Fundamentals of Energy

Summary: Every source of energy is a manifestation of the four fundamental forces of physics: gravitational, electromagnetic, the weak, and the strong nuclear forces. The most important force for human-scale systems is the electromagnetic, which comes to Earth in the form of solar radiation and is stored as chemical energy by plants.

We often think of energy as a resource that makes life easier. It allows us to heat and light our buildings, power our machines and appliances, and move ourselves and goods around the globe. This conception of energy is quite close to the actual yet abstract definition of energy. Formally, energy is the capacity to do work, and it is conserved during any change in nature. Every time a living thing or matter changes in any way, energy flows from one location to another and changes from one form to another. All these changes—all physical interactions—result from four fundamental forces: the gravitational and electromagnetic forces and the strong and weak nuclear forces. These four fundamental forces are linked to the primary energy sources used to sustain and grow human civilization.

The

Particle physics describes all physical interactions, matter, and forces within the universe in terms of 16 elementary, subatomic particles. This theory is called the Standard Model of particle physics. Scientific work on the Standard Model is ongoing today, but it began with work by Sheldon Glashow, Steven Weinberg, Abdus Salam, and Peter Higgs in the 1960s.

The model divides the 16 elementary particles into three groups: quarks, leptons, and gauge bosons. The quarks come in three “flavor” pairs of positive and negatively charged particles, grouped according to their mass. The lightest are the up (u) and down (d), then the strange (s) and charm (c), and then the top (or sometimes truth, t) and bottom (or sometimes beauty, b) quarks. Within the nucleonic particles—the proton and neutron—quarks are bound in triplets, such that a proton is an uud triplet and a neutron is a ddu triplet. Quarks may also be bound as pairs in particles called mesons.

All of the particles have complementary antiparticles, which are exactly alike in all respects, except that they have the opposite electric charges; for example, the electron’s positively charged complement is called the positron. In all interactions, charge, spin, and mass-energy must be conserved.

Gravitational Force

Physical bodies attract with a force proportional to their mass. It is the force that causes things to fall to the ground on Earth. Galileo’s work during the scientific revolution (as P. Machamer notes), Sir Isaac Newton’s Principia Mathematica (written during the Enlightenment and appearing in 1687), and Albert Einstein’s modern work on general relativity (1916) led to our current understanding of the force of gravity. Gravity is mediated by a hypothetical gauge boson called the graviton. This boson works on the mass “charge” of all massive particles. The gravitational force is the weakest of all of the four fundamental forces; however, over cosmological distances the gravitational force is highly important as the only mediator. This is true because the range of the force is infinite, as is the electromagnetic, and most macroscopic objects are electrically neutral, meaning that the electromagnetic force is irrelevant over these large distances.

Electromagnetic Force

Electric charges and currents act as sources for electric and magnetic fields that permeate nature. James Clerk Maxwell’s equations formally describe electromagnetism and show that electricity, magnetism, and optics all fall under a consistent theoretical framework. The electromagnetic force is mediated by photons and acts on the electric charge of a particle. Since particles can have either positive or negative charge, the force can be either attractive, between particles of opposite charge, or repulsive, between particles of similar charge. This attractive and repulsive nature means that particles will tend to form larger electrically neutral groups by attracting oppositely charged and repelling similarly charged. For this reason, atoms, consisting of negatively charged electrons orbiting a central positively charged nucleus of protons and neutrons, tend to be electrically neutral in themselves, or combine to form electrically neutral molecules with other atoms of opposite charge. Finally, moving magnetic fields can induce electric currents and vice versa, as Michael Faraday realized. The phenomenon of induction is key to converting between mechanical energy and electromagnetic energy.

Weak Nuclear Force

The weak nuclear force, so called because its relative strength and effective distance are much less than those of the strong nuclear force, acts to change the flavor of fermions within their pair groupings. For example, a down quark may be changed to an up quark, thereby emitting a W boson (to conserve electric charge, the boson must be negatively charged). The W boson will then normally form an electron and antielectron neutrino pair (again, the negative charge is conserved). The process described is the decay of a neutron (udd) to a proton (uud) with the emission of beta radiation (an electron and antielectron neutrino pair) and occurs spontaneously with very high probability.

Strong Nuclear Force

The strong nuclear force governs interactions between the quarks within a bound pair or triplet. It is mediated by gluons and acts on the “color charge” of quarks—red, blue, or green—which must be conserved in any interaction. Bound triplets or pairs must be color-neutral, by combining either red, blue, and green in a triplet, or forming color-anti-color pairs. The force has a range of 10–15 meters (about the scale of an atom) and has the unusual property of becoming stronger with distance, which will cause any force attempting to separate quarks, if strong enough, to produce more quarks from the vacuum.

