STATE AND EQUILIBRIUM
Consider a system not undergoing any change. At this point, all the properties
can be measured or calculated throughout the entire system, which
gives us a set of properties that completely describes the condition, or the
state,
fixed values. If the value of even one property changes, the state will change
to a different one. In Fig. 1–23 a system is shown at two different states.
Thermodynamics deals with
of the system. At a given state, all the properties of a system haveequilibrium states. The word equilibriumimplies a state of balance. In an equilibrium state there are no unbalanced
potentials (or driving forces) within the system. A system in equilibrium
experiences no changes when it is isolated from its surroundings.
There are many types of equilibrium, and a system is not in thermodynamic
equilibrium unless the conditions of all the relevant types of equilibrium
are satisfied. For example, a system is in
temperature is the same throughout the entire system, as shown in Fig.
1–24. That is, the system involves no temperature differential, which is the
driving force for heat flow.
and a system is in mechanical equilibrium if there is no change in pressure
at any point of the system with time. However, the pressure may vary within
the system with elevation as a result of gravitational effects. For example,
the higher pressure at a bottom layer is balanced by the extra weight it must
carry, and, therefore, there is no imbalance of forces. The variation of pressure
as a result of gravity in most thermodynamic systems is relatively small
and usually disregarded. If a system involves two phases, it is in
equilibrium
stays there. Finally, a system is in
composition does not change with time, that is, no chemical reactions occur.
A system will not be in equilibrium unless all the relevant equilibrium criteria
are satisfied.
thermal equilibrium if theMechanical equilibrium is related to pressure,phasewhen the mass of each phase reaches an equilibrium level andchemical equilibrium if its chemical
