Standard physics glossary

em · intermediate

Eddy Current

Loops of electric current induced within a conductor when it experiences a changing magnetic field or moves through a magnetic field, circulating in swirling patterns similar to eddies in a fluid. These currents create their own magnetic fields that generally oppose the change causing them, according to Lenz's law, producing a braking or damping effect. Eddy currents are used deliberately in magnetic braking systems but are often minimized in transformers through laminated cores to reduce wasted energy.

relativity · intermediate

Einstein's Field Equations

A set of ten interrelated equations at the heart of general relativity, formulated by Albert Einstein in 1915, that relate the curvature of spacetime to the distribution of mass and energy within it. In simple terms, they describe how matter tells spacetime how to curve, and curved spacetime tells matter how to move. Exact solutions to these equations describe phenomena including black holes, gravitational waves, and the large-scale expansion of the universe.

mechanics · intro

Elastic Collision

A collision between two or more objects in which both total momentum and total kinetic energy are conserved, meaning no kinetic energy is converted into heat, sound, or deformation. Collisions between hard, rigid objects like billiard balls approximate elastic collisions closely, while collisions between soft or sticky objects usually lose kinetic energy and are called inelastic. Elastic collisions are a useful idealization in analyzing particle interactions and gas molecule collisions.

mechanics · intro

Elastic Potential Energy

Energy stored in an object when it is deformed elastically, such as when a spring is stretched or compressed, or a rubber band is pulled taut. This energy is released and converted into kinetic energy when the deforming force is removed and the object returns to its original shape. For an ideal spring, elastic potential energy is proportional to the square of the displacement from its natural length, as described by Hooke's law.

em · intro

Electric Charge

A fundamental property of matter that causes it to experience and produce electromagnetic forces, existing in two types, positive and negative, with like charges repelling and opposite charges attracting. Electric charge is quantized, occurring only in whole-number multiples of the elementary charge carried by a single proton or electron, and it is conserved in all known physical processes. Electric charge is the source of electric fields and, when moving, magnetic fields as well.

em · intro

Electric Current

The flow of electric charge through a conductor or space, measured in amperes, where one ampere equals one coulomb of charge passing a point per second. Conventional current is defined as flowing from positive to negative terminals, though in metals it is actually negatively charged electrons that move in the opposite direction. Electric current powers virtually all modern electronic and electrical devices.

em · intro

Electric Field

A region of space surrounding an electric charge, or a distribution of charges, in which another charged object would experience a force. The electric field points away from positive charges and toward negative charges, and its strength decreases with distance according to Coulomb's law. Electric fields are used to explain how charges interact without touching, and they carry energy that can propagate as electromagnetic waves.

em · intro

Electric Potential

Also called: voltage

The amount of electric potential energy per unit charge at a point in an electric field, measured in volts. Electric potential difference, commonly called voltage, drives the flow of current between two points in a circuit; charge naturally flows from higher to lower potential in the absence of a battery or other source pushing it otherwise. Electric potential is a scalar quantity, unlike the electric field, which is a vector.

em · intermediate

Electromagnetic Induction

The generation of an electric current or voltage in a conductor caused by a changing magnetic field, discovered experimentally by Michael Faraday in 1831. Moving a magnet near a coil of wire, or changing the current in a nearby circuit, induces a voltage in the coil according to Faraday's law. This principle underlies the operation of electric generators, transformers, and many types of electric motors.

em · intro

Electromagnetic Spectrum

The complete range of electromagnetic radiation, ordered by wavelength or frequency, spanning from long-wavelength radio waves through microwaves, infrared, visible light, ultraviolet, X-rays, and finally short-wavelength gamma rays. All forms of electromagnetic radiation travel at the speed of light in a vacuum and consist of oscillating electric and magnetic fields, differing only in their wavelength and the energy carried by their photons.

em · intro

Electromagnetic Wave

A wave consisting of oscillating electric and magnetic fields that propagate through space, including light, radio waves, and X-rays, all traveling at the speed of light in a vacuum. Unlike sound waves, electromagnetic waves require no medium and can travel through the vacuum of space. James Clerk Maxwell first predicted their existence mathematically in the 1860s by combining the laws of electricity and magnetism.

particle · intro

Electron

A subatomic particle with a negative electric charge and very little mass compared to protons and neutrons, found orbiting the nucleus in atoms and responsible for chemical bonding and electric current in conductors. Electrons are classified as leptons and, as far as experiments have shown, have no internal substructure. Their behavior is governed by quantum mechanics rather than classical physics, exhibiting both particle-like and wave-like properties.

