术语表 Glossary ================== .. glossary:: :sorted: EIA Energy Information Administration. Watt \(W) is a unit of :term:`power`, defined so that 1 W is 1 J/s (one :term:`Joule` per second). power is the rate of energy, or change in energy per change in time. The units are :term:`Joules` per second (J/s), or :term:`Watts` (W). Joule \(J) is the SI unit of work or energy, and is equivalent to :term:`Newtons` times meters (N·m), or kg · m\ :sup:`2`/s\ :sup:`2` energy is defined as the capacity to do work. The SI unit is the :term:`Joule`. Newton \(N) is the SI unit of force, and is equivalent to kg · m/s\ :sup:`2` work is a mechanical expression of energy, defined as a force (:term:`Newtons`) times distance (meters) through which the force acts (along the same direction). The resulting unit is the :term:`Joule`. Big Bang is the name given to the start of the universe, about 13.8 billion years in the past. infrared radiation is the property that all objects glow in light, or :term:`electromagnetic radiation`. For objects that are not "red hot," the emission is invisible to the human eye, at longer :term:`wavelengths` than the visible spectrum. The :term:`power` radiated obeys the :term:`Stefan-Boltzmann law`. electromagnetic radiation refers to any transport of energy by electromagnetic waves, which include light, ultraviolet, infrared, X-rays, microwaves, gamma rays, and radio waves. Stefan-Boltzmann law says that the :term:`power` emitted from a surface of area, 𝐴, and temperature, 𝑇 will be 𝑃 = 𝐴𝜎𝑇\ :sup:`4`, where 𝜎 is the :term:`Stefan-Boltzmann constant`. Stefan-Boltzmann constant (𝜎) has a value of 5.67 × 10\ :sup:`-8` W/m\ :sup:`2`/K\ :sup:`4` and is used in the :term:`Stefan-Boltzmann law` relating to :term:`infrared radiation`. greenhouse gas (GHG) absorbs infrared radiation and acts as a thermal blanket in a planetary atmosphere. H\ :sub:`2`\ O, CO\ :sub:`2`, and CH\ :sub:`4` are powerful greenhouse gases. energy intensity measures the energy use of a society relative to its economic scale. A typical value may be about 5 MJ/$. decoupling is the notion that economic activities need not incur a large energy or resource cost, breaking the tendency for economic scale to be tightly coupled to physical goods. substitution refers to interchangeability between goods and services, so that an unavailable or inferior resource can be replaced by an alternative, possibly superior one. CFL compact fluorescent light. LED light emitting diode. fossil fuel refers to an energy source buried in the ground, in the form of coal (solid), petroleum (liquid), or natural gas (gaseous). Fossil fuels represent ancient solar energy captured in living matter, processed and stored underground over millions of years. doubling time is how long it takes a system or collection to double its amount under conditions of growth, such as in :term:`exponential growth`. exponential growth happens when the rate of growth—as a percentage or fraction—is constant. Jevons paradox is named after early economist William Stanley Jevons, and describes the backfire of efficiency improvements leading to *increased* usage of the associated resource due to greater demand for the more attractive, efficient technology. Also called the :term:`rebound effect`. rebound effect describes the counterintuitive process by which efficiency improvements lead to greater use of the resource as the enhanced appeal and lower cost results in more widespread adoption and use. Also called the :term:`Jevons paradox`. GDP Gross Domestic Product, effectively representing the total monetary flow of goods and serviceswithin a society, typically over a one year period. hockey stick is a term used to describe plots that suddenly shoot up after a very long time of relative inaction. Plots of human population, atmospheric CO\ :sub:`2`, energy use, all tend to show this 2 characteristic—which resembles an exponential curve. inverse function is a mathematical operation that "undoes" its counterpart, like the square root undoes the square, or the natural logarithm undoes the exponential. Gppl is a short-hand unit for giga-people, or billion people. negative feedback involves a reaction to some stimulus in the direction opposite the stimulus, performing a corrective action and leading to stability. Systems in equilibrium must have negative feedback keeping them there. carrying capacity refers to the limiting population that can be supported long-term by the environment. No consensus exists for Earth's carrying capacity for humans, though standards of living have a large influence. differential equation is an equation that relates functions and their derivatives. The subject is often sequenced after calculus within a curriculum. logistic describes a mathematical model in which rate of growth depends on how close the population is to the :term:`carrying capacity`. The resulting population curve over time is called the logistic function, or more informally, an S-curve. overshoot occurs when the :term:`negative