What is internal energy?

Internal energy refers to the total energy contained within a system, which encompasses all forms of energy associated with the microscopic components of the system, including kinetic and potential energy. This includes the energy due to the random motions of particles (which contributes to thermal energy) and interactions among particles (chemical bonds and cohesive forces).

When considering a stationary system, internal energy is particularly relevant as it represents all the energy possessed by the particles within that system regardless of external influences. This is a crucial concept in thermodynamics and helps in understanding how energy is conserved or transformed in physical processes.

The other options misinterpret or narrow the definition of internal energy. The option that states "Only thermal energy" limits the concept to just one form, ignoring potential energy aspects. The choice that describes "All energy in motion" does not accurately capture the internal energy context, as internal energy includes both the microscopic kinetic and potential energies, not just kinetic energy in motion. Lastly, the mention of "Energy from external sources" fails to align with the definition of internal energy, which is specifically the energy contained within the system itself, independent of external influences.