The human beings who populated the Earth more than 15,000 years ago were food gatherers and hunters. Their energy needs were simple: They needed enough energy to obtain and eat their food, to grow physically, and to reproduce. For these activities they at first harnessed one of the most basic sources of energy, the energy from within their own bodies. All living things (and many nonliving things) have such energy stored within them. It is called potential, or stored, energy. When this energy is actually used, it becomes actual, or kinetic, energy, which is energy being released to do work. For example, a prehistoric man who used the strength of his body to chase and capture an animal to obtain its meat for food was, in the most basic way, transforming potential energy into actual work.
Although early human energy needs were simple, capturing animals for food was not without its difficulties. Some animals were too dangerous to approach or too quick to catch. So, some of the early human beings—perhaps as they advanced in their awareness of their difficulties and of the possible means for solving them—began to craft simple tools and weapons, which they used to make hunting and capturing animals easier and more effective. Primitive tools and weapons being put to use for energy illustrate a form of energy called mechanical energy. Mechanical energy is the energy that an object possesses as a result of its motion or position. Mechanical energy can be either kinetic energy or potential energy. A primitive tool being used to cut through forest brush would be an example of an object that possesses mechanical energy. This energy is kinetic because the tool is actually doing work. On the other hand, a primitive bow on the verge of releasing an arrow to strike animal prey is an object with mechanical energy that is potential; the bow is in the position to do work.
Mechanical energy is only one form of energy. Energy also takes the form of light, heat, electrical energy, chemical energy, and nuclear energy. Mechanical energy is a type of energy usually associated with a physical object as opposed to a chemical or biological change or process. Aside from objects, other sources of energy include the Sun, moving water, wind, and substances that can be burned to give off heat.
Turning again to the example of early civilizations, the need to find new sources of energy affected these early peoples rather intensely. To ensure their survival, they needed food. And in order to obtain enough food, and to obtain it in the most efficient way possible, they needed tools and weapons. In order to create their tools and weapons, they needed to use resources from their environment, such as stones or branches—that is, natural resources. So, natural resources were essential to the very continuation of their lives. Natural resources helped to sustain their lives, enabling them to make the tools they needed to obtain food in a more efficient manner. In a rudimentary way, this situation illustrates a problem that lies at the heart of all the energy issues that human beings have ever faced: As each civilization advances, its energy needs multiply, and as these needs multiply, the civilization begins to require more and newer sources of energy to meet these needs.
During this early period, just as now, people also depended on another source of energy: the Sun. During the day sunlight could provide two other valuable forms of energy: heat and light. Heat is also called thermal energy, and light is considered a form of radiant energy, or energy traveling as electromagnetic waves. The Sun gives off both heat and light. The Sun's energy, known as solar energy, is not limited in supply but is essentially inexhaustible. It is classified as a renewable resource, a source of energy that is constantly "renewed," or replaced, by the environment. As long as renewable resources are not used up more quickly than they can be restored, their supply is essentially limitless. Aside from the Sun, wind and water are also renewable resources.
For times when the Sun was not shining, the early human beings discovered the power of fire as a source of heat and light. Perhaps they first found warmth and light through the fires that had occurred naturally—for example, when lightning struck the Earth. Eventually they discovered how to start fires themselves. To do so, however, they needed fuel. Fuel is any material or substance that is burned to provide energy. Wood was the main fuel for early civilizations, although they also used other plant matter, as well as animal waste. Organic matter that is burned, including wood and plant and animal waste, is called biomass. Biomass is also considered a renewable source of energy, since plants can always be regrown and since what living things take in for nourishment they eliminate as waste. (Biomass would actually supply most of the world's energy for thousands of years, until the 1800s, when new energy technologies, such as electricity, along with the discovery of oil, changed the whole energy paradigm.) At this time in history, fire was civilization's greatest energy discovery. It was used for lighting, cooking, heating, and even scaring away wild animals.
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