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Glossary of Classical Physics
List of definitions of terms and concepts commonly used in the study of physics
A mathematical model which seeks to describe atomic nuclei by solving the non-relativistic Schrödinger equation for all constituent nucleons and the forces that exist between them. Such methods yield precise results for very light nuclei but become more approximate for heavier nuclei.
Any system of measurement that begins at a minimum, or zero point, and progresses in only one direction. The zero point of an absolute scale is a natural minimum, leaving only one direction in which to progress, whereas an arbitrary or "relative" scale begins at some point selected by a person and can progress in both directions.
The theoretical lowest possible temperature, understood by international agreement as equivalent to 0 Kelvin or -273.15 °C (-459.67 °F). More formally, it is the theoretical lower limit of the thermodynamic temperature scale, at which enthalpy and entropy of a cooled ideal gas reach their minimum values and the fundamental particles of nature have minimal vibrational motion.
The study of the motion of air, particularly its interaction with a solid object, such as an airplane wing. It is a sub-field of fluid dynamics and gas dynamics, and many aspects of aerodynamics theory are common to these fields.
1. In meteorology, a volume of air that is defined by its temperature and water vapor content. Air masses may cover many hundreds or thousands of square miles and generally adapt to the characteristics of the surface below them. They are often classified according to their latitude and their source regions.
2. In astronomy, the "amount of air that one is looking through" when observing a star or other celestial source from a vantage point that is within Earth's atmosphere. It is formulated as the integral of air density along the light ray.
Defines the direct optical path length through the Earth's atmosphere, expressed as a ratio relative to the path length vertically upwards, i.e. at the zenith. The air mass coefficient can be used to help characterize the solar spectrum after solar radiation has traveled through the atmosphere.
An electronic device that can increase the power of a signal (a time-varying voltage or current). It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is a circuit that has a power gain greater than one.
Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves. The law of reflection says that for specular reflection the angle at which the wave is incident on the surface equals the angle at which it is reflected. Mirrors exhibit specular reflection.
The time rate of change of angular velocity. In three dimensions, it is a pseudovector. In SI units, it is measured in radians per second squared (rad/s2), and is usually denoted by the Greek letter alpha (?).. Just like angular velocity, there are two types of angular acceleration: spin angular acceleration and orbital angular acceleration, representing the time rate of change of spin angular velocity and orbital angular velocity, respectively. Unlike linear acceleration, angular acceleration need not be caused by a net external torque. For example, a figure skater can speed up her rotation (thereby obtaining an angular acceleration) simply by contracting her arms inwards, which involves no external torque.
A scalar measure of rotation rate. It refers to the angular displacement per unit time (e.g. in rotation) or the rate of change of the phase of a sinusoidal waveform (e.g. in oscillations and waves), or as the rate of change of the argument of the sine function. Angular frequency (or angular speed) is the magnitude of the vector quantity that is angular velocity. The term angular frequency vector is sometimes used as a synonym for the vector quantity angular velocity.
One revolution is equal to 2? radians, hence
How fast an object rotates or revolves relative to another point, i.e. how fast the angular position or orientation of an object changes with time. There are two types of angular velocity: orbital angular velocity and spin angular velocity. Spin angular velocity refers to how fast a rigid body rotates with respect to its centre of rotation. Orbital angular velocity refers to how fast a rigid body's centre of rotation revolves about a fixed origin, i.e. the time rate of change of its angular position relative to the origin. In general, angular velocity is measured in angle per unit time, e.g. radians per second. The SI unit of angular velocity is expressed as radians/sec with the radian having a dimensionless value of unity, thus the SI units of angular velocity are listed as 1/sec. Angular velocity is usually represented by the Greek letter omega (?, sometimes ?). By convention, positive angular velocity indicates counter-clockwise rotation, while negative is clockwise.
The electrode through which a conventional electric current flows into a polarized electrical device; the direction of current flow is, by convention, opposite to the direction of electron flow, and so electrons flow out of the anode. In a galvanic cell, the anode is the negative terminal or pole which emits electrons toward the external part of an electrical circuit. However, in an electrolytic cell, the anode is the wire or plate having excess positive charge, so named because negatively charged anions tend to move towards it. Contrast cathode.
A theory of creating a place or object that is free from the force of gravity. It does not refer to the lack of weight under gravity experienced in free fall or orbit, or to balancing the force of gravity with some other force, such as electromagnetism or aerodynamic lift.
