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a heat source into mechanical energy. Both of the preceding of correlation, the particles have a jumbled incoherent motion,

statements of the second law are concerned with a direction of which is often described as chaotic. This random, chaotic

thermodynamic processes, and the implications of this direc- motion is sometimes called thermal motion.
tion will be discussed next. Thus, there are two kinds of motion that the particles of an

object can have: (1) a coherent motion where they move together,
in step, and (2) an incoherent, chaotic motion of individual par-
ticles. These two types of motion are related to the two modes

Myths, Mistakes, & Misunderstandings of energy transfer, working and heating. The relationship is that

work on an object is associated with its coherent motion, while
It Makes Its Own Fuel? heating an object is associated with its internal incoherent motion.

Have you ever heard of a perpetual motion machine? A perpet- The second law of thermodynamics implies a direction
ual motion machine is a hypothetical device that would produce to the relationship between work (coherent motion) and heat
useful energy out of nothing. This is generally accepted as being (incoherent motion), and this direction becomes apparent as
impossible, according to laws of physics. In particular, perpetual you analyze what happens to the motions during energy conver-
motion machines would violate either the first or the second law sions. Some forms of energy, such as electrical and mechanical,
of thermodynamics. have a greater amount of order since they involve particles mov-
A perpetual motion machine that violates the first law of ing together in a coherent motion. Th e term quality of energy
thermodynamics is called a machine of the first kind. In general, is used to identify the amount of coherent motion. Energy with
the first law says that you can never get something for nothing. high order and coherence is called a high-quality energy. Energy
This means that without energy input, there can be no change in
internal energy, and without a change in internal energy, there with less order and less coherence, on the other hand, is called
can be no work output. Machines of the first kind typically use low-quality energy. In general, high-quality energy can be easily
no fuel or make their own fuel faster than they use it. If this converted to work, but low-quality energy is less able to do work.
type of machine appears to work, look for some hidden source High-quality electrical and mechanical energy can be used to
of energy. do work but then becomes dispersed as heat through energy-form

A machine of the second kind does not attempt to make en- conversions and friction. The resulting heat can be converted to
ergy out of nothing. Instead, it tries to extract either random do more work only if there is a suffi cient temperature diff erence.
molecular motion into useful work or useful energy from some The temperature differences do not last long, however, as conduc-

degraded source, such as outgoing radiant energy. The second tion, convection, and radiation quickly disperse the energy even
law of thermodynamics says this cannot happen any more than more. Thus, the transformation of high-quality energy into lower-

rocks can roll uphill on their own. This just does not happen. quality energy is a natural process. Energy tends to disperse, both
The American Physical Society states, “The American Phys-
ical Society deplores attempts to mislead and defraud the pub- from the conversion of an energy form to heat and from the heat

lic based on claims of perpetual motion machines or sources of flow processes of conduction, convection, and radiation. Both

unlimited useful free energy, unsubstantiated by experimentally processes flow in one direction only and cannot be reversed. Th is
tested established physical principles.” is called the degradation of energy, which is the transformation
Visit www.phact.org/e/dennis4.html to see a historical list of high-quality energy to lower-quality energy. In every known
of perpetual motion and free energy machines. example, it is a natural process of energy to degrade, becoming
less and less available to do work. The process is irreversible even

though it is possible to temporarily transform heat to mechani-
cal energy through a heat engine or to upgrade the tempera-
ture through the use of a heat pump. Eventually, the upgraded
THE SECOND LAW AND NATURAL
mechanical energy will degrade to heat, and the increased heat
PROCESSES will disperse through the processes of heat fl ow.

Energy can be viewed from two considerations of scale: The apparent upgrading of energy by a heat pump or heat
(1) the observable external energy of an object and (2) the internal engine is always accompanied by a greater degrading of energy

energy of the molecules, or particles that make up an object. A someplace else. The electrical energy used to run the heat pump,
ball, for example, has kinetic energy after it is thrown through for example, was produced by the downgrading of chemical or

the air, and the entire system of particles making up the ball acts nuclear energy at an electrical power plant. The overall result is

as a single massive particle as the ball moves. The motion and that the total energy was degraded toward a more disorderly state.

energy of the single system can be calculated from the laws of A thermodynamic measure of disorder is called entropy.
motion and from the equations representing the concepts of Order means patterns and coherent arrangements. Disorder
work and energy. All of the particles are moving together, in means dispersion, no patterns, and a randomized, or spread-out,
coherent motion, when the external kinetic energy is considered. arrangement. This leads to another statement about the second

But the particles making up the ball have another kind of law of thermo dynamics and the direction of natural change, that
kinetic energy, with the movements and vibrations of internal
the total entropy of the universe continually increases.
kinetic energy. In this case, the particles are not moving uni-
formly together but are vibrating with motions in many dif- Note the use of the words total and universe in this statement
ferent directions. Since there is a lack of net motion and a lack of the second law. The entropy of a system can decrease (more

106 CHAPTER 4 Heat and Temperature 4-22