CHAP. 1: BASIC TERMS [CONTENTS] 27
Obr.1.1:The volume of a system as an extensive quantity. The volumeV is the sum of the volumes
of the individual parts (i.e. sub-systems) I, II and III, i.e.V =VI+VII+VIII.1.2 Energy
There are two basic forms of energy exchange between a system and its surroundings,heat
andwork. A positive value is assigned to such energy exchange during which the system gains
energy (work or heat) from its surroundings, i.e. energy is added to the system. A negative
value indicates that the system passes energy (work or heat) to its surroundings, i.e. energy is
subtracted from the system.1.2.1 Heat
When the energy of a system changes as a result of a temperature difference between the system
and its surroundings (e.g. transfer of kinetic energy of disordered movement of molecules), we
speak about exchangedheat.
U Main unit:J.
1.2.2 Work.
Other forms of energy exchange, which are usually driven by some forces acting between the
system and its surroundings, are calledwork. Based on the type of interaction between the
system and its surroundings, we distinguish volume work [see3.1.2], electrical work, surface
work, etc.
U Main unit:J.