Category: Physics Dictionary

mathematical physics : The application of mathematics to problems in physics and the development of mathematical methods suitable for such applications and for the formulation of physical theories.

mass number: Also called atomic mass number or nucleon number.

The total number of protons and neutrons (together known as nucleons) in an atomic nucleus.

mass moment of inertia : A property of a distribution of mass in space that measures its resistance to rotational acceleration about an axis.

materials science : An interdisciplinary field incorporating elements of physics, chemistry, and engineering that is concerned with the design and discovery of new materials, particularly solids.

mass flux : The rate of mass flow per unit area. The common symbols are j, J, φ, or Φ, sometimes with subscript m to indicate mass is the flowing quantity. Its SI units are kg s−1 m−2.

mass density : See density.

mass balance: Also called material balance.

An application of the law of conservation of mass to the analysis of physical systems.

magnetism : A property of materials that respond to an applied magnetic field.

magnetic field : 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.

Maclaurin series : A representation of a function as an infinite sum of terms that are calculated from the values of the function’s derivatives at a single point.

machine element : An elementary component of a machine. There are three basic types: structural components, mechanisms, and control components.

machine : 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.

Mach number : A dimensionless quantity representing the ratio of the speed of an object moving through a fluid to the local speed of sound.

M-theory : An extension of string theory that attempts to unify seemingly contradictory mathematical formulations and which identifies 11 dimensions.

liquid crystal (LC): A state of matter which 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.

liquid : One of four classical states of matter having a definite volume but no fixed shape.

Liouville’s theorem : Phase space volume is conserved.

linear elasticity : 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.

linear algebra : The branch of mathematics concerning vector spaces, often finite or countably infinite dimensional, as well as linear mappings between such spaces.

linear actuator: A form of motor that generates a linear movement directly.

light: A form of electromagnetic radiation that occupies a certain range of wavelengths within the electromagnetic spectrum. In physics, the term sometimes refers collectively to electromagnetic radiation of any wavelength, in which case light includes gamma rays, X-rays, microwaves, and radio waves, but in common usage “light” more often refers specifically to visible light.

lepton : An elementary particle which does not undergo strong interactions but is subject to the Pauli exclusion principle. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons) and neutral leptons (better known as neutrinos).

Laplace–Runge–Lenz vector: Also abbreviated LRL vector.

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.

Kirchhoff’s equationsIn fluid dynamics, a set of equations which describe the motion of a rigid body in an ideal fluid.

laminar flow: Also called streamline flow.

Occurs when a fluid flows in parallel layers with no disruption between the layers.

Kirchhoff’s circuit laws: Also called Kirchhoff’s rules or simply Kirchhoff’s laws.

Two approximate equalities that deal with the current and voltage in electrical circuits. See Kirchhoff’s laws for other meanings of the term.

kinetic energy : The energy that a physical body possesses due to its motion, defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. The body continues to maintain this kinetic energy unless its velocity changes. Contrast potential energy.

kinematics : 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”.

Kelvin : A scale and unit of measurement of temperature. The Kelvin scale is an absolute thermodynamic temperature scale which uses absolute zero as its null point.

joule : A derived unit of energy, work, or amount of heat in the International System of Units.

isotope : A variant of a particular chemical element. While all isotopes of a given element share the same number of protons, each isotope differs from the others in its number of neutrons.

ionization : The process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions.

ionic bond : A type of chemical bond formed through an electrostatic attraction between two oppositely charged ions.

ion: An atom or molecule in which the total number of electrons is not equal to the total number of protons, giving the atom a net positive or negative electric charge.

International System of Units (SI) : The modern form of the metric system, comprising a system of units of measurement devised around seven base units and the convenience of the number ten.

impedance : The measure of the opposition that a circuit presents to a current when a voltage is applied.

ice point : A physical process that results in the phase transition of a substance from a liquid to a solid.

homeokinetics : The physics of complex, self-organizing systems.

hertz : The SI unit of frequency, defined as the number of cycles per second of a periodic phenomenon.

heat: A form of energy transferred from one body to another by thermal interaction.

half-life : 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 refer to any quantity which follows an exponential decay.

gravity : See gravitation.

gravitational wave : A ripple in the curvature of spacetime that propagates as a wave and is generated in certain gravitational interactions, travelling outward from their source.

gravitational potential : The gravitational potential at a location is equal to the work (energy transferred) per unit mass that is done by the force of gravity to move an object to a fixed reference location.

gravitational field : A model used to explain the influence that a massive body extends into the space around itself, producing a force (gravity) on another massive body. Thus, a gravitational field is used to explain and represent gravitational phenomena. It is measured in newtons per kilogram (N/kg).

gravitational energy : The potential energy associated with the gravitational field.

gravitational constant (G): Also called the universal gravitational constant and Newton’s constant.

A physical constant involved in the calculation of gravitational force between two bodies.

gravitation: Also called gravity.

