Final answer:
The sum of the kinetic energies and potential energies of all molecules in a substance is known as its internal energy, symbolized by U or E. This sum encompasses the energy due to particles' motion and interactions, and is closely related to the substance's thermal energy, which corresponds to the average kinetic energy at a given temperature.
Step-by-step explanation:
The sum of the kinetic energies and potential energies of all of the molecules in a substance is typically referred to as the internal energy of that substance. Internal energy, often symbolized as U or E, includes all forms of energy within a substance. The kinetic energy component arises from the motion of atoms or molecules, such as translations, vibrations, and rotations. Potential energy, on the other hand, is associated with the interactions between the molecules.
It's important to understand that internal energy is distinct from other forms of energy like external kinetic energy (due to motion of the system as a whole) or potential energy due to an object's position in a gravitational field. The total internal energy of a system, therefore, is the sum of all kinetic energies and potential energies of its constituent particles.
Furthermore, the concept of thermal energy is closely related to the kinetic energy of the particles in a substance. An increase in the substance's temperature is indicative of a higher average kinetic energy of its particles. This relationship between kinetic energy and temperature is a fundamental aspect of the kinetic-molecular theory of gases. Therefore, when we discuss changes in a system's internal energy, we are often referring to changes in its thermal energy, provided that the potential energy changes are negligible or cancel out.