In particle physics, every type of particle has an associated antiparticle with the same mass but with opposite physical charges such as electric charge. For example, the antiparticle of the electron is the antielectron (which is often referred to as the positron). While the electron has a negative electric charge, the positron has a positive electric charge, and is produced naturally in certain types of radioactive decay. Some particles, such as the photon, are their own antiparticle. Otherwise, for each pair of antiparticle partners, one is designated as "normal" matter (the kind comprising all matter with which humans usually interact), and the other (usually given the prefix "anti-") as antimatter.
A physical principle which states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces and acts in the upward direction at the center of mass of the displaced fluid.
A basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons.
In chemistry and physics, the ratio of the number of constituent particles in a substance, usually atoms or molecules, to the amount of substance, of which the SI unit is the mole. It is defined as exactly .
A theorem concerning diffraction which states that the diffraction pattern from an opaque body is identical to that from a hole of the same size and shape except for the overall forward beam intensity.
A structural element that is capable of withstanding load primarily by resisting bending. Beams are traditionally descriptions of building or civil engineering structural elements, but smaller structures such as truck or automobile frames, machine frames, and other mechanical or structural systems contain beam structures that are designed and analyzed in a similar fashion.
A hypothetical idealized physical body that completely absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. Perfect black bodies are imagined as substitutes for actual physical bodies in many theoretical discussions of thermodynamics, and the construction of nearly perfect black bodies in the real world remains a topic of interest for materials engineers. Contrast white body.
The type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body (an opaque and non-reflective body) held at constant, uniform temperature. The radiation has a specific spectrum and intensity that depends only on the temperature of the body.
The phenomenon by which the boiling point of a liquid (a solvent) increases when another compound is added, meaning that the resulting solution has a higher boiling point than the pure solvent. This happens whenever a non-volatile solute, such as a salt, is added to a pure solvent, such as water. The boiling point can be measured accurately using an ebullioscope.
An Imperial unit of energy defined as the amount of energy needed to heat one pound of water by one degree Fahrenheit; 1 btu is equal to about 1,055 joules. In scientific contexts the btu has largely been replaced by the SI unit of energy, the joule.
The tendency of a material to break without significant plasticdeformation when subjected to stress. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound.
A branch of mathematics that studies change and has two major sub-fields: differential calculus (concerning rates of change and slopes of curves), and integral calculus (concerning accumulation of quantities and the areas under and between curves). These two branches are related to each other by the fundamental theorem of calculus.
The ratio of the change in the electric charge of a system to the corresponding change in its electric potential. There are two closely related notions of capacitance: self capacitance and mutual capacitance. Any object that can be electrically charged exhibits self capacitance. A material with a large self capacitance holds more electric charge at a given voltage than one with low capacitance. The notion of mutual capacitance is particularly important for understanding the operations of the capacitor, one of the three elementary linear electronic components (along with resistors and inductors).
A theoretical ideal thermodynamic cycle proposed by French physicist Nicolas Léonard Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. It provides an upper limit on the efficiency that any classical thermodynamic engine can achieve during the conversion of heat into work, or conversely, the efficiency of a refrigeration system in creating a temperature difference by the application of work to the system. It is not an actual thermodynamic cycle but is a theoretical construct.
A coordinate system that specifies each point uniquely in a plane by a set of numerical coordinates, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in the same unit of length. Each reference line is called a coordinate axis or just axis (plural axes) of the system, and the point where they meet is called the origin, at ordered pair . The coordinates can also be defined as the positions of the perpendicular projections of the point onto the two axes, expressed as signed distances from the origin.
The electrode through which a conventional electric current flows out of a polarized electrical device; the direction of current flow is, by convention, opposite to the direction of electron flow, and so electrons flow into the cathode. In a galvanic cell, the cathode is the positive terminal or pole which accepts electrons flowing from the external part of an electrical circuit. However, in an electrolytic cell, the cathode is the wire or plate having excess negative charge, so named because positively charged cations tend to move towards it. Contrast anode.
The point in a body around which the resultant torque due to gravity forces vanish. Near the surface of the earth, where gravity acts downward as a parallel force field, the center of gravity and the center of mass are the same.
A branch of chemistry and physics that studies chemical processes from the point of view of physics by investigating physicochemical phenomena using techniques from atomic and molecular physics and condensed matter physics.