particle · intro

Electron Volt

Also called: eV

A unit of energy commonly used in atomic, nuclear, and particle physics, defined as the energy gained by a single electron accelerating through an electric potential difference of one volt. Because it is a very small unit, energies are often expressed in kilo-, mega-, or giga-electron-volts. The electron volt is convenient because typical atomic and subatomic energy scales come out as manageable numbers rather than extremely small numbers of joules.

particle · intermediate

Electroweak Interaction

A unified description of the electromagnetic force and the weak nuclear force, showing that at very high energies, such as those present shortly after the early hot phase of the universe, these two forces merge into a single electroweak force. At lower energies typical of everyday conditions, the two forces appear distinct. This unification, developed by Sheldon Glashow, Abdus Salam, and Steven Weinberg, is a core part of the Standard Model of particle physics.

em · intro

Elementary Charge

The smallest unit of electric charge that occurs freely in nature, carried by a single proton, with a value of approximately 1.602 times ten to the minus nineteen coulombs. The electron carries the same magnitude of charge but with a negative sign. All observed free electric charges are whole-number multiples of the elementary charge, a property known as charge quantization.

em · intermediate

EMF (Electromotive Force)

Also called: electromotive force

A measure of the energy per unit charge supplied by a source, such as a battery or generator, that drives current around a circuit, measured in volts. Despite its name, electromotive force is not actually a force but an energy-per-charge quantity, similar to voltage, though emf specifically refers to the driving energy source rather than a potential difference across a passive component.

mechanics · intro

Energy

A fundamental physical quantity representing the capacity of a system to do work or produce change, existing in many interconvertible forms including kinetic, potential, thermal, chemical, and electromagnetic energy. Energy is measured in joules and is conserved in every known physical process, though it can change form or transfer between systems. The concept of energy unifies vastly different areas of physics under a single accounting framework.

quantum · intro

Energy Level

One of the specific, discrete amounts of energy that a bound quantum system, such as an electron in an atom, is allowed to have, according to quantum mechanics. Unlike classical systems, which can have any energy value, quantum systems can only occupy these fixed levels, and transitions between levels involve absorbing or emitting a photon with energy equal to the difference between the two levels. This quantization explains the sharp spectral lines seen in atomic spectra.

thermo · intermediate

Entropy

A measure of the disorder, or more precisely the number of microscopic arrangements consistent with a system's macroscopic state, central to the second law of thermodynamics. In any isolated system, entropy tends to increase over time, which is why heat flows from hot to cold objects and not the reverse, and why processes like mixing are generally irreversible. Entropy provides a direction to time in physics, sometimes called the arrow of time.

mechanics · intro

Equilibrium

A state in which a system experiences no net change over time, because opposing influences are balanced. Mechanical equilibrium occurs when the net force and net torque on an object are both zero, so it remains at rest or moves at constant velocity. Thermal equilibrium occurs when two objects in contact reach the same temperature and no further net heat flows between them.

thermo · intermediate

Equipartition Theorem

A result from statistical mechanics stating that, in thermal equilibrium, energy is distributed equally among all the accessible degrees of freedom of a system, with each contributing an equal average share of the total thermal energy. This theorem explains, for example, why the specific heat of a gas depends on how many ways its molecules can store energy, such as through translation, rotation, and vibration. It works well for classical systems but breaks down at low temperatures where quantum effects dominate.

mechanics · intro

Escape Velocity

The minimum speed an object needs to break free from a massive body's gravitational pull without further propulsion, ignoring atmospheric drag. It depends on the mass and radius of the body being escaped; for Earth, escape velocity is about 11.2 kilometers per second, while for the Sun it is much higher. Escape velocity is derived by setting an object's kinetic energy equal to the gravitational potential energy needed to reach an infinite distance.

relativity · intro

Event Horizon

The boundary surrounding a black hole beyond which nothing, including light, can escape its gravitational pull. Once any object or information crosses the event horizon, it is causally disconnected from the outside universe and inevitably continues toward the central singularity. The size of the event horizon, described by the Schwarzschild radius for a non-rotating black hole, depends directly on the black hole's mass.

nuclear · intro

Exponential Decay

A pattern of change in which a quantity decreases at a rate proportional to its current value, resulting in the quantity shrinking by the same fraction over equal time intervals. Radioactive decay, the discharge of a capacitor, and the absorption of light passing through a material all follow exponential decay. It is mathematically described using the constant e and is characterized by a fixed half-life or time constant.