Energy

Energy comes from an Ancient Greek word meaning 'activity'. It is a quantitative property that can be passed to a body or to a physical system. We can notice energy when something can do work, or as heat and light.

Energy is a conserved quantity. This means it does not just disappear. The law of conservation of energy says that the total energy of a closed system stays the same, although it can be converted from one form to another. Scientists measure energy in the International System of Units (SI) using the joule (J).

There are many forms of energy. One is the kinetic energy of a moving object. Another is the potential energy an object stores because of where it is, such as its position in a field. Energy can also be stored in a solid object that is stretched or squeezed, which is called elastic energy. There is also chemical energy, which is linked to chemical reactions. Other forms include radiant energy carried by electromagnetic radiation, the internal energy inside a thermodynamic system, and rest energy linked to an object's rest mass. More than one of these forms can be present at the same time.

All living organisms constantly take in and release energy. The Earth's climate and ecosystems are mainly driven by radiant energy from the Sun.

Forms

The total energy of a system can be sorted into potential energy and kinetic energy, or into mixes of the two. Kinetic energy comes from the movement of an object, or from the composite motion of its parts. Potential energy is the energy an object has to move. It depends on where the object is within a field, or on what is stored in the field itself.

These two types are enough to describe all energy. Still, it is often easier to give certain mixes their own name. For example, the total of rotational movement energy and stored energy in a system is called mechanical energy. Nuclear energy is the stored energy inside an atom's nucleus. It comes from the nuclear force or the weak force, among other things.

History

Main articles: History of energy and timeline of thermodynamics, statistical mechanics, and random processes

The word energy comes from the Ancient Greek word energeia, meaning 'activity' or 'operation'. It may first have appeared in the work of Aristotle in the 4th century BC. At that time, it was a broad idea. In the late 1600s, Gottfried Leibniz proposed the idea of vis viva, or living force, which is close to today's kinetic energy. Later, Émilie du Châtelet proposed the idea of conservation of energy. In 1807, Thomas Young was possibly the first to use the word "energy" in its modern sense. In 1845, James Prescott Joule discovered the link between mechanical work and heat. These ideas helped build the field of thermodynamics, which was largely formalized by Lord Kelvin.

In 1905, Albert Einstein's theory of special relativity showed that mass can be turned into energy. This means that the total amount of mass and energy stays the same. Early in the 1900s, Niels Bohr proposed that energy in atoms comes in set levels. In 1926, Erwin Schrödinger and Werner Heisenberg developed a modern theory to explain these energy levels.

Units of measure

Main article: Units of energy

Energy can be measured in different ways. Scientists use dimensional analysis to describe the base units of energy. You can find energy by multiplying a force by a distance. In the International System of Units (SI), the unit of energy is the joule, which is a derived unit. One joule is the energy needed to push with a force of one newton over a distance of one meter.

Power is how much energy is used in a certain amount of time. Its SI unit is the watt, which equals one joule per second. A kilowatt-hour (kWh) is the energy delivered by one kilowatt of power for one hour. It equals 3.6 million joules. The CGS unit of energy is the erg. The imperial and US customary unit is the foot-pound. Other units, such as the electronvolt, food calorie, thermodynamic kilocalorie, and BTU, are used in certain areas of science and trade.

Scientific use

Main articles: Mechanics, Mechanical work, and Thermodynamics

Main article: Chemical energy

Main articles: Bioenergetics and Food energy

Main article: Metabolism

In classical mechanics, energy is a helpful idea because it is a conserved quantity. This means the total amount stays the same in a closed system, even though it can change form. Work is a form of energy. Whether work is done can depend on the reference frame of the observer. In chemistry, energy is linked to the structure of substances, and chemical reactions usually involve energy changes. Reactions that release energy are called exothermic, and endothermic reactions are the opposite. In biology, all living things need energy to grow and survive. Plants capture sunlight through photosynthesis, and animals get energy from food through cellular respiration. The molecule adenosine triphosphate (ATP) is the main energy carrier in living cells.

In geology, volcanoes, earthquakes, and mountain ranges are explained by energy changes inside the Earth. Weather, such as hurricanes and tornadoes, comes from energy from the Sun. In cosmology and astronomy, stars, supernova, and black holes are some of the universe's most powerful energy changes. In quantum mechanics, energy is described using the energy operator. In relativity, Einstein found that every massive body has rest energy. This shows that energy and mass are two forms of one underlying property.

Transformation

Energy can be transformed from one form to another. Each change has its own efficiencies. Devices that usefully change energy from one form to another are called transducers. A battery changes chemical energy into electric energy. A dam changes gravitational potential energy into the kinetic energy of water that spins a turbine. That energy then becomes electric energy through an electric generator. A heat engine changes heat into work. The Sun changes nuclear potential energy into other forms, mostly radiant energy. There are strict limits on how well heat can be turned into work. These limits are described by Carnot's theorem and the second law of thermodynamics. The direction of energy changes is often decided by entropy, which is the tendency of energy to spread evenly among all available degrees of freedom.

