The Need for Energy - Energy can be defined as the ability to move or change matter. Every living thing needs energy to live and grow. Your body gets its energy from food, but that is only a small part of the energy you use every day. Cooking your food takes energy, and so does keeping it cold in the refrigerator or the freezer. The same is true for heating or cooling your home.
Whether you are turning on a light in the kitchen or riding in a car to school, you are using energy all day long. And because billions of people all around the world use energy, there is a huge need for resources to provide all of this energy. Why do we need so much energy? The main reason is that almost everything that happens on Earth involves energy. Most of the time when something happens, energy is changing forms.
Even though energy does change form, the total amount of energy always stays the same. The Law of Conservation of Energy says that energy cannot be created or destroyed. Scientists discovered this law by noticing that any time they observed energy changing from one form to another, they could measure that the overall amount of energy did not change.
For an example of how energy changes from one form to another, think about what goes on when you kick a soccer ball. Your body gets its energy from food. When your body breaks down the food you eat, it stores the energy from the food in a form called chemical energy. But some of this stored energy has to be released to make your leg muscles move. When this happens, the energy that is released changes from chemical energy to another form, called kinetic energy.
Kinetic energy is the energy of anything in motion. Your muscles move your leg, your foot kicks the ball, and the ball gains kinetic energy by being kicked. So you can think of the action of kicking the ball as a story of energy moving and changing forms. The same is true for anything that happens involving movement or change. Potential energy is energy that is stored. Potential energy has the potential to do work or the potential to be converted into other forms of energy. An example of potential energy might be the ball you kicked, if it ended up at the top of a hill.
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. 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 a fuel for 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.
For a fuel to be useful, its energy must be released in a way that can be controlled. 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 into starting the 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, 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.
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