Any influence upon or within an oscillatory system that has the effect of reducing, restricting, or preventing its oscillations. Damping is a result of processes that dissipate the energy stored in the oscillation.
For a mathematical function of a real variable, a measurement of the sensitivity to change of the function value (output) with respect to a change in its argument (input); e.g. the derivative of the position of a moving object with respect to time is the object's velocity and measures how quickly the position of the object changes as time changes. Derivatives are a fundamental tool of calculus.
1. (fluid) Occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the immersed object will be exactly equal to the volume of the displaced fluid, so that the volume of the immersed object can be deduced if the volume of the displaced fluid is measured.
2. (vector) The shortest distance from the initial to the final position of a point. Thus, it is the length of an imaginary straight path, typically distinct from the path actually travelled by.
The change in frequency of a wave (or other periodic event) for an observer moving relative to its source. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.
Forces which act on a solid object in the direction of the relative fluid flow velocity. Unlike other resistive forces, such as dry friction, which is nearly independent of velocity, drag forces depend on velocity.
The region of space surrounding electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding.
The electrical intensity or "pressure" developed by a source of electrical energy such as a battery or generator and measured in volts. Any device that converts other forms of energy into electrical energy provides electromotive force as its output.
A field that deals with electrical circuits that involve active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies.
An adjective used to refer to a process or reaction in which a system absorbs energy from its surroundings, usually in the form of heat but also in the form of light, electricity, or sound. Contrast exothermic.
Either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts (lighter nuclei), often producing free neutrons and photons (in the form of gamma rays), and releasing a, relatively, very large amount of energy.
A model used to explain the influence that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational phenomena. It is measured in newtons per kilogram (N/kg).
The time required for a quantity to fall to half its value as measured at the beginning of the time period. In physics, half-life typically refers to a property of radioactive decay, but may be refer to any quantity which follows an exponential decay.
The International System of Units (abbreviated SI) is the modern form of the metric system. It comprises a system of units of measurement devised around seven base units and the convenience of the number ten.
The branch of classical mechanics that describes the motion of points, bodies (objects) and systems of bodies (groups of objects) without consideration of the causes of motion. The study of kinematics is often referred to as the "geometry of motion".
Two approximate equalities that deal with the current and voltage in electrical circuits.also called Kirchhoff's rules or simply Kirchhoff's laws (see also Kirchhoff's laws for other meanings of that term).
A vector used chiefly to describe the shape and orientation of the orbit of one astronomical body around another, such as a planet revolving around a star. For two bodies interacting by Newtonian gravity, the LRL vector is a constant of motion, meaning that it is the same no matter where it is calculated on the orbit; equivalently, the LRL vector is said to be conserved.
The mathematical study of how solid objects deform and become internally stressed due to prescribed loading conditions. Linear elasticity is a simplification of the more general nonlinear theory of elasticity and is a branch of continuum mechanics.
A matter in a state that has properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way.
Any powered tool consisting of one or more parts that is constructed to achieve a particular goal. Machines are usually powered by mechanical, chemical, thermal or electrical means, and are frequently motorised.
A mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field.
A set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. Maxwell's equations describe how electric and magnetic fields are generated and altered by each other and by charges and currents.
The mathematical description of an object's or substance's tendency to be deformed elastically (i.e., non-permanently) when a force is applied to it. The elastic modulus of an object is defined as the slope of its stress-strain curve in the elastic deformation region. As such, a stiffer material will have a higher elastic modulus.
A branch of physics that studies the physical properties of molecules and the chemical bonds between atoms as well as their molecular dynamics. It is closely related to atomic physics and overlaps greatly with theoretical chemistry, physical chemistry and chemical physics.
An elementary particle, technically classified as a lepton, that is similar to the electron, with unitary negative electric charge (-1) and a spin of 1/2. Muons are not believed to have any sub-structure.
The manipulation of matter on an atomic and molecular scale. A more generalized description of nanotechnology is "the manipulation of matter with at least one dimension sized from 1 to 100 nanometers".
A type of electrically neutral subatomic particle denoted by the Greek letter ? (nu). All evidence suggests that neutrinos have mass but that their mass is tiny even by the standards of subatomic particles. Their mass has never been measured accurately.
The branch of physics which involves the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviolet, and infrared light; however, because light is one of several forms of electromagnetic radiation, other forms such as X-rays, microwaves, and radio waves exhibit similar properties.
A principle in fluid mechanics which states that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the initial pressure variations remain the same.
