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FIGURE 3.12 This demonstration solar cell array converts
A radiant energy from the Sun to electrical energy, producing an
average of 200,000 watts of electric power (after conversion).

during oxidation. Chemical energy is a kind of potential energy
that is stored and later released during a chemical reaction.
Radiant energy is energy that travels through space (Fig-
ure 3.12). Most people think of light or sunlight when consider-
ing this form of energy. Visible light, however, occupies only a
small part of the complete electromagnetic spectrum, as shown
in Figure 3.13. Radiant energy includes light and all other parts
of the spectrum (see chapter 7). Infrared radiation is sometimes
called heat radiation because of the association with heating
when this type of radiation is absorbed. For example, you feel
the interaction of infrared radiation when you hold your hand
near a warm range element. However, infrared radiation is an-
B other type of radiant energy. In fact, some snakes, such as rattle-

snakes, copperheads, and water moccasins, have pits between
FIGURE 3.11 Chemical energy is a form of potential energy
their eyes that can detect infrared radiation emitted from warm
that is released during a chemical reaction. Both (A) wood and
(B) coal have chemical energy that has been stored through the animals where you see total darkness. Microwaves are another
process of photosynthesis. The pile of wood might provide fuel for a type of radiant energy that is used in cooking. As with other
small fireplace for several days. The pile of coal might provide fuel forms of energy, light, infrared, and microwaves will be consid-
for a power plant for a hundred days. ered in greater detail later. For now, consider all types of radiant
energy to be forms of energy that travel through space.
comparison. Photosynthesis is carried on in green plants. Electrical energy is another form of energy from electro-
The plants use the energy of sunlight to rearrange carbon diox- magnetic interactions that will be considered in detail later. You
ide and water into plant materials and oxygen. By leaving out are familiar with electrical energy that travels through wires to
many steps and generalizing, this reaction could be represented your home from a power plant (Figure 3.14), electrical energy
by the following word equation: that is generated by chemical cells in a flashlight, and electrical
energy that can be “stored” in a car battery.
energy + carbon dioxide + water = wood + oxygen Nuclear energy is a form of energy often discussed because
of its use as an energy source in power plants. Nuclear energy is
The plant took energy and two substances and made two differ-
another form of energy from the atom, but this time, the energy
ent substances. This is similar to raising a book to a higher shelf
involves the nucleus, the innermost part of an atom, and nuclear
in a bookcase. That is, the new substances have more energy
interactions. This will be considered in detail in chapter 13.
than the original ones did. Consider a word equation for the
burning of wood:
ENERGY CONVERSION
wood + oxygen = carbon dioxide + water + energy
Potential energy can be converted to kinetic energy and vice versa.
Notice that this equation is exactly the reverse of photosynthesis. The simple pendulum offers a good example of this conver-
In other words, the energy used in photosynthesis was released sion. A simple pendulum is an object, called a bob, sus pended

3-11 CHAPTER 3 Energy 71