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 objects that make up the system and the potential energy of the configuration of those objects.
- The atoms in an ideal gas do not interact with each other via conservative forces, and the internal structure is not considered. Therefore, an ideal 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 atoms in the gas. Relevant equation:
- 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 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 to or from the system by heating, or work done on the system. Relevant equation:
- The work done on a system by a constant or average external pressure that changes the volume of that system (for example, a piston compressing a gas in a container) is defined as:
- Pressure-volume graphs (also known as PV diagrams) are representations used to represent thermodynamic processes.
- Lines of constant temperature on a PV diagram are called isotherms.
- The absolute value of the work done on a gas when the gas expands or compresses is equal to the area underneath the curve of a plot of pressure vs. volume for the gas.
- 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), and constant pressure (isobaric), as well as processes where no energy is transferred to or from the system through thermal processes (adiabatic).