Plain English

Glossary

Every term we use, defined without gatekeeping. The real word, explained simply.

Alpha particle: A small, fast, positively charged particle thrown out by some radioactive atoms. It is actually the nucleus of a helium atom: two protons and two neutrons.
Ampere (also: amp): The Ampere is the standard international (SI) unit for measuring electric current, representing the flow of one coulomb of electric charge per second.
Astronomical Unit (also: AU): An astronomical unit (AU) is a unit of distance used in astronomy, approximately equal to the average distance from the Earth to the Sun.
Atom: The smallest unit of an element that retains the chemical identity of that element.
Atomic nucleus (also: nucleus): The tiny, dense core at the centre of an atom. It holds nearly all the atom's mass and its positive charge, with the rest of the atom mostly empty space.
Blackbody Radiation: Blackbody radiation is the electromagnetic radiation emitted by an idealized object that absorbs all incident radiation and reflects none.
Caloric Theory: An early scientific theory, popular in the 18th and early 19th centuries, which proposed that heat was an invisible, weightless fluid that flowed from hotter to colder bodies. While later disproven, it was used by Sadi Carnot in his foundational work on heat engines.
Chandrasekhar Limit: The maximum mass that a stable white dwarf star can have, which is approximately 1.4 times the mass of the Sun.
Conservation of energy: The principle that energy is never created or destroyed, only converted from one form to another. The total amount in a closed system stays the same.
Eclipse: An eclipse occurs when one celestial body passes into the shadow of another, or when one body passes in front of another from an observer's perspective, blocking its light.
Electric Charge: A fundamental property of matter that causes it to experience a force when placed in an electromagnetic field. It can be positive or negative.
Electric current (also: current): A flow of electric charge, usually electrons moving through a wire. It is what powers and connects almost every electrical device.
Electric Field: A region of space around an electrically charged particle or object within which a force would be exerted on other electrically charged particles or objects.
Electrodynamics: The branch of physics that studies the interactions between electric currents and the magnetic fields they produce.
Electromagnetic induction (also: induction): The effect where a changing magnetic field creates an electric voltage in a nearby wire. Move a magnet near a coil, or change the magnetism through it, and current flows. This is how generators turn motion into electricity.
Electromagnetic Wave (also: EM wave, light wave): A wave made of oscillating electric and magnetic fields that travel through space at the speed of light, carrying energy.
Electron: A fundamental subatomic particle with a negative electric charge, found in all atoms.
Electron Degeneracy Pressure: A quantum mechanical pressure that arises when electrons are packed so densely that they resist further compression, preventing two electrons from occupying the same quantum state.
Element: A pure substance consisting only of atoms that all have the same number of protons in their atomic nuclei.
Elementary Charge: The magnitude of the electric charge of a single proton or electron. It is the smallest possible amount of free electric charge.
Energy: The capacity to do work or cause change. It comes in many forms, including motion, heat, light, and chemical energy, and can be converted between them.
Field (also: physical field): A region of space where a physical quantity (like force) has a specific value at every point.
Fourier Series (also: Fourier decomposition): A mathematical way to represent almost any repeating pattern or signal as a sum of simple sine waves, each with its own amplitude and frequency. This process is also known as Fourier decomposition.
Frequency: How often a repeating event or wave pattern occurs in a given amount of time. Higher frequency means more repetitions per second.
Gravitation (also: Gravity): Gravitation is the fundamental force of attraction that exists between any two objects that have mass. The more massive the objects, the stronger their gravitational pull.
Heat: Energy that flows from a hotter object to a cooler one because of the temperature difference between them. Heat is energy in transit, not a substance stored inside things.
Heat Engine: A device that converts thermal energy (heat) into mechanical energy (work) by exploiting a temperature difference between a hot source and a cold sink.
Helioseismology (also: solar seismology): The study of the Sun's interior by watching the sound waves that ripple across its surface. Just as geologists read earthquakes to map the inside of the Earth, physicists read the Sun's surface vibrations to map what lies beneath it.
Interference: The phenomenon where two or more waves overlap and combine to form a new wave of greater, lower, or the same amplitude.
Inverse-Square Law: The inverse-square law describes how the strength of a physical quantity, such as gravitational force, decreases proportionally to the square of the distance from its source.
Joule (also: J): The standard unit of energy, named after James Joule. One joule is about the energy needed to lift a small apple one metre.
Light-time (also: signal delay): Light-time refers to the duration it takes for light to travel a specific distance through space.
Magnetic field: The region around a magnet or an electric current where magnetic forces act. It is what turns a compass needle and what links electricity to magnetism.
Mechanical equivalent of heat: The fixed exchange rate between mechanical work and heat, showing the two are forms of the same thing, energy. James Joule measured it precisely.
Orbit: An orbit is the curved path an object takes around another object due to gravitation. It is a continuous state of falling while also moving sideways fast enough to miss the central object.
p-mode (also: pressure mode, acoustic mode): One of the Sun's natural sound-wave patterns, where ordinary gas pressure is the force pushing the wave along (the "p" stands for pressure). These are the main waves helioseismologists measure.
Perpetual Motion Machine of the Second Kind: A hypothetical device that would continuously extract useful work from a single heat reservoir by cooling it down, without needing a colder sink to dump excess heat. Such a machine is impossible according to the Second Law of Thermodynamics.
Planck's Constant: Planck's constant is a fundamental physical constant that relates the energy of a quantum of energy to its frequency.
Quantization of Charge: The principle that electric charge does not exist in arbitrary amounts but only in discrete, integer multiples of the elementary charge.
Quantum (also: quanta): A quantum is the smallest possible discrete unit of any physical property, such as energy or matter.
Radioactivity (also: radioactive decay): The spontaneous emission of penetrating rays or particles from the nucleus of an unstable atom.
Second Law of Thermodynamics: A fundamental principle of physics stating that the total entropy (disorder) of an isolated system can only increase over time, or remain constant in ideal, reversible processes. It implies that heat naturally flows from hotter to colder bodies and that it's impossible to convert all heat into useful work.
Sine Wave: A smooth, symmetrical, repeating wave pattern, like the ripples on a pond or the path of a pendulum. It describes a regular, oscillating motion.
Solar Mass: A standard unit of mass in astronomy, equal to the mass of our Sun (approximately 2 x 10^30 kilograms).
Thermodynamic Efficiency: The ratio of the useful work output by a heat engine to the total heat energy supplied to it, indicating how effectively heat is converted into work.
Wavelength: The distance between two consecutive peaks or troughs of a wave.
White Dwarf: A very dense, compact stellar remnant, typically the size of Earth but with the mass of the Sun, formed after a star like our Sun has exhausted its nuclear fuel.
Work: In physics, work is energy transferred when a force pushes or pulls something through a distance. Lifting a weight or stirring water both do work.