Thermal equilibrium
Let us consider a system
requiring a pair of independent co-ordinates X and Y for their complete
description. If the values of X and Y remain unchanged so long as the external
factors like temperature also remains the same, then the system is said to be
in a state of thermal equilibrium.
Two systems A and B having their
thermodynamic co-ordinates X and Y and X1 and Y1
respectively separated from each other, for example, by a wall, will have new
and common co-ordinates X? and Y? spontaneously, if the wall is removed. Now
the two systems are said to be in thermal equilibrium with each other.
Zeroth law of thermodynamics
If two systems A and B are
separately in thermal equilibrium with a third system C, then the three systems
are in thermal equilibrium with each other. Zeroth
law of thermodynamics states that two systems which are individually in thermal equilibrium with a third one, are
also in thermal equilibrium with each other.
This Zeroth law was stated by
Flower much later than both first and second laws of thermodynamics.
This law helps us to define
temperature in a more rigorous manner.
Temperature
If we have a number of gaseous
systems, whose different states are represented by their volumes and pressures
V1, V2, V3 ... and P1, P2,
P3... etc., in thermal equilibrium with one another, we will have φ1 (P1,V1) = φ2 (P2, V2) = φ3 (P3, V3) and so on, where φ is a function of P and V. Hence, despite their different parameters of P and
V, the numerical value of the these functions or the temperature of these
systems is same.
Temperature may be defined as the particular property
which determines whether a system is in thermal equilibrium or not with its
neighbouring system when they are brought into contact.
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