Introduction of energy

The etymology of the term is from Greek ενέργεια, εν- means "in" and έργον means "work"; the -ια suffix forms an abstract noun. The compound εν-εργεια in Epic Greek meant "divine action" or "magical operation"; it was later used by Aristotle in a meaning of "activity, operation" or "vigour", and by Diodorus Siculus for "force of an engine

Energy is a fundamental concept in physics that is often defined as the capacity to do mechanical work — a definition that is contested by many physicists (see below). The concept of energy has applications throughout the natural sciences.

Energy is subject to a strict local conservation law. That means energy cannot be created or destroyed. The only way the energy content of a given region can change is by the flow of energy to or from adjacent regions. There is also a global law of energy conservation, which says that the total energy of the universe cannot change. The global law is a corollary of the local law (and not vice versa). Conservation of energy is related to a symmetry of the laws of physics, namely invariance with respect to a shift in time, via Noether's theorem.

In classical physics (pre-20th-century), energy was considered a scalar quantity, having no direction in space. In special relativity energy is also a scalar and is one component of more general quantity - energy-momentum 4-vector (so energy is associated with the timelike direction).[3] To say the same thing another way, energy is invariant with respect to spacelike rotations, but not invariant with respect to boosts.

The total energy of a system can be subdivided and classified in many ways. For example, it is sometimes convenient to distinguish kinetic energy from potential energy. It may also be convenient to distinguish gravitational energy, electrical energy, thermal energy, et cetera. These classifications overlap; for instance thermal energy is usually partly kinetic and partly potential energy.

It comes in different forms -- heat (thermal), light (radiant), mechanical, electrical, chemical, and nuclear energy. Energy is in everything. We use energy to do everything we do, from making a jump shot to baking our favorite cookies to sending astronauts into space -- energy is there, making sure we have the power to do it all. There are two types of energy -- stored (potential) energy and working (kinetic) energy. For example, the food you eat contains chemical energy, and your body stores this energy until you release it when you work or play. Learn more about these different forms of energy.

All forms of energy are stored in different ways, in the energy sources that we use every day. These sources are divided into two groups renewable (an energy source that can be replenished in a short period of time) and nonrenewable (an energy source that we are using up and cannot recreate in a short period of time). Renewable and nonrenewable energy sources can be used to produce secondary energy sources including electricity and hydrogen.

Renewable energy sources include solar energy, which comes from the sun and can be turned into electricity and heat. Wind, geothermal energy from inside the earth, biomass from plants, and hydropower and ocean energy from water are also renewable energy sources.

Renewable energy is energy derived from resources that are regenerative or for all practical purposes cannot be depleted.[Renewable energy sources contribute approximately 29.3% of human energy use worldwide. The prime source of renewable energy is solar radiation, i.e. sunlight. The Earth-Atmosphere system supports approximately 5.4 x 1024 joules per year in the solar radiation cycle (Sorensen, 2004).

Mankind's traditional uses of wind, water, and solar power are widespread in developed and developing countries; but the mass production of electricity using renewable energy sources has become more commonplace only recently, reflecting the major threats of climate change due to pollution, exhaustion of fossil fuels, and the environmental, social and political risks of fossil fuels and nuclear power. Many countries and organizations promote renewable energies through taxes and subsidies.

However, we get most of our energy from nonrenewable energy sources, which include the fossil fuels oil, natural gas, and coal. They're called fossil fuels because they were formed over millions and millions of years by the action of heat from the Earth's core and pressure from rock and soil on the remains (or "fossils") of dead plants and animals. Another nonrenewable energy source is the element uranium, whose atoms we split (through a process called nuclear fission) to create heat and ultimately electricity.

We use all these energy sources to generate the electricity we need for our homes, businesses, schools, and factories. Electricity "energizes" our computers, lights, refrigerators, washing machines, and air conditioners, to name only a few uses.

We use energy to run our cars and trucks. Both the gasoline used in our cars, and the diesel fuel used in our trucks are made from oil. The propane that fuels our outdoor grills and makes hot air balloons soar is made from oil and natural gas.