Energy exists in various forms such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, nuclear, etc. In thermodynamics, it is considered that the various forms of energy make up total energy of a system. This total energy can be represented into two groups—macroscopic and microscopic. The macroscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame, such as kinetic and potential energies. The microscopic forms of energy are those related to the molecular structure of a system and the degree of the molecular activity, and they are independent of outside reference frames. The sum of all the microscopic forms of energy is called the internal energy of a system and is denoted by U. The macroscopic energy of a system is related to motion and the influence of some external factors such as gravity, magnetism, electricity, surface tension, etc.
The energy that a system possesses as a result of its motion relative to some reference frame is called kinetic energy. The energy that a system possesses as a result of its elevation in a gravitational field is called potential energy. In the absence of effect of external factors, the total energy of a system consists of the kinetic, potential, and internal energies.
That is
where m is mass of the system, v is velocity of the system, h is height from reference point, and U is internal energy of the system. The change in the total energy E of a stationary system is equal to the change in its internal energy, U since the changes in kinetic, and potential energies in stationary close system are negligible.
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