Physics · Ch 8 — Heat and Thermodynamics
Isobaric Process
Isobaric Process
An isobaric process occurs at constant pressure while temperature, volume and internal energy all vary. From with fixed, , giving and a straight - line through the origin; on a P-V diagram it is a horizontal line (Figures 8.33-8.35). The work done is the simple rectangular area (equivalently from the equation of state), positive for expansion and negative for compression (Figure 8.36); applying the first law gives . Real-life isobaric examples include cooking food in an open vessel (Figure 8.34, always at fixed atmospheric pressure above the food) and a piston loaded with fixed weights, where the pressure never changes even as heat is supplied (Figure 8.33). Comparing a …
What this figure shows. A gas cylinder with a movable piston loaded with a fixed set of masses, shown in an 'Initial' state with the piston at volume Vi and a 'Final' state with the piston having risen to a larger volume Vf after heat is supplied. Because the masses on top of the piston stay the same throughout, the pressure the piston exerts on the gas (atmospheric pressure plus the weight of the masses divided by piston area) stays fixed the whole time, which is exactly what keeps this process at constant pressure …
What this figure shows. A cooking pot sitting open (uncovered) on a stove, with heat being applied from below. Because the top of the pot is open to the atmosphere, the pressure directly above the food inside stays fixed at atmospheric pressure throughout the entire cooking process, however long heat continues to be supplied -- making this one of the most common everyday isobaric proces …
What this figure shows. Two small P-V diagrams, each showing a single horizontal straight line parallel to the volume axis (since pressure is constant throughout). Diagram (a) shows the line running from a larger initial volume Vi down to a smaller final volume Vf, representing isobaric compression. Diagram (b) shows the line running from a smaller initial volume Vi up to a larger final volume Vf, representing isobaric expansion. Both diagrams sit at the same fixed height P on the pressure axis, visually confirming that an isobaric process always trac …
What this figure shows. A P-V diagram showing a horizontal isobaric line at pressure P running from Vi to Vf, with the rectangular region beneath the line (between the line and the V-axis, bounded on the sides by verticals at Vi and Vf) shaded to represent the work done. Because the top edge of this region is perfectly flat (constant pressure), the shaded region is an exact rectangle of height P and width (Vf−Vi), directly giving the simple formula W = PΔV without needing any calculus, unlike the curved isothe …