Vijay Shankar Sharma
Textbook physics terms. Not a BFUT reinterpretation. 371 terms. Textbook meanings only.
371 terms. Page 2 of 8.
mechanics · intro
The net force that keeps an object moving along a curved path, directed toward the center of curvature at every point along the motion. For an object moving in a circle at constant speed, this force is what continuously changes the direction of the object's velocity without changing its speed. Gravity provides the centripetal force that keeps planets in orbit, while tension provides it for a ball on a string swung overhead.
thermo · intro
A gas law stating that, at constant pressure, the volume of a fixed amount of gas is directly proportional to its absolute temperature: heating a gas causes it to expand, and cooling it causes it to contract, as long as pressure stays fixed. Named after French scientist Jacques Charles, it is one of the basic relationships combined to form the ideal gas law.
optics · intermediate
An optical distortion in which a lens fails to focus all wavelengths of light at exactly the same point, because the refractive index of the lens material varies slightly with wavelength. This causes colored fringes to appear around the edges of objects in images produced by simple lenses. Photographers and telescope makers reduce chromatic aberration using combinations of lenses made from different materials, called achromatic doublets.
mechanics · intro
The branch of physics that describes the motion of everyday objects, from falling apples to orbiting planets, using the laws formulated primarily by Isaac Newton in the seventeenth century. It successfully predicts motion for objects much larger than atoms and moving much slower than the speed of light, and remains the practical foundation for engineering. At very small scales or very high speeds, it is superseded by quantum mechanics and relativity.
particle · intro
A particle detector consisting of a sealed container filled with a supersaturated vapor, in which charged particles passing through leave visible trails of condensed droplets, similar to the way an airplane leaves a contrail. It was one of the earliest tools used to observe and photograph subatomic particles, including the discovery of the positron in 1932. Modern particle detectors have largely replaced it, but it remains a popular tool for physics education.
optics · intermediate
A property of waves, especially light waves, in which they maintain a constant, predictable phase relationship with each other over time and space. Coherent light sources, such as lasers, produce waves that can interfere in stable, predictable patterns, which is essential for applications like holography and interferometry. Ordinary light bulbs produce incoherent light, with rapidly and randomly varying phase relationships between different light waves.
cosmology · intermediate
An open question in cosmology asking why the present-day energy densities of matter and dark energy happen to be of roughly the same order of magnitude, given that the two components evolve very differently over cosmic time, with matter density dropping much faster than dark energy density as the universe expands. We appear to be observing the universe at a special epoch when the two are comparable, rather than a time when one overwhelmingly dominates, and the standard Lambda-CDM model does not explain this timing from first principles.
quantum · intermediate
The scattering of a photon by a charged particle, usually an electron, in which the photon loses energy and increases in wavelength, while the electron gains kinetic energy. Discovered by Arthur Compton in 1923, this effect provided strong evidence that light behaves as discrete particles, photons, carrying momentum, rather than only as a continuous wave. The shift in wavelength depends on the scattering angle.
thermo · intro
Also called: heat conduction
The transfer of heat through a material by direct molecular and atomic collisions, without any bulk movement of the material itself. Metals conduct heat efficiently because their free electrons carry thermal energy quickly through the material, while materials like wood or air conduct heat poorly and are used as insulators. Thermal conduction is described quantitatively by Fourier's law, relating heat flow to temperature gradient.
em · intro
A material that allows electric charge, or in some contexts heat, to flow through it easily, because it contains charge carriers, typically electrons, that are free to move. Metals such as copper and silver are excellent electrical conductors due to their loosely bound outer electrons. Conductors are distinguished from insulators, which strongly resist the flow of charge, and semiconductors, whose conductivity lies between the two and can be controlled.
mechanics · intro
A fundamental principle stating that the total energy of an isolated system remains constant over time, though it can change form, such as converting between kinetic, potential, thermal, and other types of energy. No known physical process violates this law when all forms of energy, including mass-energy, are properly accounted for. It underlies analysis across every branch of physics, from simple falling objects to nuclear reactions.
mechanics · intro
A fundamental principle stating that the total momentum of an isolated system remains constant unless an external force acts on it. This applies to collisions, explosions, and rocket propulsion alike; when a rocket expels exhaust gas backward, the rocket itself is pushed forward so that total momentum is preserved. Conservation of momentum follows directly from Newton's third law when applied to a system of interacting objects.
waves · intro
The reinforcement of waves that occurs when two or more waves overlap while in phase, so that their crests align with crests and troughs align with troughs, producing a combined wave of greater amplitude than either wave alone. It is one of the two basic outcomes of wave superposition, the other being destructive interference. Constructive interference produces the bright fringes seen in patterns from double-slit experiments and thin films.
thermo · intro
The transfer of heat through the bulk movement of a fluid, such as air or water, as warmer, less dense regions rise and cooler, denser regions sink, creating a circulating flow. Convection drives weather patterns in the atmosphere, currents in the ocean, and the boiling of water in a pot. It differs from conduction, which transfers heat without bulk motion of the material.
