The study of heat, thermodynamics, and
statistical mechanics
Heat is a form of internal energy associated with the random motion of the
molecular constituents of matter or with radiation. Temperature is an average of a part of the internal energy
present in a body (it does not include the energy of molecular binding or of molecular rotation).
The lowest possible energy state of a substance is defined as the absolute zero (−273.15 °C, or −459.67 °F) of temperature. An
isolated body eventually reaches uniform temperature, a state known as
thermal equilibrium, as do two or more bodies placed in contact. The
formal study of states of matter at (or near) thermal equilibrium is
called thermodynamics; it is capable of analyzing a large variety of
thermal systems without considering their detailed microstructures.
First law
The first law of
thermodynamics is the energy conservation principle of mechanics (i.e., for all
changes in an isolated system, the energy remains constant) generalized to
include heat.
Second law
The second law of
thermodynamics asserts that heat will not flow from a place of lower temperature to
one where it is higher without the intervention of an external device (e.g., a
refrigerator). The concept of entropy involves the measurement of the state of
disorder of the particles making up a system. For example, if tossing a coin
many times results in a random-appearing sequence of heads and tails, the
result has a higher entropy than if heads and tails tend to appear in
clusters. Another formulation of the second law is that the entropy of an
isolated system never decreases with time.
Third law
The third law of thermodynamics states that the entropy at the absolute
zero of temperature is zero, corresponding to the most ordered possible state.
The science of statistical mechanics derives bulk properties
of systems from the mechanical properties of their molecular constituents,
assuming molecular chaos and applying the laws of probability. Regarding each possible configuration of the
particles as equally likely, the chaotic state (the state of maximum entropy)
is so enormously more likely than ordered states that an isolated system will
evolve to it, as stated in the second law of thermodynamics. Such reasoning,
placed in mathematically precise form, is typical of statistical
mechanics, which is capable
of deriving the laws of
thermodynamics but goes beyond them in describing fluctuations (i.e., temporary
departures) from the thermodynamic laws that describe only average behaviour.
An example of a fluctuation phenomenon is the random motion of small particles
suspended in a fluid, known as Brownian motion.
Quantum statistical mechanics plays a major role in many other modern fields
of science, as, for example, in plasma physics (the study of fully ionized gases), in
solid-state physics, and in the study of stellar structure. From a microscopic
point of view the laws of thermodynamics imply that, whereas the total quantity
of energy of any isolated system is constant, what might be called the quality
of this energy is degraded as the system moves inexorably, through the
operation of the laws of chance, to states of increasing disorder until it
finally reaches the state of maximum disorder (maximum entropy), in which all
parts of the system are at the same temperature, and none of the state’s energy
may be usefully employed. When applied to the universe as a whole, considered
as an isolated system, this ultimate chaotic condition has been called the “heat death.”
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