Balancing Forces in the Nucleus

Because of the existence of electrically positively charged protons within the nucleus, there exists an incredibly strong repulsive electromagnetic force. This is balanced within the nucleus by the existence of neutrons, which have no electrical charge but add to the attractive strong nuclear force. The elemental type of nucleus (whether that of hydrogen, helium, or another element) is governed by the number of protons; for example, carbon has six protons. Isotopes of elements have the same number of protons but different numbers of neutrons. Some isotopes are more stable than others; for instance, carbon-14 (with six protons and eight neutrons) is radioactive, whereas carbon-12 is stable. When the number of protons and neutrons within the nucleus allows balance between the strong nuclear and electromagnetic forces, the nucleus is stable.

For large nuclei, since the strong nuclear force acts over only a very short range, more neutrons are required to counteract the electromagnetic force. This means that the balance between protons and neutrons is in favor of neutrons. However, neutrons have a high probability of decaying into protons. If this occurs, the nucleus is unstable and balance is restored by the emission of an alpha particle (a helium nucleus of two protons and two neutrons). In smaller nuclei, the balance between protons and neutrons must be equal. If there are more protons (as is the case in radioactive carbon-14), the electromagnetic force is too strong and the balance is restored by the decay of a proton (via the weak nuclear force) into a neutron and the emission of an beta particle (in this case, in the form of a positron—an anti-electron). If the balance is in favor of neutrons, a neutron will decay into a proton, given the high probability of this occurrence with the emission of a beta particle, in this case in the form of an electron (as above). When a nucleus is in a high-energy state, it may emit gamma radiation (a very high-energy, short-wavelength photon), which will not alter the composition of protons or neutrons.

Within stars, the presence of many protons (hydrogen nuclei) at huge pressures and temperatures results in many collisions between particles. If these collisions can overcome the electromagnetic repulsion between protons, helium nuclei (two protons and two neutrons) may be formed (in a process called fusion) via the decay of two protons into neutrons (the weak nuclear pathway) and the emission of beta particles and neutrinos. The helium nuclei represents a lower energy state than the four protons individually, and hence energy is released.

Relationship Between Spatial Scale and Dominating Force

Both of the nuclear forces have a limited spatial range, which limits their influence to interactions on the subatomic scale. The electromagnetic and gravitational forces both have an infinite range and so have effects on macro scales. At large spatial scales, objects tend to form electrically neutral combinations, such that the gravitational force dominates. The rest of this article shall deal primarily with the electromagnetic and gravitational forces.

Energy Fields and Potentials

Although all the forces are described in terms of the interaction of particles, the electromagnetic and magnetic forces may be more usefully thought of in terms of fields whereby the presence of charges (positive and negative electric charges in the case of the electromagnetic and mass in the case of gravitational force) give rise to a field with a potential gradient, which then influences the behavior of other objects within that field. The classic example is of massive objects deforming the gravitational field, likened to a bowling ball placed on a rubber sheet. The sheet bends such that other objects will be pulled toward the bowling ball. The magnitude of the force between the objects is a function of both their mass and their separation. Despite its smaller radius, the surface gravity of the moon is about one-sixth that of the Earth because of its smaller mass. In the case of the electromagnetic force, there are both attractive and repulsive aspects, such that the presence of a negative charge will pull in (or be pulled toward) other positive charges and push away (or be pushed away by) other negative charges.

Linking Forces to Energy

All of the energy sources currently exploited (and some potential sources for the future) are manifestations of the four forces. The US Energy Information Administration lists energy sources, in order of use from greatest to least, as oil (both conventional and unconventional sources), natural gas (again coming in both conventional and unconventional sources), coal, nuclear energy, hydroelectric power, biomass, wind, geothermal power, and solar power (both solar thermal and photovoltaic, or PV). Energy sources under research and development include tidal energy, wave energy, and ocean thermal energy conversion (OTEC).

The electromagnetic force governs not only electric and magnetic but also chemical interactions. The combustion of a fuel (both fossil fuels and biofuels) is entirely describable in terms of the creation and destruction of electronic (in the sense of electrons) bonds between molecules. As such, this force governs any of the energy sources that are described in terms of thermodynamics, including all of the fossil fuels, biomass, solar thermal power, and geothermal power (although geothermal power can be said to be a result of gravitational energy, both because it is heat energy left over from the cooling of the planet and the interaction of radioactivity—hence the weak nuclear force—and gravity as the heating and convection of material through the Earth’s mantle). The electromagnetic force also governs photovoltaic interactions, whereby light is converted directly into electricity.