feedback` in a system is delayed. After surpassing the equilibrium, oscillation may ensue. Green Revolution refers to the modernization of agricultural practices worldwide beginning around 1950, when fossil fuels transformed both fertilization and mechanization. demographic transition refers to the process in which an undeveloped country initially having high birth rate and high death rate transitions to low death rates followed by low birth rates as medical and resource conditions improve. terraforming is the speculative idea of transforming the atmosphere and environment of a planet hostile to human life into one that is suited to human needs. heat loss rate as used in this book is the power per Δ𝑇 (in ◦C) required to maintain a temperature differential. Units are W/◦C, and typical houses might be a few hundred W/◦C. electromagnetic spectrum refers to the sweep of wavelengths or frequencies of :term:`electromagnetic radiation`, including light, ultraviolet, infrared, X-rays, microwaves, gamma rays, and radio waves. photon is the smallest indivisible particle of light: a minimum quantum packet of energy. Each photon has a well defined energy, which can also be expressed as a :term:`wavelength` or :term:`frequency`. wavelength measures the length of a wave from crest to crest or trough to trough, and can apply to waves in water, air (sound), or electromagnetic waves (light). The symbol 𝜆 (lambda) is often used to denote wavelength. The units are length (m), often expressed in :term:`microns` (𝜇m). micron (𝜇m) is 10\ :sup:`−6` meters, or a micro-meter. frequency characterizes the number of cycles per second in a periodic phenomenon (often in wave phenomena). The units are Hertz, or 1/s. R/P ratio or reserves-to-production ratio is a means to assess time remaining for a resource of quantity R units, being used (produced) at a rate of P units per year. The result is years available at the present rate, absent discovery of additional resources or change in rate of use. Annual Energy Review AER is compiled by the U.S. :term:`EIA`, capturing energy use and trends for all sources and sectors. British thermal unit Btu is a unit of energy in the Imperial unit system, defined as the amount of energy required to heat one pound of water by 1 °F. It is equivalent to 1,055 :term:`Joules`. SI Système International. thermal equivalent is a construct used to compare :term:`thermal energy` sources like coal, oil, and natural gas to sources like solar, wind, and hydroelectricity, which do not derive from thermal sources. Usually in the context of electricity production, multiplying by about 2.7 puts non-thermal sources into thermal-equivalent terms. kinetic energy is the energy of motion, given by 1/2 𝑚𝑣\ :sup:`2` for a mass, 𝑚, at velocity, 𝑣. If input units are kg and m/s, the resulting unit will be :term:`Joules`. calorie (cal) is a unit of energy, defined as the amount of energy required to heat one gram (1 g, 1 mL, 1 cm\ :sup:`3`) of water by 1 ℃. It is equivalent to 4.184 :term:`Joules`. heat capacity is the amount of energy it takes to raise an object's temperature by 1◦C. The specific heat capacity is the heat capacity divided by mass, becoming an intrinsic property of the material. Water's specific heat capacity is 4,184 J/kg/℃, intimately tied to the definition of the kilocalorie. kilowatt-hour (kWh) is a unit of energy, constructed as a power (kilowatts) times time (hours). It is equivalent to 3,600,000 :term:`Joules`, or 3.6 MJ. Therm is a unit of energy defined as 100,000 :term:`British thermal units`, and is equivalent to 1.055 × 10\ :sup:`8` :term:`Joules`. voltage is a measure of electric potential energy, expressed in units of :term:`Volts`. Volt (V) is a unit of voltage, or electric potential, and can be thought of as the electrical analog to gravitational potential energy, and is also somewhat like pressure in a fluid system. charge is a measure of the degree to which a particle or object is influenced by electromotive forces. Electric charge can be positive or negative, so that like charges repel and opposites attract. The unit for electric charge is the :term:`Coulomb`. Coulomb (C) is the SI unit of electric charge. An :term:`electron` has a charge of −1.6 × 10\ :sup:`-19` C and a proton has a charge of +1.6 × 10\ :sup:`-19` C current is a measure of charge flow, expressed in the SI unit of :term:`Amps`. Ampere Amp (A, or Amp) is the :term:`SI` unit of :term:`current`, defined such that one Ampere is the same as one :term:`Coulomb` per second (1 A = 1 C/s) AC alternating current. Planck's constant is a fundamental constant of nature associated with quantum mechanics and the world of the very small. In :term:`SI` units, its value is h = 6.626 × 10\ :sup:`-34` J · s electron-volt (eV) is a unit of energy, defined as the energy (work) it takes to push a charge of one fundamental charge unit (see entry for :term:`Coulomb`) through an electric potential of one Volt. 1 eV is equivalent to 1.6 × 10\ :sup:`-19` :term:`Joules`. mole is a number