Internal Energy

• The total energy possessed by all the microscopic models of motion is called internal energy (U). Internal energy is the energy associated with the molecular structure and molecular activity of the constituent particles of the system.
• In general, the individual molecules of a system will move around with a certain velocity, vibrate about each other with a frequency and rotate about an axis during their random motion. Associated with these motions are translational, vibrational and rotational kinetic energies, the sum of which constitutes the kinetic energy of a molecule.
• The portion of the internal energy of a system, associated with the kinetic energy of the molecules of the molecules is called the sensible energy. The average velocity and activity of the molecules are proportional to temperature of the system. Thus, at higher temperatures the molecules will possess higher kinetic energies and as a result the system will possess a higher internal energy. The internal energy is also associated with the intermolecular forces of a system. These are the forces that bind the molecules to each other and they 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 to a gas. This is described as the phase-change process. Because of the added energy, a system in a gas phase is at a higher internal energy level than when it exists in the solid or the liquid phase. The internal energy associated with the phase of a system is called latent energy.
• The internal energy associated with the bonds in a molecule is called 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 form of energy (as it is associated with the random motion of molecules), whereas kinetic energy and potential energy are organized forms of energy.
• A major 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 several particles like atoms, molecules etc. which are in continuous motion.
• A molecule possesses energy due to translation, rotational and vibrational models of motion.
• The energy posses by a molecule in the above models cannot be evaluated through macroscopic measurements and hence they are named as microscopic models of energy.

The total energy of the system is given by the equation