8.4 The Energy Equation
The sum total of energy of a system may be divided into two parts—(1) the internal energy and (2) the external energy.
The internal energy consists of the kinetic energy of molecular motion and potential energy of intermolecular forces. The external energy of a system is measured by the work which would be performed by the system in expanding to a state of infinite rarity starting from its present state. This work is performed by the pressure exerted by the body.
The work done by an expanding fluid can be easily calculated as: let us suppose that a small element of surface dS of the fluid moves through a distance dn in the outward direction normally. If p is the pressure of the fluid, the work done in this displacement ...
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