Where does energy come from? What are the sources of energy? How do we get energy?
Most of the energy that we use really comes from just one source. If you think about some of the important transformations of energy, you will understand why this is true. Your body gets muscular energy and heat by oxidizing food and releasing its chemical energy. This food was made by green plants with the help of sunlight. Radiant energy from the sun was transformed into chemical energy and stored in the food. So the sun is the real source of the muscular energy and the heat energy in your body.
Some machines are run by muscular energy, but many of them are driven by mechanical energy. This energy is often supplied by some kind of engine that burns fuel. Like food, all fuels contain chemical energy that was once stored in green plants. Coal was formed from the dead bodies of huge fems and other green plants that lived millions of years ago. Gasoline, kerosene, and fuel oil are all made from petroleum. It was also formed millions of years ago, probably by tiny green plants or by tiny animals that ate them. Radiant energy from the sun was transformed into chemical energy and stored in the plants from which the fuels were made. When any fuel is burned, the chemical energy is released and changed into heat energy. In an engine, heat is transformed into mechanical energy that can be used to run machines. So the sun is also the real source of the heat from fuels and the mechanical energy from engines.
Fans, vacuum cleaners, mechanical refrigerators, and many other devices are driven by electric motors. An electric motor changes electrical energy into mechanical energy. The electrical energy comes through wires from a powerhouse. In the powerhouse are generators that make an electric current flow through the wires. A generator changes mechanical energy into electrical energy. The mechanical energy is often supplied by an engine that burns fuel. If so, the electrical energy that turns the motor really comes from the sun.
However, some generators are supplied with mechanical energy from water wheels or even windmills. Wind is moving air. Like running water, it has kinetic energy because of its motion. The sun is the real source of this energy, too. When the sun shines on the earth, radiant energy is changed into heat energy. Heat makes air expand and become lighter. The wind blows because the air at some place is warmed more than the air at another place. Gravity pulls down with more force on the heavier, cooler air. As this air sinks to the earth, it pushes against the lighter, warmer air. So the warmer air is forced outward and upward. The current of air moving along the earth is the wind.
Radiant energy is also changed into heat energy when the sun shines on the water in oceans, lakes, and streams. Heat makes water evaporate. Wind carries the water vapor high into the air. The water vapor has potential energy because it has been lifted against the force of gravity. When the water vapor is cooled enough, it condenses and falls back to earth as rain or snow. Then water from the rain or melted snow flows downhill on its way back to the oceans. The potential energy in the water vapor is transformed into the kinetic energy of running water. So no matter whether an engine, a water wheel, or a windmill is used to supply the mechanical energy, the sun is the real source of the electrical energy from a generator. If you trace almost any transformation of energy back far enough, you will always find that it began with some change in the radiant energy from the sun.
The sun is a huge ball of extremely hot, glowing gases. Though it is 93 million miles away, it provides practically all the energy that we use. In fact, it provides far more energy than we have learned how to use directly. Enough radiant energy falls on the earth in 3 minutes to supply all the energy that we need in one year. Yet only a tiny part of the suns energy ever reaches the earth. Most of the energy radiates into space. For many years, scientists have been puzzled about how the sun can keep on giving out enormous amounts of energy. It was once thought that the sun was actually burning. But if this were true, the sun would have burned itself out long ago. Scientists are now quite sure that the heat and light of the sun are produced by releasing the atomic energy of certain elements, mainly hydrogen and carbon.