of atoms or molecules, tuned so that one mole of the carbon-12 :term:`isotope` is exactly 12.000 grams. It takes 6.022 × 10\ :sup:`23` atoms for this to happen, which is called :term:`Avogadro's number`. Avogadro's number is 𝑁\ :sub:`A` = 6.022 × 10\ :sup:`23`, pertaining to one :term:`mole` of particles (e.g., atoms, molecules). isotope is what we call atoms that have various nuclear configurations for the same element. That is, variants of a :term:`nucleus` having the same number of :term:`protons` but differing numbers of :term:`neutrons`, and therefore differing mass number. nucleus is at the center of an atom, composed of :term:`protons` and :term:`neutrons` and spanning ∼ 10\ :sup:`-15` m. The vast majority (99.97%) of an atom's mass is in the positively charged nucleus, which attracts a cloud of negative-charge :term:`electrons` to complete the neutral atom. electron is a fundamental particle typically found in the outer parts of atoms, surrounding the :term:`nucleus`. Electrons have negative charge equal and opposite to that of :term:`protons`, but are 1,836 times lighter than the proton, at 0.511 MeV. proton is one of two basic building blocks of atomic nuclei, the other being the :term:`neutron`. Protons have positive charge, equal and opposite to that of the :term:`electron`. Protons have a mass of 938.272 MeV, or 1.0072765 :term:`a.m.u.`. Protons are made up of three quarks: 2 up and 1 down. neutron is one of two basic building blocks of atomic nuclei, the other being the :term:`proton`. Neutrons have no electric charge, and a mass of 939.565 MeV, or 1.008665 :term:`atomic mass unit` (a.m.u.). Neutrons are made up of three quarks: 1 up and 2 down. atomic mass unit a.m.u. is defined so that a single neutral carbon atom, consisting of 6 :term:`protons`, 6 :term:`neutrons`, and 6 :term:`electrons` has exactly 12.00000 a.m.u. In other units, it is 931.4941 MeV or 1.66054 × 10\ :sup:`-27` kg. This unit sometimes goes by the name: Dalton. geothermal refers to thermal energy within the earth, both from the original heat of formation and from :term:`radioactive decay`. radioactive decay involves a change in the nucleus of an atom, most commonly in the form of :term:`alpha decay`, :term:`beta decay`, or :term:`gamma decay`. thermal energy is the energy of heat, and is really just randomized :term:`kinetic energy` (motion) of atoms and molecules vibrating and zipping around. alpha decay (𝛼) happens when a nucleus emits an :term:`alpha particle`, otherwise known as a :sup:`4`\ He nucleus. alpha particle (𝛼) is a :sup:`4`\ He (helium) nucleus, ejected from a larger nucleus in an alpha decay It therefore consists of two protons and two neutrons. beta decay (𝛽) happens when a nucleus emits either an :term:`electron` (𝛽−) or a :term:`positron` (𝛽+). positron is an elementary particle of anti-matter, and specifically an anti-electron, having the same mass and opposite charge as the :term:`electron` and will annihilate with an electron into :term:`gamma rays`. gamma ray (𝛾) is a high-energy :term:`photon`, as may be generated by a :term:`gamma decay` or by annihilation of an :term:`electron` and :term:`positron`. gamma decay (𝛾) is when a :term:`nucleus` in an energetically excited state emits a high-energy :term:`photon`. heat pump is a device that moves thermal energy from a cold environment to a hotter one, against normal flow. Some energy input is required to drive this reverse flow, but thermodynamic principles permit a small amount of input energy to drive a larger amount of thermal energy transfer. U-value describes the insulating quality of a wall or similar barrier, in terms of how many Watts move through each square meter of surface area for each 1 ◦C difference in temperature across the barrier. Units are W/m\ :sup:`2`/◦C, and smaller numbers mean better insulation. The U-value is an inverse measure to the :term:`R-value`, numerically 5.7/𝑅. R-value describes the thermal resistance, or insulating quality of a wall or similar barrier. It is an inverse to the :term:`U-value`, numerically 5.7/𝑈. Units are ◦F ·ft\ :sup:`2` · hr/Btu, and larger numbers translate to better insulation. duty cycle refers to the percentage of time something is "active." For example, a refrigerator may be on 40% of the time to maintain internal temperature, in which case its duty cycle is 40%. heat engine is a device that converts :term:`thermal energy` into another form, usually mechanical motion. Automobile engines are a common example, as are power plants that create steam from a thermal source that itself drives a :term:`turbine` and :term:`generator`. turbine is essentially fan blades on a rotating shaft, which can be compelled to move by a flow of air, water, or steam through the blades. generator converts mechanical motion (rotation, typically) into electrical current, generally by the relative motion of wire loops and a strong magnetic field. Boltzmann constant is a fundamental constant of nature associated with thermodynamics. In SI units, it