Energy, in physics, the capacity to do work. (Energy in an economic sense is discussed in the article Energy Resources.) Work is done whenever an object is moved. The object can be set into motion only when a force, such as a push or pull, is applied to it. The force can be furnished only by some form of energy, such as heat, electrical energy, or the energy of the atom. Anything that occupies space is called matter, and all matter has energy. Matter and energy are the two fundamental concepts of physical science. They are different forms of the same thing, and one can be converted into the other.

Energy is either potential (stored) or kinetic (moving). Potential energy is the energy that a body has because of its position, composition, or state. For example, potential energy is contained by a raised ball (by virtue of its position), a stick of dynamite (by virtue of its composition), and a compressed spring (by virtue of its state). Kinetic energy is the energy a body has because of its motion or activity. When a raised ball is dropped, its potential energy changes into kinetic energy as it falls; as it bounces up from the ground, some of its kinetic energy changes back into potential energy.

Forms of Energy

All energy may be classified into one of several major groups.

Chemical Energy

is the energy associated with a substance's chemical composition. Chemical energy is liberated when a substance undergoes chemical change.

Electric Energy

is the energy associated with electric charges.

Heat Energy

is the internal energy of an object or substance; it results from the motion of the molecules that make up the object or substance.

Mechanical Energy

is the energy that is associated with the motion of objects. This form of energy is transferred from one object to another by m chanical forces. For example, when a hammer strikes a nail, mechanical energy is transferred from the hammer to the nail.

Nuclear, or Atomic, Energy

is bound up in the nucleus of an atom. This energy is produced by the powerful forces holding the particles of the nucleus together. When the nucleus is disturbed, this energy is released in the form of radiations such as gamma rays, alpha particles, and beta rays.

Radiant Energy

is transmitted by electromagnetic waves. It includes radio waves, visible light, ultraviolet radiation, infrared radiation, gamma rays, and X rays. Radiant energy is the subject of the quantum theory and the branch of physics known as wave, or quantum, mechanics.

Conservation of Energy

Energy cannot be created or destroyed. Sometimes energy seems to be created or destroyed, but what is actually happening is that one form of energy is being converted into another. The illustration Conversion of Energy shows an example of how energy can be changed into different forms. Even in nuclear reactions, energy is not created or destroyed. A nuclear reaction merely transforms the energy of matter into heat and other forms of energy.

In converting one form of energy into another, some energy is ordinarily wasted in overcoming friction. This wasted energy is not destroyed, however, but is converted into heat energy that cannot be used.

How Energy Is Measured

The unit of energy in the SI (the metric system) is the joule. It is defined as the work done by a force of one newton acting through a distance of one meter. The corresponding unit in the traditional English system of measurement is the foot-poundal. .) All forms of energy can be expressed in terms of the joule, but for some forms of energy other units are frequently used.

Chemical Energy

like heat energy, is commonly measured in calories and Btu's.

Electric Energy

used commercially is usually measured in kilowatt-hours.

Heat Energy

is measured in joules for most scientific work, but is also commonly measured in calories and British thermal units, or Btu's.

Mechanical Energy

is usually measured in joules or foot-poundals. These units are absolute units (units independent of the force of gravity). However, gravitational units, such as the foot-pound, or the foot-pound-force, are sometimes used. A foot-pound is the amount of energy needed to lift one pound a distance of one foot.

Nuclear Energy

is commonly measured in electron volts. The mass of a subatomic particle is frequently expressed in terms of its equivalent in energy given in electron volts.

Radiant Energy

Radiant energy is usually measured in joules.