Within the hydrologic cycle, incoming energy from the sun—in the form of infrared (IR), visible, ultraviolet (UV), and X-ray photons, all manifestations of the electromagnetic force—evaporate water, which then rises through the atmosphere. After the water vapor condenses and collects (again, a manifestation of the electromagnetic force), it falls out of the sky as precipitation, which then forms streams and rivers through which it flows to the ocean. This downward leg is governed entirely by the gravitational force. Hydropower makes use of the work done by the sun in lifting vast amounts of water in order to power turbines. The amount of energy available is a function of the distance the water falls (called the head), the gravitational pull of the Earth, and the amount of water that flows.

Geothermal energy is the slow release by the Earth of heat generated by radioactive decay of elements within the Earth’s mantle. The emission of these high-speed alpha and beta particles and gamma radiation thermally excites the surrounding material. If these particles are also unstable, this may cause further radioactive decay.

Nuclear energy from fission makes use of elements with large nuclei for fuel. The fuel nuclei are bombarded with neutrons, which make the nucleus unstable. The nucleus undergoes fission by splitting into smaller nuclei and emitting up to three neutrons. These neutrons are utilized to generate heat and to enable the fission of more fuel nuclei. The heat generated is used to power a turbine and generate electricity, much like in a fossil fuel power station. The fuel most often used for nuclear power generation is uranium-235. Since this element makes up less than 1 percent of naturally occurring uranium (the bulk being made of the more stable isotope uranium-238), the stable isotope is usually enriched to increase the concentration of U-235 to enable the fission process.

Research is under way to replicate fusion. Different fuel pathways are being investigated, including deuterium (a hydrogen-2 nucleus with one proton and one neutron) and tritium (a hydrogen-3 nucleus) collisions. The positively charged deuterium or tritium fuels are squeezed by large magnetic fields to induce fusion to occur, and the heat thereby generated is used to turn a turbine and generate electricity. If the electrical energy produced is greater than the energy required to produce the magnetic fields (via electromagnets), then the process offers positive net energy.

Tidal forces are also a manifestation of the gravitational force. Water on the Earth closest to the moon is acted upon more strongly by the pull of the moon than the bulk of the Earth. Water on the far side from the moon feels the least pull, such that there are two bulges in the water, one toward and one away from the moon. This gives rise to two tides per day as the Earth rotates through these two bulges. The sun also has an effect on the tide (about one-third that of the moon), meaning that tides are increased when the three bodies (sun-Earth-moon) are aligned, such as at full moon (called a spring tide), and decreased when a right angle is formed, such as at new moon (called a neap tide). In certain places on the Earth, the topology of the land acts to amplify the effects of the tide, leading to large tidal ranges—as much as 32.8 feet (10 meters) in some locations.

Wind energy exploits differential (electromagnetic) heating of the Earth’s surface by incoming solar radiation. As air over a “hot” region rises, it pulls in air from the surrounding regions. This is particularly noticeable in coastal regions because of the different thermal mass of land and water. During the day, solar radiation increases the temperature of the land more than the water, creating an onshore breeze. After sunset, the land loses heat more quickly, creating an onshore breeze. Differential heating between the equator and the poles also creates global weather systems. These are added to by the Coriolis effect, caused by the rotation of the Earth.

Solar energy generation comes in two types: that using the direct insolation from the sun to generate heat (and sometimes thereby to generate electricity) and photovoltaic (PV) panels, which generate electricity directly. Thermal solar comes in a number of configurations, from small, flat panels for generating low-grade heat (generally less than 100 degrees Celsius for residential or commercial settings) to large-scale arrays of concentrating troughs, dishes, or mirrors focused onto a central tower. PV cells are made from a number of semiconductive materials (most often high-purity silicon), which conduct electrons when exposed to photons of a particular wavelength.

Wave energy exploits the energy carried in waves to operate a mechanical system to generate electricity. There are a wide variety of designs in development. Surface waves are generated when the wind moves across the surface of a body of water. Once small waves have been created, the effect may be amplified by the differential Bernoulli effect between the moving air and the peaks and troughs of the waves. Due to the greater density of water, the power density of waves is often greater than that of wind. Ocean thermal energy conversion (OTEC) exploits the difference in temperature between the surface water (often around 20 degrees Celsius in the tropics) and the cold, deep water (typically that greater than 0.62 mile, or 1 kilometer deep, which may be near 0 degrees Celsius). This temperature range may be used to drive a heat engine and generate electricity.