mechanics · intermediate
An apparent deflection of moving objects observed within a rotating reference frame, such as the surface of the rotating Earth, even though no real sideways force is acting on the object. It causes winds and ocean currents to curve, deflecting to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The effect is named after French scientist Gaspard-Gustave de Coriolis, who described it mathematically in 1835.
cosmology · intro
Also called: CMB
Faint microwave radiation that permeates the entire observable universe almost uniformly, representing the earliest light able to travel freely once the universe cooled enough for neutral atoms to form, about 380,000 years after the start of the standard hot early expansion phase. First detected in 1965, the cosmic microwave background carries tiny temperature variations that encode detailed information about the universe's composition, geometry, and history, and it stands as one of the strongest pieces of evidence supporting the standard Big Bang cosmological model.
particle · intro
A high-energy particle, mostly protons and atomic nuclei, that travels through space and strikes Earth's atmosphere, originating from sources such as the sun, supernova remnants, and other high-energy astrophysical events. When cosmic rays collide with atoms in the upper atmosphere, they produce cascades of secondary particles that can be detected at ground level. Cosmic rays are also a natural source of background radiation on Earth.
cosmology · intermediate
Also called: lambda
A term introduced into Einstein's field equations of general relativity, representing a constant energy density that fills space uniformly and is the simplest theoretical explanation for the observed accelerating expansion of the universe within the standard Lambda-CDM model. Einstein originally introduced the term in 1917 to allow for a static universe, later calling it his greatest blunder once the universe was found to be expanding, though it was revived decades later to explain cosmic acceleration.
cosmology · intermediate
A major unresolved discrepancy in theoretical physics between the extremely small value of the cosmological constant inferred from cosmological observations and the vastly larger value predicted by calculating the vacuum energy density using quantum field theory, a mismatch that can span more than one hundred and twenty orders of magnitude depending on the calculation. This enormous discrepancy is often described as one of the worst theoretical predictions in the history of physics and remains a significant unsolved problem connecting cosmology and quantum theory.
em · intro
A law describing the electric force between two stationary charged objects, stating that the force is proportional to the product of the two charges and inversely proportional to the square of the distance between them. Like charges repel while opposite charges attract. Formulated by French physicist Charles-Augustin de Coulomb, this law forms the foundation of electrostatics and closely parallels Newton's law of universal gravitation in mathematical form.
cosmology · intermediate
The precise average density of matter and energy that a universe would need to have for its overall spatial geometry to be exactly flat, according to the equations of general relativity applied to cosmology. Comparing the universe's actual measured density to this critical value determines its overall geometric shape: a density above critical would produce positively curved space, below critical would produce negatively curved space, and observations show the actual universe is very close to critical density and therefore very close to flat.
waves · intro
The gradual reduction in the amplitude of an oscillation over time, usually caused by energy loss to friction, air resistance, or electrical resistance. A swinging pendulum in air eventually comes to rest due to damping, while a car's shock absorbers are specifically designed to add damping and smooth out bouncing after a bump. Systems can be underdamped, critically damped, or overdamped depending on how quickly oscillation is suppressed.
cosmology · intro
A mysterious form of energy that permeates all of space and is currently understood to be driving the accelerating expansion of the universe, first discovered through observations of distant Type Ia supernovae in the late 1990s. Within the standard Lambda-CDM cosmological model, dark energy is modeled as a cosmological constant and accounts for roughly 68 percent of the universe's total energy content, though its underlying physical nature remains one of the biggest open questions in modern physics.
cosmology · intro
A form of matter that does not emit, absorb, or reflect light, and has never been directly detected, but whose gravitational influence is inferred from its effects on visible matter, such as the rotation speeds of galaxies, the motion of galaxy clusters, and the pattern of fluctuations in the cosmic microwave background. Within the standard Lambda-CDM cosmological model, dark matter accounts for roughly 27 percent of the universe's total energy content, far outweighing ordinary visible matter.
quantum · intermediate
The wavelength associated with any moving particle, proposed by physicist Louis de Broglie in 1924, calculated by dividing Planck's constant by the particle's momentum. This idea extended the wave-particle duality already known for light to all matter, suggesting that electrons and even larger objects have an associated wave nature. The de Broglie wavelength of everyday objects is far too small to observe, but it becomes significant for electrons and other subatomic particles.
nuclear · intermediate
A value that characterizes how quickly a radioactive isotope decays, representing the probability per unit time that any given nucleus will decay. It is directly related to the isotope's half-life; a larger decay constant means faster decay and a shorter half-life. The decay constant appears in the exponential decay equation used to predict how much of a radioactive sample remains after a given time.
waves · intro
A logarithmic unit used to express the ratio between two values of power or intensity, most commonly used to measure the loudness of sound. Because human hearing spans an enormous range of intensities, the logarithmic decibel scale compresses this range into more manageable numbers; a 10 decibel increase corresponds to a tenfold increase in sound intensity. Normal conversation is roughly 60 decibels, while a jet engine can exceed 140 decibels.
