Energy

All human life depends upon the energy in the universe. Most of the energy on earth comes from the sun. It travels from the sun to the earth in the rays that the sun gives off. The sun's rays are needed so that plants can make food. The food that plants make is the food on which all the animals in the world depend in order to live. Animals and human beings use the energy found in food to operate their bodies and muscles. The sun's energy is stored up in coal, wood, and oil, which people burn to do work for them. The sun evaporates the water which falls as rain. This causes rivers to flow and produces other energy that people can use. The amount of energy falling on the earth's surface each year is equal to that supplied by 250 million million short tons (227 million million metric tons) of coal.

Energy is one of the two fundamental ideas in physics. The other is matter. These two ideas are not completely separate. Many physicists believe that energy and matter are merely two aspects of the same thing, much as ice, water, and water vapor are three different aspects of water. The tiny electric particles (electrons) inside the atom give off the energy we know as light. A body loses part of its mass when it releases energy. It gains mass when it absorbs energy.

A new stage in energy development came with the invention of the steam engine. Steam could be used to develop energy to run machines. The discovery of electricity created an even more important way of using energy. In a similar way, so did the invention of the gasoline engine. A new era of the use of energy began with the application of nuclear energy.

People find many ways to release energy to do work. They change the energy in a waterfall into electrical energy. They can turn this electrical current into radio waves that carry ideas thousands of miles or kilometers. They can release the energy in gasoline by burning it to power automobiles. People burn coal to turn water into steam and, in turn, use steam to generate electrical energy. The nucleus of some atoms can produce millions of times more energy per pound or kilogram of material than can be made available by chemical means.

Kinetic energy is the energy of movement. The rock on the edge of the cliff had to be placed in that position and the gun had to be loaded. If you stretch an elastic band between your fingers, it represents potential energy. Let the elastic band snap back into its normal position, and the potential energy is changed to kinetic energy as long as the band keeps moving. The word kinetic comes from a Greek word meaning to move. A moving train or a high speed electron in a TV tube exhibits kinetic energy.

Changes in energy. Energy is constantly changing from the potential to the kinetic state and back again. Every change in the universe represents the change of energy from one form into another. This process is called the transformation of energy. For example, let us analyze what happens when a boy shoots a stone from a slingshot. When he pulls back the rubber band, the potential energy in the boy is changed to the potential energy in the stretched band. When he releases the band, the potential energy of the band is changed to kinetic energy of the thrown stone. When the stone strikes a target, the kinetic energy becomes heat energy.

The pendulum is one example of the way energy changes and is conserved. When a pendulum reaches the top of its swing, it has potential energy because of its position. As the pendulum swings through the lowest point of its swing, it has only kinetic energy. This energy becomes potential energy as the pendulum again reaches the top of its swing. The pendulum would swing back and forth forever if it were not for friction and the resistance of the air. But the energy used up in overcoming such friction is not lost. It is simply changed to heat.

Because energy and matter are now known to be closely related, the law of the conservation of energy has been interpreted in a new light. Energy cannot be created or destroyed. But it may be developed from matter and turned into matter.