SourcePercentagePetroleum37Natural gas25Coal21Nuclear9Hydroelectric2.5Wood2Biofuels1.8Wind0.9Waste0.5Geothermal0.2Solar 0.1* Based on a total of 107 EJ

Exploiting Energy Gradients

In all cases, extracting energy from the environment requires exploitation of a naturally occurring energy gradient. These occur because of (sometimes minute) differential conditions between regions in the universe. The Sun is obviously a great source of high-quality, low-entropy energy, which floods the Earth. This has given rise to plants, which exploit this resource, and animals, which feed on plants. Plants store solar energy within their structure as highly ordered carbon-oxygen bonds that are far from chemical equilibrium with the surrounding atmosphere. Combustion (of fossil fuels and biofuels) breaks these bonds to release the energy stored. Differential heating of the land and oceans can be exploited to drive turbines. Hydropower and tidal power exploit gravitational gradients. Geothermal power exploits naturally occurring thermal gradients, and nuclear power exploits instability in the balance between electromagnetic and strong nuclear forces within the nuclei of radioactive fuels.

Relative Comparability of Energy Sources

Energy sources are far from equivalent in the service they provide. Obviously, some sources are replenished on the timescale at which they are being used by humanity, and as such they are renewable. Sources that are exploited more quickly than the timescale at which they are replenished are said to be nonrenewable. These include the fossil fuels (coal, oil, and gas) and fissile nuclear fuels. Some energy sources represent stores of energy to be used as and when required; others are intermittent and must be supplemented by means of storage, if so desired.

Net Energy Analysis

Energy is also required for the extraction and conversion of energy. Energy requirements may include direct inputs of energy (the fuel needed to power an oil rig) but also the energy embodied in any materials used (the energy required to manufacture the oil rig). If an energy source produces more energy than is used in its delivery, the delivery process is said to have a positive net energy. The ratio of the energy delivered by a process to the energy required in its delivery is called variously the net energy ratio, or NER, the energy-return-on-(energy)-investment, or ERO(E)I, and the energy yield ratio, or EYR, which may vary greatly among energy sources.

Energy sources differ also in their energy density, either by unit mass (megajoules divided by kilograms, or MJ/kg) or by unit volume (megajoules divided by cubic meters, or MJ/m³). Nuclear fuels offer the greatest energy densities, followed by oil, gas, and coal, and then by biomass. Renewable energy sources may be ranked in terms of their power density (megajoules divided by square meters, or MJ/m²).

and Energy Quality

Not all forms of energy are of equal use to humankind. An equal amount of energy enters the Earth’s atmosphere in the form of insolation as leaves it in the form of “waste” heat; however, the incoming solar radiation is of much higher quality (is more usable) than the outgoing infrared radiation. A measure of energy quality, exergy is the amount of work that may be extracted as a system comes into equilibrium with its surroundings.

The exergy of 100 joules of low-temperature steam is much lower than the exergy of 100 joules worth of oil. Since most produce electricity directly, renewable energy sources are often rated in terms of the electricity that they produce. A joule of electricity has an exergy of 1 joule, since conversion of electricity to work is presumed to be completely efficient. Fossil fuels are often rated in terms of their higher (or lower) heating values (HHV or LHV), which have a lower exergy due to inefficiencies in conversion to work. The exergy perspective is important, since many of the economic roles of energy are in its ability to produce work, as opposed to heat, directly.

Conclusion

Whenever nature undergoes a change, energy transforms from one form to another and is transferred from one location to another or exchanged between material objects. These physical interactions are mediated by one or more of the four fundamental forces: electromagnetism, gravitation, weak-nuclear, and strong-nuclear. In 2024, the United States obtained about 60 percent of its primary energy from fossil fuel sources: petroleum, natural gas, and coal. It follows that this energy system is dominated by the electromagnetic force, because the chemical energy present in these hydrocarbons are a manifestation of the electromagnetic force. When these chemical bonds are broken to form stronger bonds with oxygen, infrared radiation is released to heat engine cycles that drive turbines and dynamos, which generate electricity—again, thermodynamic phenomena driven by electromagnetic forces. Society also utilizes the gravitational force to harness the flow of water, whose kinetic energy was obtained from gravitational potential energy. Fundamentally, research in clean energy technologies—turbines, solar PV, energy storage, fission, and fusion—focuses on harnessing the four forces in a manner that permits safe, environmentally responsible, economically viable, and thermodynamically efficient conversion of various forms of energy into electrical energy for society’s use.

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