astro · intermediate
A quantum mechanical pressure that arises because fermions, such as electrons or neutrons, cannot occupy the same quantum state, a restriction known as the Pauli exclusion principle. When matter is compressed to extremely high densities, this pressure resists further compression even at zero temperature. Degeneracy pressure supports white dwarf stars against gravitational collapse, and a related form supports neutron stars, until gravity overwhelms it in the most massive collapsing stars.
thermo · intermediate
The number of independent ways a system can store energy or the number of independent coordinates needed to fully describe its configuration. A single free particle moving in three dimensions has three translational degrees of freedom, while a diatomic gas molecule also has rotational and vibrational degrees of freedom. In statistical mechanics, each degree of freedom contributes a predictable share of a system's total thermal energy.
mechanics · intro
The amount of mass contained within a given volume of a substance, usually expressed in kilograms per cubic meter or grams per cubic centimeter. Density determines whether an object floats or sinks in a given fluid, and it varies with temperature and pressure for gases and, to a lesser extent, liquids and solids. Water has a density of about 1,000 kilograms per cubic meter, which serves as a common reference point.
waves · intro
The cancellation of waves that occurs when two or more waves overlap while out of phase, so that the crest of one wave aligns with the trough of another, reducing or eliminating the combined amplitude. It is one of the two basic outcomes of wave superposition, the other being constructive interference. Noise-cancelling headphones use destructive interference deliberately, generating a sound wave that cancels out unwanted ambient noise.
nuclear · intro
Also called: heavy hydrogen
An isotope of hydrogen whose nucleus contains one proton and one neutron, making it roughly twice as massive as ordinary hydrogen, which has no neutron. It occurs naturally in small quantities in ordinary water and is used in nuclear fusion research, certain nuclear reactors, and as a tracer in chemistry and biology. Its chemical symbol is often written as D or 2H.
em · intermediate
A weak form of magnetism exhibited by all materials, in which an external magnetic field induces a very slight magnetization that opposes the applied field, causing a small repulsive force. Diamagnetism is typically far weaker than paramagnetism or ferromagnetism and is only clearly observable in materials that lack these stronger effects, such as water, wood, and most organic compounds. It arises from changes in electron orbital motion in response to the applied field.
waves · intro
The bending and spreading of waves as they pass through a narrow opening or around an obstacle, most noticeable when the size of the opening or obstacle is comparable to the wavelength of the wave. Diffraction explains why sound can be heard around corners and why light passing through a narrow slit produces a pattern of bright and dark bands rather than a sharp shadow. It is a defining feature that distinguishes wave behavior from simple particle behavior.
optics · intermediate
An optical component consisting of a surface with many closely and regularly spaced parallel lines or slits, used to split and diffract light into its component wavelengths. Because each wavelength diffracts at a slightly different angle, a diffraction grating produces a spectrum similar to a prism, but with generally higher resolution. Diffraction gratings are widely used in spectrometers to analyze the composition of light from sources ranging from lab samples to distant stars.
thermo · intro
The gradual spreading out of particles, such as molecules of a gas or solute in a liquid, from regions of higher concentration to regions of lower concentration, driven by random molecular motion. Diffusion continues until concentration is uniform throughout the available space, and its rate depends on temperature, particle size, and the medium involved. The smell of perfume spreading through a room is a familiar everyday example of diffusion.
math-methods · intro
A technique in physics for checking the consistency of equations and estimating relationships between quantities by tracking the fundamental dimensions involved, such as length, mass, and time, rather than their numerical values. If an equation's terms do not have matching dimensions, the equation must be wrong. Dimensional analysis is often used as a quick sanity check on calculations and can also help derive the general form of physical relationships before detailed theory is developed.
em · intermediate
A pair of equal and opposite charges, or equivalent poles, separated by a small distance, which together create a characteristic field pattern in the surrounding space. An electric dipole consists of a positive and negative charge, common in polar molecules like water, while a magnetic dipole, such as a bar magnet, has a north and south pole. Dipoles interact with external fields, tending to align with them, and radiate energy when they oscillate.
em · intro
Also called: DC
An electric current that flows steadily in a single direction, as opposed to alternating current, which periodically reverses direction. Batteries and solar cells produce direct current naturally, and it is the form of electricity used internally by most electronic devices, even those plugged into an alternating current outlet, after passing through a rectifier. Direct current was central to Thomas Edison's early electrical power systems in the late nineteenth century.
optics · intermediate
The phenomenon in which the refractive index of a material, and therefore the angle at which light bends, varies with the wavelength of the light passing through it. Dispersion causes white light passing through a prism to spread out into its constituent colors, since different wavelengths refract by slightly different amounts. It is also responsible for the chromatic aberration seen in simple lenses and for the formation of rainbows in water droplets.
waves · intro
The change in observed frequency or wavelength of a wave caused by relative motion between the source and the observer. Sound from an approaching ambulance siren rises in pitch and drops as it passes and recedes, which is a familiar example. The same principle applies to light, where it produces the redshift and blueshift used by astronomers to measure the motion of stars, galaxies, and other celestial objects.
em · intermediate
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
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
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
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
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
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
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
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
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.