Isobaric – when the gas is held at a constant pressure
Isochoric – when the gas is held at a constant volume
Isothermal – when the gas is held at a constant temperature
Adiabatic – no heat flows in and out of the container
These fundamental processes are modeled on PV diagrams and follow ideal gas laws. Each process has its unique PV diagram, as shown below.
Pressure is plotted along the vertical axis, and volume is plotted along the horizontal axis.
The pressure increases from bottom to top, and the volume increases from left to right.
An arrow can be added to the curve to indicate the direction of the process.
Identify the process: The first step is to understand for which thermodynamic process we are drawing a PV diagram.
Look for indicators: Identify signs that say something about the process. For example, “the gas pressure doubles”, ”the volume remains constant”, and “the temperature decreases”. These will be useful in determining the direction of the process.
Learn the direction: Understand the direction in which the process occurs. For example, if a gas compresses, it goes from low pressure to high pressure or high volume to low volume. Also, if compression is in one direction, expansion is in the opposite direction.
Calculate unknown variables: Each thermodynamic process is governed by an ideal gas law describing the relationships between pressure, volume, and temperature. Use the appropriate equation to determine the unknown variable. These quantities will help to understand the direction.
Arrange the state variables: After determining the unknown quantities, arrange the state variables according to the order of their states. For example, state 1 has pressure P1, volume V1, and temperature T1, and state 2 has pressure P2, volume V2, and temperature T2. Suppose a system goes from state 1 to 2. Then, draw a line connecting the two states after identifying the process from step 1.
Below is an example of a PV diagram during adiabatic compression of an ideal gas. Each point on the curve represents a thermodynamic state with unique P, V, and T values.
The sign conversion used for W is as follows:
W > 0: Work is done on the system or compression
W < 0: Work is done by the system or expansion
W = 0: No work is done or isochoric process
