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248 CHAPTER 8 Conservation of Energy

Online 8.3 OTHER FORMS OF ENERGY
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Concept
Tutorial If the forces acting on a particle are conservative, then the mechanical energy of the par-
ticle is conserved. But if some of the forces acting on the particle are not conservative,
then the mechanical energy of the particle—consisting of the sum of the kinetic energy
and the net potential energy of all the conservative forces acting on the particle—will
not remain constant. For instance, if friction forces are acting, they do negative work
and thereby decrease the mechanical energy of the particle.
However, it is a remarkable fact about our physical universe that whenever mechan-
ical energy is lost by a particle or some other body, this energy never disappears—it is merely
changed into other forms of energy. Thus, in the case of friction, the mechanical energy
lost by the body is transformed into kinetic and potential energy of the atoms in the
body and in the surface against which it is rubbing.The energy that the atoms acquire
in the rubbing process is disorderly kinetic and potential energy—it is spread out
among the atoms in an irregular, random fashion. At the macroscopic level, we per-
ceive the increase of the disorderly kinetic and potential energy of the rubbed sur-
faces as an increase of temperature. Thus, friction produces heat or thermal energy.
(You can easily convince yourself of this by vigorously rubbing your hands against
each other.)
Heat is a form of energy, but whether it is to be regarded as a new form of energy
or not depends on what point of view we adopt. Taking a macroscopic point of view,
we ignore the atomic motions; then heat is to be regarded as distinct from mechani-
cal energy.Taking a microscopic point of view, we recognize heat as kinetic and poten-
tial energy of the atoms; then heat is to be regarded as mechanical energy. (We will
further discuss heat in Chapter 20.)
Chemical energy and nuclear energy are two other forms of energy. The former is
kinetic and potential energy of the electrons within the atoms; the latter is kinetic and
potential energy of the protons and neutrons within the nuclei of atoms. As in the case
of heat, whether these are to be regarded as new forms of energy depends on the point
of view.
Electric and magnetic energy are forms of energy associated with electric charges
and with light and radio waves. (We will examine these forms of energy in Chapters 25
and 31.)
Table 8.1 lists some examples of different forms of energy. All the energies in
Table 8.1 are expressed in joules, the SI unit of energy. However, for reasons of tradi-
tion and convenience, some other energy units are often used in specialized areas of
physics and engineering.
The energy of atomic and subatomic particles is usually measured in electron-
volts (eV), where
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1 electron-volt 1 eV 1.60 10 J (8.26)
HERMANN VON HELMHOLTZ
Electrons in atoms typically have kinetic and potential energies of a few eV.
(1821–1894) Prussian surgeon, biologist,
mathematician, and physicist. His scientific The energy supplied by electric power plants is usually measured in kilowatt-hours
contributions ranged from the invention of the (kW
h), where
ophthalmoscope and studies of the physiology 6
and physics of vision and hearing to the mea- 1 kilowatt-hour 1 kW
h 3.60 10 J (8.27)
surement of the speed of light and studies in
The electric energy used by appliances such as vacuum cleaners, hair dryers, or toast-
theoretical mechanics. Helmholtz formulated
ers during one hour of operation is typically 1 kilowatt-hour.
the general Law of Conservation of Energy,
treating it as a consequence of the basic laws of And the thermal energy supplied by the combustion of fuels is often expressed in
mechanics and electricity. kilocalories (kcal):