Write down the first law of thermodynamics for the isobaric process.

The first law of thermodynamics says that the amount of heat Q transferred to the system is equal to the sum of the change in the internal energy ΔU of the thermodynamic system and the work A that the system performs on external bodies:
Q = ΔU + A.
For an isobaric process, the pressure is constant (p = const), therefore, the work done by the gas is: A = p (V2 – V1) = p ΔV.
We substitute this expression into the first law:
Q = ΔU + p ΔV.
Answer: For an isobaric process, the first law of thermodynamics has the form Q = ΔU + p ΔV.