A tabular display of the chemical elements organised on the basis of their atomic numbers, electron configurations, and recurring chemical properties. Elements are presented in order of increasing atomic number (number of protons).
The natural science that involves the study of matter and its motion through space and time, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.
A measure of the expectation that an event will occur or that a statement is true. Probabilities are given a value between 0 (will not occur) and 1 (will occur). The higher the probability of an event, the more certain one can be that the event will occur.
A wheel on an axle that is designed to support movement of a cable or belt along its circumference; one of six classical simple machines. Pulleys are used in a variety of ways to lift loads, apply forces, and transmit power.
A branch of physics dealing with physical phenomena at microscopic scales, where the action is on the order of the Planck constant. Quantum mechanics departs from classical mechanics primarily at the quantum realm of atomic and subatomic length scales. Quantum mechanics provides a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter.
An elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei.
Any nuclide possessing excess nuclear energy to the point that it is unstable. Such excess energy is emitted through any of several processes of radioactive decay, resulting in a stable nuclide or sometimes another unstable radionuclide which can then undergo further decay. Certain radionuclides can occur naturally; many others can be produced artificially in nuclear reactors, cyclotrons, particle accelerators, or radionuclide generators.
The change in direction of a wave as it passes from one transmission medium to another or as a result of a gradual change in the medium. Though most commonly used in the context of refraction of light, other waves such as sound waves and fluid waves also experience refraction.
An idealization of a solid body in which deformation is neglected. In other words, the distance between any two given points of a rigid body remains constant in time regardless of the external forces exerted on it. Even though such an object cannot physically exist due to relativity, objects can normally be assumed to be perfectly rigid if they are not moving near the speed of light.
Any simple physical quantity that can be described by a single number (as opposed to vectors, tensors, etc., which are described by several numbers such as magnitude and direction) and is unchanged by coordinate system rotations or translations (in Newtonian mechanics) or by Lorentz transformations or central-time translations (in relativity).
The general physical process by which some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localised non-uniformities in the medium through which they pass.
A mechanical device that changes the direction or magnitude of a force. In general, a set of six classical simple machines identified by Renaissance scientists drawing from Greek texts on technology are collectively defined as the simplest mechanisms that can provide mechanical advantage (also called leverage).
A tube in an inverted U shape that causes a liquid to flow uphill without pumps, powered by the fall of the liquid as it flows down the tube under the pull of gravity. The term may also more generally refer to a wide variety of devices involving the flow of liquids through tubes.
The tendency of a solid, liquid, or gaseous chemical substance (called a solute) to dissolve in another solid, liquid, or gaseous substance (called a solvent) to form a homogeneous solution of the solute in the solvent. The solubility of a solute fundamentally depends on the specific solvent as well as on temperature and pressure.
A universal physical constant defined as exactly 299,792,458 metres per second, a figure that is exact because the length of the metre is defined from this constant and the international standard for time. When not otherwise qualified, the term "speed of light" usually refers to the speed of light in a vacuum as opposed to the speed of light through some physical medium.
The branch of mechanics concerned with the analysis of loads (force and torque, or "moment") on physical systems in static equilibrium, that is, in a state where the relative positions of subsystems do not vary over time, or where components and structures are at a constant velocity.
The process of transformation directly from the solid phase to the gas phase without passing through an intermediate liquid phase. Sublimation is an endothermic phase transition that occurs at temperatures and pressures below a substance's triple point in its phase diagram.
A state in which there is no net flow of thermal energy between two physical systems when the systems are connected by a path permeable to heat. A system may also be said to be in thermal equilibrium with itself if the temperature within the system is spatially and temporally uniform. Systems in thermodynamic equilibrium are always in thermal equilibrium, but the converse is not always true.
The ability of a material to absorb energy and plastically deform without fracturing. Material toughness is defined as the amount of energy per unit volume that a material can absorb before rupturing. It is also defined as the resistance to fracture of a material when stressed.
Any of a variety of mathematical inequalities asserting a fundamental limit to the precision with which certain pairs of physical properties of a particle, such as position x and momentum p, can be known simultaneously.
An instrument used for measuring the difference in electrical potential between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit.
A difference in wind speed and direction over a relatively short distance in the atmosphere. Wind shear can be broken down into vertical and horizontal components, with horizontal wind shear seen across fronts and near the coast, and vertical shear typically near the surface, though also at higher levels in the atmosphere.