A natural phenomenon by which physical bodies attract each other with a force proportional to their masses.

gamma ray : A form of electromagnetic radiation of very high frequency and therefore very high energy.

fusion : A nuclear reaction in which two or more atomic nuclei join together, or “fuse”, to form a single heavier nucleus.

fundamental forces: Also called fundamental interactions.

freezing point : The temperature at which a substance changes state from liquid to solid.

lever : A type of machine consisting of a beam or rigid rod pivoted at a fixed hinge or fulcrum; one of six classical simple machines.

force (F) : Any interaction that, when unopposed, will change the motion of a physical body. A force has both magnitude and direction, making it a vector quantity. The SI unit used to measure force is the newton.

fission : 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 relatively large amounts of energy.

entropy : A quantity which describes the randomness of a substance or system.

fermion : A type of particle that behaves according to Fermi–Dirac statistics, obeys the Pauli exclusion principle, and possesses half-integer spin. Fermions include all quarks and leptons, as well as all composite particles made of an odd number of these (such as all baryons and many atoms and nuclei). Fermions constitute one of two main classes of particles, the other being bosons.

falling bodies : Objects that are moving towards a body with greater gravitational influence, such as a planet.

exothermic : An adjective used to refer to a process or reaction that releases energy from a system, usually in the form of heat but also in the form of light, electricity, or sound. Contrast endothermic.

escape velocity : The velocity at which the kinetic energy plus the gravitational potential energy of an object is zero. It is the speed needed to “escape” from a gravitational field without further propulsion.

endothermic : 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.

energy : The ability to do work.

electronics : A field that deals with electrical circuits that involve active electrical components such as vacuum tubes, transistors, diodes, and integrated circuits as well as associated passive interconnection technologies.

electronegativity : A chemical property that describes the tendency of an atom or a functional group to attract electrons (or electron density) towards itself.

electronvolt (eV) : A unit of energy equal to approximately 1.6×10⁻¹⁹ joule. By definition, it is the amount of energy gained by the charge of a single electron moved across an electric potential difference of one volt.

electron paramagnetic resonance:Also called electron spin resonance (ESR) and electron magnetic resonance (EMR).

A method for studying materials with unpaired electrons which makes use of the Zeeman effect. It shares some basic principles with nuclear magnetic resonance (NMR).

electron : A subatomic particle with a negative elementary electric charge.

electromotive force: Also abbreviated emf.

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.

electromagnetic radiation: Also abbreviated EM radiation or EMR.

A form of energy emitted and absorbed by charged particles, which exhibits wave-like behavior as it travels through space.

electromagnetic field: Also abbreviated EM field or EMF.

A physical field produced by moving electrically charged objects.

electromagnet : A type of magnet in which the magnetic field is produced by the flow of electric current.

electricity: The set of physical phenomena associated with the presence and flow of electric charges.

electrical resistance : The opposition to the passage of an electric current through an electrical element.

electrical network : An interconnection of electrical elements such as resistors, inductors, capacitors, voltage sources, current sources, and switches.

electrical insulator : Any material whose internal electric charges do not flow freely and which therefore does not conduct an electric current under the influence of an electric field.

electrical conductor : Any material which contains movable electric charges and therefore can conduct an electric current under the influence of an electric field.

electric power: The rate at which electric energy is transferred by an electric circuit.

electric field: The region of space surrounding electrically charged particles and time-varying magnetic fields. The electric field represents the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding.

electric current : A flow of electric charge through a conductive medium.

electric circuit : An electrical network consisting of a closed loop, giving a return path for the current.

electric charge : A physical property of matter that causes it to experience a force when near other electrically charged matter. There are two types of electric charge: positive and negative.

elasticity : The tendency of a material to return to its original shape after it is deformed.

dynamics : The branch of classical mechanics that studies forces and torques and their effects on motion, as opposed to kinematics, which studies motion without reference to these forces.

ductility : A solid material’s ability to deform under tensile stress; this is often characterized by the material’s ability to be stretched into a wire.

drag: 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.

Doppler effect : 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.

distance : A numerical description of how far apart objects are.

displacement : 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.

dielectric : An electrical insulator that can be polarized by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the material as they would in a conductor but only shift slightly from their equilibrium positions, with positive charges displaced in the direction of the field’s flow and negative charges displaced in the opposite direction; this creates an internal electric field that reduces the larger field within the dielectric material.

derivative: 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.

density: Also called mass density.

A physical property of a substance defined as its mass per unit volume.

deformation: 1.  (mechanics)2.  (engineering)

deflection : The degree to which a structural element is displaced under a load. It may refer to an angle or a distance.

DC motor : A mechanically commutated electric motor powered by direct current.

Damping ratio: 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.

cyclotron : A type of particle accelerator in which charged particles accelerate outwards from the center along a spiral path.

curvilinear motion: The motion of a moving particle or object that conforms to a known or fixed curve. Such motion is studied with two coordinate systems: planar motion and cylindrical motion.

cube of theoretical physics : See cGh physics.

critical mass : The smallest amount of fissile material needed for a sustained nuclear chain reaction.

crest : The point on a wave with the maximum value or upward displacement within a cycle.