9.4: The First Law of Thermodynamics

Describe the internal energy of a system

The internal energy of a system is the sum of the kinetic energy of the constituent objects within the system and the potential energy of the configuration of the objects that make up the system.

The molecules in an ideal gas do not interact with each other via conservative forces and the internal structure is not considered. Therefore, the gas does not have internal potential energy.

The internal energy of an ideal monatomic gas is the sum of the kinetic energies of the constituent molecules within the gas.

Changes to a system’s internal energy can result in changes to the internal structure and internal behavior of that system without changing the motion of the system’s center of mass.

Describe the behavior of a system using thermodynamic processes

The first law of thermodynamics is a restatement of the conservation of energy that accounts for energy transferred into or out of a system by work, heating, or cooling.

• For an isolated system, the total energy is constant.

• For a closed system, the change in internal energy is the sum of energy transferred by heating or cooling and the energy transferred to or from the system by work on the system.

The work done on a system by a constant or average external pressure that changes the volume of that system is defined as:

P-V graphs are representations used to analyze the thermodynamic processes of a system.

• Plots of pressure vs. volume of an ideal gas that maintains a constant temperature follow specific curves called isotherms.

• The magnitude of work done when a gas is compressed or when it expands is equal to the area underneath a graph of pressure vs. volume.

Special cases of thermal processes depend on the relationship between the configuration of the system, the nature of the work done on the system, and the system’s surroundings. These include:

•  constant volume (isovolumetric), 

• constant temperature (isothermal), 

• constant pressure (isobaric), and 

• processes where no energy is transferred to or from the system through thermal processes (adiabatic).