this chemical energy is released as heat.
The lit match gives off enough heat to set the twigs on fire. This heat is enough energy to
start changing the chemical energy in the wood (and the oxygen in the air, which the wood
needs in order to burn) into heat. What happens is that the heat from the match breaks
chemical bonds in the twigs. When these bonds break, the atoms in the twigs are free to
move around and form new bonds. When the atoms form new bonds, they release more
heat. This heat causes more and more of the wood to change its stored chemical energy into
heat. So, what started as a fairly small amount of heat from the match turns into a much,
much larger amount of heat from the wood. The same thing is true for any fuel. We have
to add some energy to the fuel to get it started. But once the fuel starts burning, it keeps
changing its chemical energy into heat. As long as the conditions are right, the fuel will keep
turning its energy into heat until the fuel is all gone.
Types of Energy Resources
Energy resources can be put into two categories—either renewable or nonrenewable. Re-
sources that arenonrenewableare used faster than they can be replaced (Figure5.2). Other
resources that are calledrenewablewill never run out. In most cases, these resources are
replaced as quickly as they are used.
In a way, the difference between nonrenewable and renewable resources is like the difference
between ordinary batteries and rechargeable ones. If you have a flashlight at home that uses
ordinary batteries, and you accidentally leave the flashlight on all night long, you will need
to buy new batteries once the ones in the flashlight have died. The energy in the ordinary
batteries is nonrenewable. But if the flashlight has rechargeable batteries, you can put them
in a battery charger and use them in the flashlight again. In this way, the energy in the
rechargeable batteries is “renewable.”
Fossil fuels are the most common example of nonrenewable energy resources. Renewable
energy resources include solar, water, and wind power. If you traced the energy in all of
these resources back to its origin, you would find that almost all energy resources—not just
solar energy—come from the sun. Fossil fuels are made of the remains of plants and animals
that stored the sun’s energy millions of years ago. These plants and animals got all of their
energy from the sun, either directly or indirectly. The sun heats some areas more than
others, which causes wind. The sun’s energy drives the water cycle, which moves water over
the surface of the Earth. Both wind and water power can be used as renewable resources.
Types of Nonrenewable Resources
Fossil fuels, which include coal, oil, and natural gas are nonrenewable resources. Millions of
years ago, plants used energy from the sun to form sugars, carbohydrates, and other energy-
rich carbon compounds that were later transformed into coal, oil, or natural gas. The solar
energy stored in these fuels is a rich source of energy, but while fossil fuels took millions of