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A constant pressure process is called an isobaric process. An example is a gas in a container sealed with a piston that is free to slide up and down.
If heat is added the temperature goes up and the system expands, so work is done.
The full First Law applies:
ΔEint = Q - W
The P-V diagram for this process is a horizontal line, so the work done is simply:
W = P ΔV = nR ΔT
For a monatomic ideal gas:
| ΔEint | = |
|
nR ΔT |
Plugging this into the First Law gives:
Q = ΔEint + W
| Q | = |
|
nR ΔT | + | nR ΔT | = |
|
nR ΔT |