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Physics Pathway Glossary

Glossary compliments of Paul G. Hewitt,

enhancements by Michael Christel & Dean Zollman

© 2009, Paul G. Hewitt



(You can also click on any glossary entry to look it up on Wikipedia.)


A (a) Abbreviation for ampere. (b) When in lowercase italic a, the symbol for acceleration.

aberration Distortion in an image produced by a lens or mirror, caused by limitations inherent to some degree in all optical systems. See spherical aberration and chromatic aberration.

absolute zero  Lowest possible temperature that any substance can have; the temperature at which the atoms of a substance have their minimum kinetic energy. The temperature of absolute zero is –273.15°C, which is –459.7°F and 0 kelvin.

absorption lines Dark lines that appear in an absorption spectrum. The pattern of lines is unique for each element.

absorption spectrum  Continuous spectrum, like that generated by white light, interrupted by dark lines or bands that result from the absorption of light of certain frequencies by a substance through which the light passes.

ac Abbreviation for alternating current.

acceleration (a)  Rate at which an object’s velocity changes with time; the change in velocity may be in magnitude (speed), or direction, or both.


acceleration due to gravity  (g) Acceleration of a freely falling object. Its value near the Earth’s surface is about 9.8 meters per second each second.

achromatic lenses See chromatic aberration.

acoustics Study of the properties of sound, especially its transmission.

action force One of the pair of forces described in Newton’s third law. See also Newton’s laws of motion, Law 3.

additive primary colors Three colors of light - red, blue, and green - that when added in certain proportions will produce any color of the spectrum.

adhesion Molecular attraction between two surfaces making contact.

adiabatic Term applied to expansion or compression of a gas occurring without gain or loss of heat.

adiabatic process Process, often of fast expansion or compression, wherein no heat enters or leaves a system. As a result, a liquid or gas undergoing an expansion will cool, or undergoing a compression will warm.

air resistance Friction, or drag, that acts on something moving through air.

alchemist Practitioner of the early form of chemistry called alchemy, which was associated with magic. The goal of alchemy was to change base metals to gold and to discover a potion that could produce eternal youth.

alloy Solid mixture composed of two or more metals or of a metal and a nonmetal.

alpha particle Nucleus of a helium atom, which consists of two neutrons and two protons, ejected by certain radioactive nuclei.

alpha ray Stream of alpha particles (helium nuclei) ejected by certain radioactive nuclei.

alternating current (ac) Electric current that rapidly reverses in direction. The electric charges vibrate about relatively fixed positions, usually at the rate of 60 hertz.

AM Abbreviation for amplitude modulation.

ammeter A device that measures current. See galvanometer.

ampere (A)  SI unit of electric current. One ampere is a flow of one coulomb of charge per second: 6.25 × 1018 electrons (or protons) per second.

amplitude For a wave or vibration, the maximum displacement on either side of the equilibrium (midpoint) position.

amplitude modulation  (AM)  Type of modulation in which the amplitude of the carrier wave is varied above and below its normal value by an amount proportional to the amplitude of the impressed signal.

amu Abbreviation for atomic mass unit.

analog signal Signal based on a continuous variable, as opposed to a digital signal made up of discrete quantities.

aneroid barometer Instrument used to measure atmospheric pressure; based on the movement of the lid of a metal box, rather than on the movement of a liquid.

angle of incidence Angle between an incident ray and the normal to the surface it encounters.

angle of reflection Angle between a reflected ray and the normal to the surface of reflection.

angle of refraction Angle between a refracted ray and the normal to the surface at which it is refracted.

angular momentum Product of a body’s rotational inertia and rotational velocity about a particular axis. For an object that is small compared with the radial distance, it is the product of mass, speed, and radial distance of rotation.

antimatter Matter composed of atoms with negative nuclei and positive electrons.

antinode Any part of a standing wave with maximum displacement and maximum energy.

antiparticle Particle having the same mass as a normal particle, but a charge of the opposite sign. The antiparticle of an electron is a positron.

antiproton Antiparticle of a proton; a negatively charged proton.

apogee Point in an elliptical orbit farthest from the focus around which orbiting takes place. See also perigee.