has a value of 𝑘\ :sub:`B` = 1.38 × 10\ :sup:`-23` J/K. entropy is a measure of how many ways a system can be configured for some fixed energy level. The entropy of a closed system cannot decrease. conservation of energy says that energy is never created or destroyed, only shifting from one form to another. coefficient of performance COP refers to the energy gain by a heat pump, usually in the context of heating rather than cooling. It is identical to 𝜀 , as defined in Eq. 6.11 energy efficiency ratio EER refers to the energy gain by a heat pump, usually in the context of cooling rather than heating. Its units are odd, defining how many British thermal units (thermal energy) may be moved per Watt-hour of input energy, but relating to 𝜀 (defined in Eq. 6.10 (p. 95)) by a simple cool numerical factor: EER = 3.41𝜀 . Sometimes seen as SEER to represent a seasonal average EER cool value. heating seasonal performance factor HSPF refers to the energy gain by a heat pump in the context of heating, but in the same units as the :term:`EER` so that HSPF is :term:`COP` times 3.41, numerically. solar constant measures 1,360 W/m\ :sup:`2`, and is the power flux of the sun at the top of Earth's atmosphere. It is not technically a constant, but is very stable. blackbody is a term describing the radiative qualities for thermal emission of light (infrared radiation for "normal" temperatures, becoming visible for very hot objects). A perfect blackbody is not reflective (i.e., "black" at the wavelengths of interest) and emits energy as a function of wavelength according to the :term:`Planck spectrum`. Planck spectrum describes a mathematically precise spectrum of light emission from a :term:`blackbody`, fully defined by the temperature of the blackbody. radiative forcing is used to describe the areal power (in W/m\ :sub:`2`) of absorbed solar energy and :term:`infrared radiation` to space. In equilibrium, a balance exists so that the net radiative forcing is zero. climate sensitivity parameter relates a change in :term:`radiative forcing` to the net temperature change once all the feedback mechanisms have acted. The units are °C per W/m\ :sub:`2`, and a typical value is 0.8. parts per million ppm is a unit used to measure small contributions. One ppm is 0.0001%. parts per million by mass (ppm\ :sub:`m`) is a parts per million measure in terms of fractional mass. For instance, a gram is 1 ppm\ :sub:`m` of a metric ton (1,000 kg). parts per million by volume (ppm\ :sub:`v`) is a parts per million measure in terms of fractional volume occupied. For instance, a cubic millimeter (1 𝜇m, or micro-liter) is 1 ppm\ :sub:`v` of a liter. Astronomical Unit AU is a unit of distance, equal to the average Earth-Sun distance of 149.6 million kilometers (1.496 × 10\ :sup:`11` m). asymmetric risk describes a condition where given the choice to pursue action B for fear of some future condition instead of the normal action A, the downside of being correct about the threat and not taking action B is far more disastrous than being wrong about the threat and pursuing route B unnecessarily. band gap is the energy difference between the conduction band and the valence band, determining how much energy is needed to promote an :term:`electron` out of an atom and into conduction. barrel (bbl) is a unit of volume used primarily for petroleum. It is exactly 42 U.S. gallons, amounting to 159 L of volume. A commonly used measure of energy is barrels of oil equivalent (b.o.e.), amounting to 6.1 GJ of combustion energy. Betz limit is a theoretical maximum amount of :term:`kinetic energy` power that can be removed from wind without slowing the wind too much. It computes to 16/27, or 59%, and is independent of technology. biofuel describes a liquid chemical fuel derived from biologically grown plants: algae, sugar, corn, rapeseed, etc. The two most common forms are :term:`ethanol` and biodiesel. biomass is a generic term for biological matter, but in the energy context usually means firewood or dung that may be burned for :term:`thermal energy`. birth rate quantifies the number of births per 1,000 people per year, typically. Numbers tend to be in the 10-50 range. boiling water reactor is a type of nuclear :term:`fission` reactor in which water surrounding the :term:`fuel rod` acts both as a :term:`moderator` and as the means of transporting heat away from the nuclear fuel. breeder reactor is a nuclear :term:`fission` reactor that transforms non-:term:`fissile` nuclei into ones that are fissile by means of :term:`neutron` capture and subsequent :term:`radioactive decay`. breeding see :term:`breeder reactor`. Calorie (cal) is a unit of energy, defined as the amount of energy required to heat one kilogram (1 kg, 1 L, 1,000 cm\ :sup:`3`) of water by 1 °C. It is equivalent to 4,184 :term:`Joules`. capacity factor is the fraction of energy delivered by an installation compared to what it would deliver if operating continuously at peak operating ("nameplate") capacity. caprock is a geological feature