Energy that has been stored since the Big Bang is released when a trigger is available. One example is nucleosynthesis, which used energy from the gravitational collapse of supernovae to form heavy elements such as uranium and thorium. Their stored energy is released by nuclear decay, which powers fission bombs and civil nuclear power. A chemical explosion changes chemical potential energy into kinetic and thermal energy in a very short time. A simple pendulum swings back and forth as its energy changes between potential and kinetic forms. Without friction or other losses, it would keep swinging forever. In an isolated system, energy cannot be created or destroyed, so the starting and final energy are equal.

Albert Einstein derived the formula E = mc² in 1905. It shows mass–energy equivalence between relativistic mass and energy. J. J. Thomson, Henri Poincaré, and Friedrich Hasenöhrl also derived similar formulas (see Mass–energy equivalence § History). Because the speed of light squared is so large, a small amount of mass can release huge amounts of energy, as seen in nuclear reactors and nuclear weapons. Large changes between matter and energy are studied in nuclear physics and particle physics. Matter can change fully into light when it meets antimatter. Thermodynamics also divides changes into irreversible processes, where energy spreads out and cannot be fully recovered, and reversible ones. As time passes, more energy becomes trapped in irreversible states. This has led to the idea of the heat death of the universe.

Conservation of energy

Main article: Conservation of energy

Energy cannot be created or destroyed. This idea is called the law of conservation of energy. In the first law of thermodynamics, the energy of a closed system stays the same unless energy moves in or out as work or heat. No energy is lost when it moves. In heat engines, the second law of thermodynamics says that a working system always loses some energy as waste heat. This sets a limit called the available energy. Energy can change from mechanical forms into thermal energy without this limit. In a 1961 lecture, Richard Feynman explained that there is a quantity called energy that does not change, even as nature changes. He shared this idea in The Feynman Lectures on Physics.

Most kinds of energy follow strict local rules. Energy can only move between nearby regions of space. The total energy of the universe also cannot change. As shown by Noether's theorem, this comes from translational symmetry of time. This means that yesterday, today, and tomorrow are physically the same. Energy is linked to time as a canonical conjugate. This link leads to the uncertainty principle, which is different from energy conservation. Every basic force has its own type of potential energy. Energy also adds to the mass of a system. In quantum mechanics, energy is written using the Hamiltonian operator. Its uncertainty is similar to the Heisenberg uncertainty principle, but the two are not the same. This is because energy and time are not conjugate variables. In particle physics, virtual particles, which carry momentum, help create the fundamental forces, also called fundamental interactions. Virtual photons help cause the force between electric charges described by Coulomb's law, spontaneous radiative decay, the Casimir force, and the Van der Waals force.

Energy transfer

Energy transfer can be studied in closed systems, which do not exchange matter. Some energy moves when conservative forces act over a distance. This moved energy is called work when it is done by one system on another. The rest of the energy, which does no work, is called heat. Energy can move in many ways. It can travel as electromagnetic energy carried by photons, pass between objects that bump into each other as kinetic energy, move through tidal interactions, or spread through touching materials as thermal energy. Energy is always conserved, which means the total amount never changes. In thermodynamics, the first law describes this transfer for closed systems.

Heat can sometimes be ignored. This happens in fast processes involving gases, which are poor conductors of heat, or when the thermal efficiency of the transfer is high. Such adiabatic processes are simpler to describe.

This simpler method is used to define the joule, for example. Beyond closed systems, open systems can gain or lose energy when matter moves in or out. A car engine, for example, gains energy when a mixture of air and fuel is put into it. It does not need any added work or heat for this to happen.

Thermodynamics

Internal energy is the total of all the tiny forms of energy inside a system. It is the energy needed to create that system. It includes potential energy, which comes from things like how molecules and crystals are arranged, and the kinetic energy of moving particles. Thermodynamics mainly looks at how internal energy changes, not its exact value, because that value cannot be found on its own. The first law of thermodynamics says that the total energy of a system and its surroundings always stays the same. It also says that heat flow is one way energy moves. Adding heat to a system and doing work on it both change its internal energy. This simple version leaves out chemical, electrical, nuclear, and gravitational effects.

A mass on a spring, called a harmonic oscillator, keeps switching between kinetic and potential energy. Over many cycles, its energy is shared about equally between the two on average. This is an example of the equipartition principle, which says that the energy of a system with many parts is shared equally among them on average. Entropy measures how evenly energy is spread out in a system. This idea is important for the second law of thermodynamics. The second law is simple only for systems near a balanced equilibrium state. For systems that are not balanced, the rules are still debated. One guiding idea is the principle of maximum entropy production.