Archimedes principle Relationship between buoyancy and displaced fluid: An immersed object is buoyed up by a force equal to the weight of the fluid it displaces.

armature Part of an electric motor or generator where an electromotive force is produced. Usually the rotating part.

astigmatism Defect of the eye caused when the cornea is curved more in one direction than in another.

atmospheric pressure Pressure exerted against bodies immersed in the atmosphere resulting from the weight of air pressing down from above. At sea level, atmospheric pressure is about 101 kPa.

atom Smallest particle of an element that has all the element’s chemical properties. Consists of protons and neutrons in a nucleus surrounded by electrons.

atomic bonding Linking together of atoms to form larger structures, such as molecules and solids.

atomic mass number Number associated with an atom, equal to the number of nucleons (protons plus neutrons) in the nucleus.

atomic mass unit  (amu) Standard unit of atomic mass. It is based on the mass of the common carbon atom, which is arbitrarily given the value of exactly 12. An amu of one is one-twelfth the mass of this common carbon atom.

atomic number Number associated with an atom, equal to the number of protons in the nucleus, or, equivalently, to the number of electrons in the electron cloud of a neutral atom.

aurora borealis Glowing of the atmosphere caused by ions from above the atmosphere that dip into the atmosphere; also called northern lights. In the southern hemisphere, they are called aurora australis.

average speed  Path distance divided by time interval.


Avogadro’s number 6.02 × 1023 molecules.

Avogadro’s principle Equal volumes of all gases at the same temperature and pressure contain the same number of molecules, 6.02 × 1023 in one mole (a mass in grams equal to the molecular mass of the substance in atomic mass units).

axis (pl. axes) (a) Straight line about which rotation takes place. (b) Straight lines for reference in a graph, usually the x-axis for measuring horizontal displacement and the y-axis for measuring vertical displacement.

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barometer Device used to measure the pressure of the atmosphere.

beats Sequence of alternating reinforcement and cancellation of two sets of superimposed waves differing in frequency, heard as a throbbing sound.

bel Unit of intensity of sound, named after Alexander Graham Bell. The threshold of hearing is 0 bel (10–12 watts per square meter). Often measured in decibels (dB, one-tenth of a bel).

Bernoulli’s principle Pressure in a fluid decreases as the speed of the fluid increases.

beta particle Electron (or positron) emitted during the radioactive decay of certain nuclei.

beta ray Stream of beta particles (electrons or positrons) emitted by certain radioactive nuclei.

Big Bang Primordial explosion that is thought to have resulted in the creation of our expanding universe.

bimetallic strip Two strips of different metals welded or riveted together. Because the two substances expand at different rates when heated or cooled, the strip bends; used in thermostats.

binary code Code based on the binary number system (which uses a base of 2). In binary code any number can be expressed as a succession of ones and zeros. For example, the number 1 is 1, 2 is 10, 3 is 11, 4 is 100, 5 is 101, 17 is 10001, etc. These ones and zeros can then be interpreted and transmitted electronically as a series of “on” and “off” pulses, the basis for all computers and other digital equipment.

bioluminescence Light emitted from certain living things that have the ability to chemically excite molecules in their bodies; these excited molecules then give off visible light.

biomagnetism Magnetic material located in living organisms that may help them navigate, locate food, and affect other behaviors.

black hole Concentration of mass resulting from gravitational collapse, near which gravity is so intense that not even light can escape.

blind spot Area of the retina where all the nerves carrying visual information exit the eye and go to the brain; this is a region of no vision.

blue shift Increase in the measured frequency of light from an approaching source; called the blue shift because the apparent increase is toward the high-frequency, or blue, end of the color spectrum. Also occurs when an observer approaches a source. See also Doppler effect.

boiling Change from liquid to gas occurring beneath the surface of the liquid; rapid vaporization. The liquid loses energy, the gas gains it.

bow wave V-shaped wave produced by an object moving on a liquid surface faster than the wave speed.