of impermeable rock that can trap oil, gas, or steam below it. chain reaction is a self-feeding process that keeps itself going. In the context of nuclear :term:`fission`, :term:`neutrons` released by the fission precipitate the next fission event, and so on. Chart of the Nuclides is a Periodic Table on steroids, listing the properties of every known :term:`nuclide` including mass or energy, abundance (if stable), :term:`half life` (if unstable), decay mode, :term:`neutron` cross section, nuclear spin, and other salient properties; see https://people.physics.anu.edu.au/~ecs103/chart/. chemical energy is energy stored in chemical bonds, like gasoline or wood that might be burned, or in the food we eat. compound describes a particular combination of :term:`elements` that construct a particular molecule. For instance, H\ :sub:`2`O is the compound we know as water. concentrated solar power CSP (CSP) refers to a form of :term:`solar thermal` (ST) energy, employing troughs or "power towers" or any technique that focuses solar :term:`power` to create high temperatures, often then used to generate electricity. conduction band is the energy level a step up from that of :term:`electrons` in the valence band. Electrons in the conduction band are very loosely bound and freely wander about the crystal, hopping from one atom to the next, and therefore able to contribute to a :term:`current`. confinement in the context of :term:`fusion` refers to the trapping and holding of a high-temperature :term:`plasma`, usually by magnetic means. conservation of mass-energy extends :term:`conservation of energy` to include mass, so that the combined mass-plus-energy of a closed system is never created or destroyed, only shifting from one form to another (mass-energy exchange via E = mc\ :sup:`2`). control rod is used in a nuclear :term:`fission` reactor to absorb :term:`neutrons` so that the :term:`chain reaction` does not get out of control and cause a :term:`meltdown`. coupled refers to the tight connection often seen between energy/resource use and economic scale (as measured, for instance, by :term:`GDP`). critical mass is the mass of :term:`fissile` material (assumed to be in spherical form) above which a self-sustained :term:`chain reaction` will occur. Below this, the material poses no danger. Right at critical mass, the material will limp along in a slow chain reaction. Above this threshold—super-critical—an :term:`exponential growth` runaway detonation will occur, and is the basis of nuclear weapons. For \ :sup:`235`U, critical mass is 50 kg (a bit smaller than a volleyball), and for \ :sup:`239`Pu, it is 10 kg, and about the size of an American softball. D-D fusion uses deuterons (\ :sup:`2`H nuclei) as the fuel for :term:`fusion`, achieving an :term:`energy density` of 87 million kcal/g. D-T fusion combines a :term:`deuteron` (\ :sup:`2`H nucleus) and a triton (\ :sup:`3`H) as the fuel for :term:`fusion`, achieving an :term:`energy density` of 80 million kcal/g. death rate quantifies the number of deaths per 1,000 people per year, typically. Numbers tend to be in the 5-20 range. decay chain refers to a consecutive series of :term:`radioactive decay` events. deuterium is an :term:`isotope` of hydrogen, in which the :term:`nucleus` (called a :term:`deuteron`) contains one :term:`proton` and one :term:`neutron`. deuteron is the :term:`nucleus` of :term:`deuterium`, consisting of one :term:`proton` and one :term:`neutron`. dietary energy factor is the quantitative energy impact of a set of dietary choices compared to a vegetarian diet. A typical American diet has a dietary energy factor around 2, meaning it takes twice as much energy as would a vegetarian diet. This term is not in universal use. doping is a process by which deliberate impurities are introduced into a :term:`semiconductor` in order to change its properties with respect to transport of :term:`electrons` or :term:`holes`. Ecological Economics is a field that builds economic theory on top of the notion that the planet offers finite resources and flows. A principle aim is that of a steady-state economy capable of indefinite planetary compatibility. Electric Power Monthly EPM (EPM) is compiled by the U.S. :term:`EIA`, capturing electricity production and usage at the state level from all energy sources. element pertains to a single atom on the Periodic Table. For instance, hydrogen, helium, and carbon are all elements. energy density describes how concentrated energy is in a substance, quantified as energy per unit mass. In chemical contexts, anything around 10 kcal/g or higher is considered energy-dense, while substances at about 5 kcal/g or lower are poor. Carbohydrates and proteins are middling, around 4 kcal/g, while fat is 9 kcal/g, and therefore among the more energy-dense substances. energy trap refers to a phenomenon in which energy shortage motivates aggressive pursuit of alternative energy schemes, but that pursuit requires substantial energy investment—forcing an even more acute