The Zeroth Law of Thermodynamics
Introduction
** Thermodynamics
is based on a few statements called laws that have broad application to
physical and chemical systems.
** As
simple as these laws are, it took many years of observation and experimentation
before they were formulated and recognized as scientific laws.
** Three
such statements that we will eventually discuss are the first, second, and
third laws of thermodynamics.
** However,
there is an even more fundamental idea that is usually assumed but rarely
stated because it is so obvious. Occasionally this idea is referred to as the
zeroth law of thermodynamics, since even the first law depends on it. It has to
do with one of the variables that was introduced in the previous section,
temperature.
What is temperature?
** Temperature
is a measure of how much kinetic energy the particles of a system have.
** The
higher the temperature, the more energy a system has, all other variables
defining the state of the system (volume, pressure, and so on) being the same.
** Since
thermodynamics is in part the study of energy, temperature is a particularly
important variable of a system.
** We
must be careful when interpreting temperature, however. Temperature is not a
form of energy. Instead, it is a parameter used to compare amounts of energy of
different systems.
Energy transfer between two systems
** Consider
two systems, A and B, in which the temperature of A is greater than the
temperature of B (Figure).
** Each
is a closed system, which means that matter cannot move in or out of each
system but energy can.
** The
state of each system is defined by quantities like pressure, volume, and
temperature.
**
The two systems are brought together and physically joined but kept separate
from each other, as shown. For example, two pieces of metal can be brought into
contact with each other, or two containers of gas can be connected by a closed
stopcock.
** Despite
the connection, matter will not be exchanged between the two systems or with
the surroundings.
** What
about their temperatures, TA and TB ? What is always observed is that energy
transfers from one system to another.
** As
energy transfers between the two systems, the two temperatures change until the
point where TA = TB. At that point, the two systems are said to be at thermal
equilibrium.
** Energy
may still transfer between the systems, but the net change in energy will be
zero and the temperature will not change further.
** The
establishment of thermal equilibrium is independent of the system size. It
applies to large systems, small systems, and any combination of large and small
systems.
** The
transfer of energy from one system to another due to temperature differences is
called heat.
** We
say that heat has flowed from system A to system B. Further, if a third system
C is in thermal equilibrium with system A, then TC = TA and system C must be in
thermal equilibrium with system B also.
** This
idea can be expanded to include any number of systems, but the basic idea
illustrated by three systems is summed up by a statement called the zeroth law
of thermodynamics:
What is the Zeroth Law of Thermodynamics?
** The
zeroth law of thermodynamics states that:
"If two systems
(of any size) are in thermal equilibrium with each other and a third system is
in thermal equilibrium with one of them, then it is in thermal equilibrium with
the other also."
** The
zeroth law introduces a new idea. One of the variables that defines the state
of our system (the state variables) changes its value. In this case, the
temperature has changed. We are ultimately interested in how the state
variables change and how these changes relate to the energy of our system.
** The
final point with respect to the system and its variables is the fact that the
system does not remember its previous state. The state of the system is
dictated by the values of the state variables, not their previous values or how
they changed.
** Consider
the two systems in shown Figure:
System
A: goes to a higher temperature before settling on T = 200 temperature units.
System
B: goes directly from the initial conditions to the final conditions.
Therefore,
the two states are the same. It does not matter that the first system was at a
higher temperature; the state of the system is dictated by what the state
variables are, not what they were, or how they got there.
Reference:
physical Chemistry /David W. Ball / Cleveland State University /2011 .
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