Illustration 20.3: Thermodynamic Processes

Please wait for the animation to completely load.

There is a time delay—since the system must be in equilibrium—before the change of state occurs.

In this animation N = nR (i.e., k_B = 1). This, then, gives the ideal gas law as PV = NT.  Restart.

There are many ways for a gas to go from one state (as described by its pressure, volume, number of atoms and temperature) to another.  There can be heat flow from the environment to the gas or from the gas to the environment.  Work can be done on the gas or by the gas.  How the gas goes from one state to another is determined by the heat flow and the work done.  However, the change in internal energy depends only on the temperature change (not on how the temperature change occurs).  In other words, the change in internal energy is process independent, but the work and heat required are process dependent.

To make things easier, we categorize a set of processes that have names that describe the type of process.  We illustrate them for the case when the number of atoms in the gas remains constant (there is no requirement that the number of atoms remains constant, but for purposes of this Illustration, we assume a sealed container).

• Isobaric: The pressure on the gas remains constant.  This means that as the temperature of the gas changes, so will the volume.  For example, a balloon that is put in the refrigerator shrinks in size.
• Isochoric: The volume of the gas remains constant.  This means that any temperature change is accompanied by a change in pressure.  For example, the steam in a pressure cooker increases in pressure as the temperature increases.
• Isothermal: The temperature of the gas remains constant.  As the volume increases, the pressure decreases.  For example, a balloon in a vacuum chamber increases in volume as the pressure in the chamber decreases.
• Adiabatic: The volume, temperature, and pressure all change.  This is a rapid process in which no heat is exchanged with the environment, for example, compressing a piston on a bicycle pump or syringe quickly.

One way to describe the state of a gas is with a PV diagram.  You could just as easily use a PT or VT diagram, but we use a PV diagram because it is easy to see the work done by the gas.  The work is simply the area under the curve (the red region in the graphs).  If you know some calculus, this is because work is given by W = ∫ P dV (and an integral is a calculation of the area under the curve).

A gas does not have to follow any of these special, "named" methods of changing state (Unknown Process).  For an ideal gas, as long as PV = NT remains true through the process, everything is fine.  It is simply a bit harder to  mathematically describe the process and, therefore, harder to calculate the work and heat.

When you get a good-looking graph, right-click on it to clone the graph and resize it for a better view.

Illustration authored by Anne J. Cox.
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