but voluntary energy shortage, which is politically difficult. enriched see :term:`enrichment`. enrichment refers to the process of increasing the concentration of a particular :term:`isotope` within a sample of an :term:`element`. Usually, this term is applied to the concentration of \ :sup:`235`U from its natural 0.7% to 3–5% for power plants or >90% (typically ~94%) for weapons. Environmental Economics is an offshoot of :term:`neo-classical economics` that adds a layer of pricing to capture "externalities," or environmental costs not normally included in market price. EROEI Energy Returned on Energy Invested: a measure of how profitable an energy source is in terms of energy, expressed as a ratio. For instance, a 10:1 EROEI means 10 units were extracted or produced for an investment of 1 unit, leaving a net gain of 9 units of energy. 1:1 is break-even, deriving no net energy benefit. estimated total resource is an educated extrapolation of :term:`proven reserve` trying to characterize the amount of resource that may be ultimately found and extracted. ethanol (C\ :sub:`2`H\ :sub:`5`OH) is a liquid alcohol frequently produced as a :term:`biofuel` having an :term:`energy density` of ~7 kcal/g. eV :term:`electron-volt`. feedback is the response of a system when a change is made that itself influences the change: either counteracting it as in :term:`negative feedback` or amplifying it as in :term:`positive feedback`. fill factor is a generic term describing the fraction of total area occupied. For instance a polka-dot pattern of circles on a piece of fabric might have a fill factor of 50%. fissile describes a :term:`nucleus` that is prodded into :term:`fission` by a (slow) thermal :term:`neutron`. The three fissile :term:`nuclides` of interest are \ :sup:`233`U, \ :sup:`235`U, and \ :sup:`239`Pu. fission is a nuclear process in which a heavy :term:`nucleus` splits into two lighter nuclei. Only \ :sup:`233`U, \ :sup:`235`U, and \ :sup:`239`Pu are usually considered as accessible :term:`nuclides` that are :term:`fissile` in the presence of slow (thermal) :term:`neutrons`. flexitarianism is the practice of pursuing dietary choices based on quantitative assessment of energy costs in an effort to keep the :term:`dietary energy factor` low, without enforcing complete strictness, enjoying the occasional deviation on special occasions or just to avoid being a pain to others. fracking is slang for hydraulic fracturing, a technique used to extract "tight" oil and gas resources locked up in less permeable rock formations. High-pressure fluids are used to create cracks in the rock that allow oil and/or gas to flow. fuel rod is a long cylinder having a high-enough concentration of :term:`fissile` material to be used in a nuclear :term:`fission` reactor. fusion is a nuclear process in which two light nuclei merge to form a larger :term:`nucleus`. Repulsion of the charges in the nuclei make it exceedingly hard to achieve, requiring temperatures of many millions of degrees. galaxy is a collection of stars held together by mutual gravitational attraction, generally numbering in the billions of stars. gravitational potential energy is the energy stored in a mass, m, lifted a height, h, above some reference in the presence of gravity, g ≈ 10 m/s\ :sup:`2`. The energy amounts to mgh, and will be in :term:`Joules` if the inputs are in kg, m, and s. grid tied refers to a :term:`photovoltaic` system connected to the local electrical utility grid, enabling export of solar production by day and use of utility electricity by night. half life is the time after which half a sample of :term:`radioactive` nuclei will have undergone :term:`radioactive decay`. After N half-life periods, the remaining fraction will be 1/2\ :sup:`N`. heat of fusion is the energy barrier associated with either forming (fusing) or melting a solid from a liquid. In the case of water (ice), the heat of fusion is 335 J per gram. heat of vaporization is the energy barrier associated with turning a liquid into gas. In the case of water going to water vapor, the heat of vaporization is about 2,260 J per gram. heavy oil refers to oil that is very viscous—closer to tar than to gasoline. Heavy oil is more difficult to extract, process, and obtain gasoline via refinement. Hertz (Hz) is the :term:`SI` unit for :term:`frequency`, and is equivalent to cycles per second, or 1/s. hole in the context of :term:`semiconductors` is the absence of an :term:`electron`—or an electron vacancy. When another electron fills the hole, it leaves behind another hole, and it is as if the hole moved—effectively like a positive charge able to roam through the crystal. HST Hubble Space Telescope. hydrocarbon is a chain of carbon and hydrogen atoms such as the alkanes (methane, ethane, propane, butane, octane, etc.) having chemical formula C\ :sub:`n`H\ :sub:`2n+2`, where n = 1 for methane, 2 for ethane, 8 for octane, etc. hydrological cycle is the solar-driven process by which evaporation of water from the surface (bodies