Notes on Internal Energy

• The total energy of all microscopic motion models is referred to as internal energy (U). Internal energy is the energy associated with the molecular structure and molecular activity of the system's constituent particles.
• In general, the individual molecules of a system move around with a certain velocity, vibrate about each other with a frequency, and rotate about an axis during their random motion. Translational, vibrational, and rotational kinetic energies are associated with these motions, the sum of which constitutes a molecule's kinetic energy.
• The portion of the internal energy of a system associated with the kinetic energy of the molecules is called sensible energy. The average velocity and activity of the molecules are proportional to the system's temperature. Thus, at higher temperatures, the molecules possess higher kinetic energies, and as a result, the system has a higher internal energy. The internal energy is also associated with the intermolecular forces of a system. These forces bind the molecules to each other and are the strongest in solids and weakest in gases.
• When sufficient kinetic energy is added to the molecules of a solid or liquid, they overcome the intermolecular forces and break away, turning the system into a gas. This is described as the phase-change process. Due to the added energy, a system in the gas phase has a higher internal energy level than when it exists in a solid or liquid phase. The internal energy associated with a system's phase is referred to as latent energy.
• The internal energy associated with the bonds in a molecule is referred to as the chemical or bond energy. During a chemical reaction, such as a combustion process, some chemical bonds are destroyed while others are formed. As a result, the internal energy changes.
• Internal energy is highly disorganized (as it is associated with the random motion of molecules), whereas kinetic and potential energy are organized forms of energy.
• A primary task of an engineer is to find means of converting the disorganized form of energy into organized forms of energy. At a microscopic level, matter consists of various particles, such as atoms and molecules, in continuous motion. A molecule possesses energy due to translation, rotational, and vibrational motion.
• The energy a molecule possesses in the above models cannot be evaluated through macroscopic measurements; hence, they are referred to as microscopic models of energy.

The total energy of the system is given by the equation.

Energy = Kinetic Energy + Potential Energy + Internal Enegy

E = K.E + P.E + U