isobaric process
/ eye-so-BAR-ik /
An isobaric process is a change that happens at constant pressure. The root baro- means 'weight' or 'pressure' (as in barometer), so isobaric means 'same pressure'. The everyday picture is a gas under a piston that is free to slide and carries a fixed weight on top: as you heat the gas it expands and lifts the piston, but the pressure pushing back stays the same throughout because the load never changes.
Because the pressure P is constant, the work done by the gas is easy to compute: W = P ΔV, pressure times the change in volume. Feed this into the first law and Q = ΔU + P ΔV: the heat you add both raises the internal energy and pays for the expansion work. This is why it takes more heat to warm a gas at constant pressure than at constant volume — some of the heat is spent pushing the surroundings back rather than raising the temperature.
Heating water in an open pan, or any process open to the steady push of the atmosphere, is essentially isobaric, held at about 101000 Pa (one atmosphere). On a PV diagram an isobaric process is a simple horizontal line, and the area beneath it, P times ΔV, is exactly the work done. Charles's law — volume proportional to absolute temperature — describes how an ideal gas behaves along such a constant-pressure path.
Heat a gas at a steady pressure of 100000 Pa so its volume grows by 0.002 m^3. The gas does W = P ΔV = 100000 x 0.002 = 200 J of work lifting the piston, on top of whatever heat went into raising its internal energy.
At constant pressure the work is just pressure times the volume change.
Do not confuse constant pressure with constant force: the piston's weight sets the pressure, and the gas can still do work because the piston moves through a distance as the volume changes.