Constant Pressure Process (Isobaric Process)

When the gas is heated at a constant pressure, its temperature and volume will increase. Since there is a change in its volume, the heat supplied is utilized in increasing the internal energy of the gas and also for doing some external work.

i) P,V,T Relationship:
                                    V/T = Constant
ii) Work done:

                                    W_1-2 = P (V₂-V₁) = mR (T₂-T₁)

iii) Internal Energy:
                                    U₂-U₁ = mC_v (T₂-T₁)

iv) Heat Supplied:
                                    Q_1-2 = mC_P (T₂-T₁)

v) Enthalpy:

                                    H₂-H₁ = mC_P (T₂-T₁)

Notes:

• When the gas is cooled at constant pressure, there will be a compression. The temperature and volume will decrease during cooling and work is said to be done on the gas.
• During expansion or heating process, work is done by the gas (i.e W_1-2 is + ve); internal energy of the gas increases (i.e dU is + ve) and heat is supplied to the gas (i.e Q_1-2 is + ve).
• During compression or cooling process, work is done on the gas (i.e W_1-2 is - ve); internal energy of the gas decreases (i.e dU is - ve) and heat is rejected by the gas (i.e Q_1-2 is - ve).