Page 49 - Environment: The Science Behind the Stories

well. A sparrow in flight expends energy to propel its body Mechanical energy, such as the energy stored in a compressed
through the air (change of position). When the sparrow lays spring, is yet another type of potential energy. Kinetic energy
an egg, its body uses energy to create the calcium-based can also express itself in different forms, including thermal
eggshell and color it with pigment (change in composition). energy, light energy, sound energy, and electrical energy—all
The sparrow sitting on its nest transfers energy from its of which involve the movement of atoms, subatomic particles,
body to heat the developing chicks inside its eggs (change molecules, or objects.
of temperature).

Energy comes in different forms Energy is always conserved,
but it changes in quality
Energy manifests itself in different ways and can be converted
from one form to another. Two major forms of energy that sci- Although energy can change from one form to another, it
entists commonly distinguish are potential energy, energy of cannot be created or destroyed. Just as matter is conserved
position; and kinetic energy, energy of motion. Consider river (p. 41), the total energy in the universe remains constant and
water held behind a dam. By preventing water from moving thus is said to be conserved. Scientists refer to this principle
downstream, the dam causes the water to accumulate potential as the first law of thermodynamics. The potential energy of
energy. When the dam gates are opened, the potential energy the water behind a dam will equal the kinetic energy of its
is converted to kinetic energy as the water rushes downstream. eventual movement downstream. Likewise, we obtain energy
Energy conversions take place at the atomic level every from the food we eat and then expend it in exercise, apply it
time a chemical bond is broken or formed. Chemical energy toward maintaining our body and all its functions, or store
is essentially potential energy stored in the bonds between it in fat. We do not somehow create additional energy or
atoms. Bonds differ in their amounts of chemical energy, end up with less energy than the food gives us. Any particu-
depending on the atoms they hold together. Converting mol- lar system in nature can temporarily increase or decrease in
ecules with high-energy bonds (such as the carbon–carbon energy, but the total amount in the universe always remains
bonds of fossil fuels) into molecules with lower-energy constant.
bonds (such as the bonds in water or carbon dioxide) releases While the overall amount of energy is conserved in any
energy by changing potential energy into kinetic energy and conversion of energy, the second law of thermodynamics
produces motion, action, or heat. Just as automobile engines states that the nature of energy will change from a more-
split the hydrocarbons of gasoline to release chemical energy ordered state to a less-ordered state as long as no force coun-
and generate movement, our bodies split glucose molecules teracts this tendency. That is, systems tend to move toward
in our food for the same purpose (Figure 2.9). increasing disorder, or entropy. For instance, a log of fire-
Besides occurring as chemical energy, potential energy wood—the highly organized and structurally complex prod-
can occur as nuclear energy, the energy that holds atomic uct of many years of tree growth—transforms in the campfire
nuclei together. Nuclear power plants utilize this energy when to a residue of carbon ash, smoke, and gases such as carbon
they break apart the nuclei of large atoms within their reactors. dioxide and water vapor, as well as the light and the heat of

Potential energy Kinetic energy
Figure 2.9 Energy is released
when potential energy is con-
Food verted to kinetic energy. Potential
molecules energy stored in sugars (such as
glucose) in the food we eat, com-
bined with oxygen, becomes kinetic
energy when we exercise, releasing
carbon dioxide, water, and heat as
by-products.

+ + +
C 6 H 12 O 6 O 2 CO 2 H 2 O
48 Glucose Oxygen Carbon dioxide Water Heat

M02_WITH7428_05_SE_C02.indd 48 12/12/14 2:53 PM