(T1) are the internal energies of the gas at the initial and final states, V1 is the initial volume, and V2 is the final volume of the gas.

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