isobaric

Meaning: Isobaric means constant pressure (iso = same, baric = pressure). In an ideal gas, heating at fixed pressure makes the gas expand, and cooling makes it contract, because V/T stays constant (Charles's law). Heat added is split between raising internal energy and doing expansion work, as the first law says: ΔU = Q − W.

Equation: W = PΔV = P(Vf − Vi), where W is the work done by the gas (J), P is the constant pressure (Pa), and V is volume (m³).

On the MCAT: Expect a PV diagram in which an isobaric step appears as a horizontal line. Work is the area under that line, positive when the gas expands (volume increases to the right) and negative when it is compressed. Questions often compare isobaric, isothermal, isochoric and adiabatic paths between states, or ask you to find the sign of W, Q or ΔU. They may also give pressure and two volumes and ask for the work. A piston loaded with a fixed weight, or a container open to the atmosphere, signals constant pressure.

Common confusion: Isobaric does not mean constant volume (that is isochoric) or constant temperature (isothermal). Because T changes when V changes at fixed P, an ideal gas has a nonzero ΔU in an isobaric process. Isobaric work is also not zero, unlike isochoric work, where ΔV = 0.