Boyle’s law The product of pressure and volume is a constant for a given mass of confined gas regardless of changes in either pressure or volume individually, as long as temperature remains unchanged.

P1V1 = P2V2

breeder reactor Nuclear fission reactor that not only produces power but produces more nuclear fuel than it consumes by converting a nonfissionable uranium isotope into a fissionable plutonium isotope. See also nuclear reactor.

British thermal unit  (BTU)  Amount of heat required to change the temperature of 1 pound of water by 1 Fahrenheit degree.

Brownian motion Haphazard movement of tiny particles suspended in a gas or liquid resulting from bombardment by the fast-moving molecules of the gas or liquid.

BTU Abbreviation for British thermal unit.

buoyancy Apparent loss of weight of an object immersed or submerged in a fluid.

buoyant force Net upward force exerted by a fluid on a submerged or immersed object.

butterfly effect Situation in which a very small change in one place can amplify into a large change somewhere else.

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C Abbreviation for coulomb.

cal Abbreviation for calorie.

calorie (cal)  Unit of heat. One calorie is the heat required to raise the temperature of one gram of water 1 Celsius degree. One Calorie (with a capital C) is equal to one thousand calories and is the unit used in describing the energy available from food; also called a kilocalorie (kcal).

1 cal = 4.184 J  or  1 J = 0.24 cal

capacitor Device used to store charge in a circuit.

capillarity Rise of a liquid in a fine, hollow tube or in a narrow space.

carbon dating Process of determining the time that has elapsed since death by measuring the radioactivity of the remaining carbon-14 isotopes.

Carnot efficiency Ideal maximum percentage of input energy that can be converted to work in a heat engine.

carrier wave High-frequency radio wave modified by a lower-frequency wave.

Celsius scale Temperature scale that assigns 0 to the melt-freeze point for water and 100 to the boil-condense point of water at standard pressure (one atmosphere at sea level).

center of gravity  (CG) Point at the center of an object’s weight distribution, where the force of gravity can be considered to act.

center of mass Point at the center of an object’s mass distribution, where all its mass can be considered to be concentrated. For everyday conditions, it is the same as the center of gravity.

centrifugal force Apparent outward force on a rotating or revolving body.

centripetal force Center-directed force that causes an object to follow a curved or circular path.

CG Abbreviation for center of gravity.

chain reaction Self-sustaining reaction that, once started, steadily provides the energy and matter necessary to continue the reaction.

charge See electric charge.

charging by contact Transfer of electric charge between objects by rubbing or simple touching.

charging by induction Redistribution of electric charges in and on objects caused by the electrical influence of a charged object close by but not in contact.

chemical formula Description that uses numbers and symbols of elements to describe the proportions of elements in a compound or reaction.

chemical reaction Process of rearrangement of atoms that transforms one molecule into another.

chinook Warm, dry wind that blows down from the eastern side of the Rocky Mountains across the Great Plains.

chromatic aberration Distortion of an image caused when light of different colors (and thus different speeds and refractions) focuses at different points when passing through a lens. Achromatic lenses correct this defect by combining simple lenses made of different kinds of glass.

circuit Any complete path along which electric charge can flow. See also series circuit and parallel circuit.

circuit breaker Device in an electric circuit that breaks the circuit when the current gets high enough to risk causing a fire.

coherent light Light of a single frequency with all photons exactly in phase and moving in the same direction. Lasers produce coherent light. See also incoherent light and laser.

complementarity Principle enunciated by Niels Bohr stating that the wave and particle aspects of both matter and radiation are necessary, complementary parts of the whole. Which part is emphasized depends on what experiment is conducted (i.e., on what questions one puts to nature.)

complementary colors Any two colors of light that, when added, produce white light.

component Parts into which a vector can be separated and that act in different directions from the vector. See resultant.

compound Chemical substance made of atoms of two or more different elements combined in a fixed proportion.

