Energy, Fuel, and Heat
As you have learned, energy is the ability to move or change matter. To put it another
way, you could say that energy is the ability to do work. But what makes energy available
whenever you need it? If you have ever accidentally unplugged a lamp while you were using
it, you have seen that the lamp does not have a supply of energy to keep itself lit. When
the lamp is plugged into an outlet, it has the source of energy it needs—electricity. The
electricity comes from a power plant, and the power plant has to have energy to produce
this electricity (Figure5.1). The energy to make the electricity comes from a fuel, which
stores the energy and releases it when it is needed. A fuel is any material that can release
energy in a chemical change. The food you eat acts as afuelfor your body. You probably
hear the word fuel most often when someone refers to its use in transportation. Gasoline
and diesel fuel are two fuels that provide the energy for most cars, trucks, and buses. But
many different kinds of fuel are used to meet the wide variety of needs for energy storage.
Forafueltobeuseful,itsenergymustbereleasedinawaythatcanbecontrolled. Controlling
the release of the energy makes it possible for the energy to be used to do work. When a fuel
is used for its energy, the fuel is usually burned, and most of the energy is released as heat.
The heat can be used to do work. For an example of how the energy in a fuel is released
mostly as heat, think about what happens when someone starts a fire in a fireplace. First,
the person strikes a match and uses it to set some small twigs on fire. After the twigs have
burned for a while, they get hot enough to make some larger sticks burn. The fire keeps
getting hotter, and soon it is hot enough to burn whole logs.
You might think at first that the heat comes from starting the fire. After all, someone struck
a match to start the fire, and then the fire just spread, right? But if you think about this
fire in terms of energy, almost all of the heat comes from the energy that has been stored
in the wood. In other words, the wood is the fuel for the fire. There is a reason why it is
easy to be confused about the source of the fire’s heat. The reason is that some energy has
to be put intostartingthe fire before any energy can come out of the fire. At first, there is
energy stored in the head of the match as chemical energy. When someone strikes a match,