Electricity is a form of energy caused by the presence of electrical charges in matter. All matter consists of atoms, and atoms themselves contain charged particles.

Electric Motor

An electric motor is a device used to convert electrical energy to mechanical energy. Electric motors are extremely important in modern-day life.


An electrocardiogram (pronounced ee-lek-troe-KAR-dee-oh-gram) is a recording of the electrical activity within the heart that is obtained by placing various electrodes on the skin surface. From this painless, quick, and inexpensive test, doctors are able to evaluate a person's heart rate and rhythm and to detect if something is wrong.


Electrolysis is a process by which electrical energy is used to produce a chemical change. Perhaps the most familiar example of electrolysis is the decomposition (breakdown) of water into hydrogen and oxygen by means of an electric current.

Electromagnetic Field

An electromagnetic field is a region in space in which electric and magnetic forces interact. A magnetic compass will detect a magnetic field when held close to an electric wire carrying current to a lit lightbulb.

Electromagnetic Induction

The term electromagnetic induction refers to the generation of an electric current by passing a metal wire through a magnetic field. The discovery of electromagnetic induction in 1831 was preceded a decade earlier by a related discovery by Danish physicist Hans Christian Oersted (1777–1851).

Electromagnetic Spectrum

The term electromagnetic spectrum refers to all forms of energy transmitted by means of waves traveling at the speed of light. Visible light is a form of electromagnetic radiation, but the term also applies to cosmic rays, X rays, ultraviolet radiation, infrared radiation, radio waves, radar, and microwaves.


Electromagnetism is the force involving the interaction of electricity and magnetism. It is the science of electrical charge, and its rules govern the way charged particles of atoms interact.


The electron is a subatomic (smaller than an atom) particle that carries a single unit of negative electricity. All matter consists of atoms that, in turn, contain three very small particles: protons, neutrons, and electrons.


Electronics is the branch of physics (the science of matter and energy) that deals with the flow of electrons and other carriers of electric charge. This flow of electric charge is known as electric current, and a closed path through which current travels is called an electric circuit.

Element, Chemical

A chemical element can be defined in one of two ways: experimentally or theoretically. Experimentally, an element is any substance that cannot be broken down into any simpler substance.

El Niño

El Niño (pronounced el-NEEN-yo) is the name given to a change in the flow of water currents in the Pacific Ocean near the equator. El Niño—Spanish for "the child" because it often occurs around Christmas—repeats every three to five years.

Embryo and Embryonic Development

The term embryo applies to the earliest form of life, produced when an egg (female reproductive cell) is fertilized by a sperm (male reproductive cell; semen). The fertilized egg is called a zygote.

Endangered Species

An endangered species is any animal or plant species whose very survival is threatened to the point of extinction. Once extinct, a species is no longer found anywhere on Earth.

Endocrine System

The endocrine system is the human body's network of glands that produce more than 100 hormones to maintain and regulate basic bodily functions. Hormones are chemical substances carried in the bloodstream to tissues and organs, stimulating them to perform some action.


Energy is the capacity to do work. In science, the term work has a very special meaning.


Engineering is the art of applying science, mathematics, and creativity to solve technological problems. The accomplishments of engineering can be seen in nearly every aspect of our daily lives, from transportation to communications to entertainment to health care.

Environmental Ethics

Environmental ethics is a branch of philosophy that considers the moral relations between human beings and their natural environment. As a field of study, it assumes that humans have certain responsibilities to the natural world, and it seeks to help people and their leaders become aware of them and to act responsibly when they do things that impact the natural world.


An enzyme is a biological catalyst. A catalyst is a chemical compound that speeds up the rate of some chemical reaction.

Equation, Chemical

A chemical equation is a shorthand method for representing the changes that take place during a chemical reaction.

Equilibrium, Chemical

Chemical equilibrium (plural equilibria) is a dynamic condition (meaning it is marked by continuous change) in which the rate at which two opposing chemical changes is the same. As an example, consider the reaction in which ammonia gas (NH3) is made from the elements nitrogen (N2) and hydrogen (H2).


Erosion is the general term for the processes that wear down Earth's surfaces, exposing the rocks below. The natural forces responsible for this endless sculpting include running water, near-shore waves, ice, wind, and gravity.


Europe is the world's sixth largest continent. Together with its adjacent islands, it occupies an area of about 4,000,000 square miles (10,360,000 square kilometers), roughly 8 percent of the world's land area.


Eutrophication (pronounced you-tro-fi-KAY-shun) is a natural process that occurs in an aging lake or pond as that body of water gradually builds up its concentration of plant nutrients. Cultural or artificial eutrophication occurs when human activity introduces increased amounts of these nutrients, which speed up plant growth and eventually choke the lake of all of its animal life.


Evaporation is the name given to the process in which a liquid is converted to the gaseous state. Everyone is familiar with the process of evaporation.


The term evolution in general refers to the process of change. For example, one can describe the way in which a section of land evolves over time.

Excretory System

The excretory system is a system of organs that removes waste products from the body. When cells in the body break down proteins (large molecules that are essential to the structure and functioning of all living cells), they produce wastes such as urea (a chemical compound of carbon, hydrogen, nitrogen, and oxygen).

Expansion, Thermal

Thermal expansion is the change in size of an object as its temperature changes. Normally, as the temperature increases, the size of an object also increases.