of water or moist land) forms clouds, and the clouds deliver rain back to the surface. insolation is the annual average solar flux reaching flat, level ground for a particular location. A typical number is 200 W/m\ :sup:`2`, but can range from half that at high latitudes to about 300 W/m\ :sup:`2` for arid areas at lower latitudes. ISS International Space Station. junction describes an interface between two :term:`semiconductors` that have different :term:`doping`. Junctions are the basis of :term:`photovoltaic`, diodes, light emitting diodes (:term:`LEDs`), transistors, and many light detectors. Kill-A-Watt is the name of a relatively inexpensive device that can measure instantaneous :term:`power` in :term:`Watts` and accumulated energy in kWh of electrical appliances. The name is a pun on units. kilocalorie kcal (kcal) is a unit of energy, equivalent to 1,000 :term:`calories`, defined as the amount of energy required to heat one kilogram (1 kg, 1 L, 1,000 cm\ :sup:`3`) of water by 1 °C. It is equivalent to 4,184 :term:`Joules`. kWh :term:`kilowatt-hour`. life-cycle CO2 emission is an assessment of how much CO\ :sub:`2` is released from an energy source when considering the entire enterprise—including manufacture/construction, operation, etc. See the Wikipedia page on List of life-cycle greenhouse gas emissions. liquefied natural gas LNG (LNG) is cryogenically-cooled natural gas (methane) at −160 °C that can be stored much more compactly than the gaseous form, making it suitable to transport. macro-economics concerns itself with the allocation of goods and services across the marketplace, optimizing supply and demand, aiming to minimize surplus or deficits. mass number (A) is simply the total number count of :term:`protons` and :term:`neutrons` (:term:`nucleons`) in a :term:`nucleus`. For example, a carbon atom having 6 protons and 6 neutrons has A = 12. meltdown refers to a failure mode of nuclear :term:`fission` reactors, in which the :term:`chain reaction` becomes uncontrolled due to too many :term:`neutrons` triggering new fission events (as may happen if :term:`control rod` is absent or insufficiently deployed). MeV is a mega-:term:`electron-volt`, or 10\ :sup:`6` eV. In :term:`Joules`, it is equivalent to 1.6 × 10\ :sup:`-13` J. Nuclear masses are often expressed in MeV/c\ :sup:`2` terms, where 1 :term:`a.m.u.` is equivalent to 931.4941 MeV. micro-economics concerns itself with the production of goods, including raw resources, marketing, and distribution. moderator in the context of nuclear :term:`fission` is a material used to slow down :term:`neutrons` speeding out from the break-up so that they can become thermal neutrons and stimulate subsequent fission events in a :term:`chain reaction`. Light atoms like water are a good choice for absorbing the neutron impacts. molar mass is the mass of one :term:`mole` of an :term:`element` or :term:`compound`. The molar mass for carbon, for instance, is 12 grams. The number is often found on a Periodic Table, in addition to the proton number for the element. neo-classical economics is the prevailing economic regime practiced today, driven by supply and demand, fueled by growth, market investment, and focus on :term:`micro-economics` and :term:`macro-economics`. neutrino is a fundamental particle associated with the :term:`weak nuclear force` that has almost no mass, travels near the speed of light, and interacts so weakly with matter that it could pass through light-years of rock before being likely to hit anything. Neutrinos from the sun stream through our bodies constantly, day and night, since Earth is transparent to them. nuclear binding energy is the energy associated with the strong nuclear force that holds a :term:`nucleus` together against charge repulsion. Typical levels are 8 MeV per :term:`nucleon`. nuclear energy derives from reconfiguring the nuclei of atoms, releasing tremendous :term:`thermal energy` that can be harnessed in a :term:`heat engine`. nucleon is either of the two building blocks of a :term:`nucleus`, meaning that it is either a :term:`proton` or a :term:`neutron`. triton is the nucleus of :term:`tritium`, consisting of one :term:`proton` and two :term:`neutrons`. nuclide is any bound arrangement of :term:`protons` and :term:`neutrons`. Every :term:`nucleus` of every :term:`isotope` is one of the possible nuclides, designated, for instance as \ :sup:`12`C, C-12, or \ :sup:`12`\ C. payback time is how long it takes to recuperate an investment by removing a chronic cost. For example, spending $3,000 to no longer pay an annual $300 charge has a payback time of 10 years. photosynthesis is the process by which living matter captures sunlight and stores some of it as :term:`chemical energy`. Effectively, it takes CO\ :sub:`2` out of the atmosphere, combines the carbon with water to make sugars, releasing oxygen back into the air. photovoltaic PV (PV) is a :term:`semiconductor` technology by which light directly drives an electrical :term:`current` by interacting