Second process. Now, we release the lock on the piston. This allows the gas to expand, doing work by moving the piston. The internal energy
of the gas and its temperature decrease. A hole in the insulating wall opens to the cold reservoir, and 1074 J of heat also flow out of the
engine. As you can see above, the gas’s temperature and pressure are now less than they were when the cycle started.Third process. The insulating walls to the reservoirs are again closed, so no heat flows. The piston pushes down, returning the gas to its initial
volume. This is a lesser amount of work than was done when the piston moved up, because the gas pressure is always less during this
process than during the expansion process. This work done on the gas increases its temperature to its initial value. The pressure returns to its
initial state.
This is one of many possible engine cycles. We will examine some specific engine cycles in more detail later. For now, we focus on the
thermodynamic processes that can make up part of an engine cycle.Second process
Gas does work on piston
Pressure and temperature decrease
Heat flows outThird process
Piston does work on gas
Engine returns to initial state
Cycle complete20.6 - Pressure-volume graphs and heat engines
In an engine, the pressure and volume of the gas change as heat is transferred to the engine and the engine does work. These changes can
be tracked with what is called a pressure-volume graph. In a pressure-volume graph, pressure is plotted on the vertical axis and volume on the
horizontal axis. You see two engine processes diagramed above, one in blue and the other in red. They illustrate how two different processes
can cause a gas to move between the same initial and final states.We will now describe these processes step-by-step. The gas starts at 2.43×10^5 Pa of pressure, 1.00×10í^2 cubic meters of volume, and a
temperature of 293 K.Pressure-volume graphs
Multiple paths between same pressure-
volume points(^378) Copyright 2007 Kinetic Books Co. Chapter 20