Different Forms of Stored Energy

Total energy stored in a system is represented by E. Basically there are two models in which energy may be stored in a system

1. Macroscopic energy mode
2. Microscopic energy mode

The macroscopic energy mode includes the macroscopic kinetic energy and potential energy of a system. Let us consider a fluid element of mass m having the center of mass velocity (Fig. 1.11). The macroscopic kinetic energy E_K of the fluid element by virtue of its motion is given by

Fig. 1.11 Macroscopic and Microscopic energy in a system

If the elevation energy of the fluid element from an arbitrary datum is z, then the macrosscopic potential energy E_p by virtue of its position is given by

The microscopic energy mode refers to the energy stored in the molecules and atomic structure of the system, which is called the molecular internal energy or simply internal energy U. Matter is composed of molecules. Molecules are in random thermal motion with an average velocity v, constantly colliding with one another and with the walls. Due to a collision, the molecules may subjected to rotation as well as vibration. They can have translational kinetic energy, rotational kinetic energy, vibrational energy, electronic energy, chemical energy and nuclear energy (Fig 1.12).

Fig. 1.12 Various components of internal energy stored in a molecule (a) translational kinetic energy, (b) rotational kinetic energy, (c) nuclear energy, (d) vibrational energy and (e) electron spin