with :term:`electrons` in the material. plasma is a gas hot enough to strip :term:`electrons` from atoms to create a highly-ionized medium, such as the gas comprising the sun. positive feedback involves a reaction to some stimulus in the same direction as the stimulus, thus amplifying the effect. Positive feedback leads to an unstable, runaway process—like :term:`exponential growth`. predicament describes a seemingly intractable situation: more than a problem, but possibly a tangled set of interconnected problems. Predicaments require responses rather than tidy solutions. proliferation is used to describe widespread distribution of dangerous nuclear materials, which becomes difficult to control if they exist in abundance due to increased reliance on nuclear energy. proven reserve pertains to the amount of resource known to exist, having been discovered and surveyed to a degree of certainty. Quadrillion Quad is 10\ :sup:`15` British thermal units, and is equivalent to 1.055 × 10\ :sup:`18` :term:`Joules`. radiation is a broad term that can describe light (e.g., :term:`electromagnetic radiation`, :term:`infrared radiation`, :term:`gamma rays`) or particles from :term:`radioactive decay` or cosmic origin. High-energy radiation of any form can cause damage to materials and biological tissues (DNA being perhaps most critical). radioactive describes a :term:`nucleus`, or :term:`nuclide` that is unstable and will undergo :term:`radioactive decay` with some :term:`half life`. recombination is when an :term:`electron` in the :term:`conduction band` of a :term:`semiconductor` finds a vacancy (:term:`hole`) for it to settle into. By disappearing from the conduction band, it is no longer available to contribute to :term:`current`, and the energy it had becomes unrecoverable. rule of 70 is a quick estimation tool: divide 70 by the percentage growth rate to get the doubling time. Its units depend on how the time over which percentage growth is expressed—like 2% per day or 7% per year, for instance. The rule works most accurately for smaller growth rates, under 10%. sea level rise is one of the inevitable consequences of climate change, as land-bound ice melts and ocean water thermally expands. sector refers to a domain of activity, typically dividing into residential, commercial, industrial, and transportation. semiconductor is a material poised between being a good conductor of electrical :term:`current` and an insulator (not passing current). Silicon is the most commonly used semiconductor. solar system refers to our own star, the sun, and the planets that surround it, including Earth. solar thermal ST (ST), also called :term:`concentrated solar power` (CSP), typically refers to troughs or "power towers" or any technique that focuses solar :term:`power` to create high temperatures, often then used to generate electricity via a :term:`heat engine` and :term:`generator`. spectrum describes a distribution, often associated with light. In this context, a light spectrum specifies how much light is present as a function of :term:`wavelength` or :term:`frequency`. The :term:`Planck spectrum` is a good example. stoichiometry amountstothecountingofatomsandbalancingformulasinchemicalreactionstoreflectthe survival of everyatom in a reaction: none createdordestroyed. strong nuclear force is the force that binds nucleons together in a nucleus, overcoming the electrical repulsion of protons. thermal expansion describes how materials expand, or swell, as temperature increases. Typical rates of expansion are in the range of 10–100 ppm per °C. thermal neutron is a :term:`neutron` whose :term:`kinetic energy` (speed) is no greater than it would naturally possess based on the temperature of its surroundings. Sometimes it is called a "slow" neutron because it is not traveling faster than thermal jostling would establish. tokamak is the name of a donut-shaped chamber in which high-temperature :term:`plasma` can be confined, and potentially used to generate :term:`fusion`. transmutation describes the transformation of a :term:`nucleus` into a different one, usually via :term:`neutron` absorption—possibly followed by :term:`radioactive decay`. tritium is an :term:`isotope` of hydrogen, in which the :term:`nucleus` (called a triton) contains one :term:`proton` and two :term:`neutrons`. valence band is the energy level of outer :term:`electrons` bound to an atom. Valence electrons stay home, as opposed to :term:`electrons` in the :term:`conduction band`. watt-hour Wh (Wh) is a unit of energy, constructed as a :term:`power` (watts) times time (hours). It is equivalent to 3,600 :term:`Joules`, or 3.6 kJ. weak nuclear force joins gravity, electromagnetism, and the strong nuclear force as one of nature's four fundamental forces, responsible for :term:`beta decay` and :term:`neutrino` interactions. Wien law describes the :term:`wavelength` for which the :term:`Planck spectrum` is at maximum brightness. It is roughly 2.9 mm divided by the :term:`blackbody` temperature, in Kelvin.