compression (a) In mechanics, the act of squeezing material and reducing its volume. (b) In sound, the region of increased pressure in a longitudinal wave.

concave mirror Mirror that curves inward like a “cave.”

condensation Change of phase of a gas into a liquid; the opposite of evaporation.

conduction (a) In heat, energy transfer from particle to particle within certain materials, or from one material to another when the two are in direct contact. (b) In electricity, the flow of electric charge through a conductor.

conduction electrons Electrons in a metal that move freely and carry electric charge.

conductor (a) Material through which heat can be transferred. (b) Material, usually a metal, through which electric charge can flow. Good conductors of heat are generally good electric charge conductors.

cones See retina.

conservation of angular momentum When no external torque acts on an object or a system of objects, no change of angular momentum takes place. Hence, the angular momentum before an event involving only internal torques is equal to the angular momentum after the event.

conservation of charge Principle that net electric charge is neither created nor destroyed but is transferable from one material to another.

conservation of energy Principle that energy cannot be created or destroyed. It may be transformed from one form into another, but the total amount of energy never changes.

conservation of energy for machines Work output of any machine cannot exceed the work input.

conservation of momentum In the absence of a net external force, the momentum of an object or system of objects is unchanged.

            mu(before event) = mu(after event)

conserved Term applied to a physical quantity, such as momentum, energy, or electric charge, that remains unchanged during interactions.

constructive interference Combination of waves so that two or more waves overlap to produce a resulting wave of increased amplitude. See also interference.

convection Means of heat transfer by movement of the heated substance itself, such as by currents in a fluid.

converging lens Lens that is thicker in the middle than at the edges and refracts parallel rays of light passing through it to a focus. See also diverging lens.

convex mirror Mirror that curves outward. The virtual image formed is smaller and closer to the mirror than the object. See also concave mirror.

cornea Transparent covering over the eyeball, which helps focus the incoming light.

correspondence principle If a new theory is valid, it must account for the verified results of the old theory in the region where both theories apply.

cosmic ray One of various high-speed particles that travel throughout the universe and originate in violent events in stars.

cosmology Study of the origin and development of the entire universe.

coulomb (C)  SI unit of electrical charge. One coulomb is equal to the total charge of 6.25 × 1018 electrons.

Coulomb’s law Relationship among electrical force, charges, and distance: The electrical force between two charges varies directly as the product of the charges (q) and inversely as the square of the distance between them. (k is the proportionality constant 9 × 109 Nxm2/C2) If the charges are alike in sign, the force is repulsive; if the charges are unlike, the force is attractive.


crest One of the places in a wave where the wave is highest or the disturbance is greatest in the opposite direction from a trough. See also trough.

critical angle Minimum angle of incidence for which a light ray is totally reflected within a medium.

critical mass Minimum mass of fissionable material in a nuclear reactor or nuclear bomb that will sustain a chain reaction. A subcritical mass is one in which the chain reaction dies out. A supercritical mass is one in which the chain reaction builds up explosively.

crystal Regular geometric shape found in a solid in which the component particles are arranged in an orderly, three-dimensional, repeating pattern.

current See electric current.

cyclotron Particle accelerator that imparts high energy to charged particles such as protons, deuterons, and helium ions.

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dark matter Unseen and unidentified matter that is evident by its gravitational pull on stars in the galaxies—comprising perhaps 90% of the matter of the universe.

dB Abbreviation for decibel. See bel.

dc Abbreviation for direct current.

DDT Abbreviation for the chemical pesticide dichloro diphenyl trichloroethane.

de Broglie matter waves All particles have wave properties; in de Broglie’s equation, the product of momentum and wavelength equals Planck’s constant.

decibel (dB)  One-tenth of a bel.

de-excitation See excitation.

density Mass of a substance per unit volume. Weight density is weight per unit volume. In general, any item per space element (e.g., number of dots per area).



destructive interference Combination of waves so that crest parts of one wave overlap trough parts of another, resulting in a wave of decreased amplitude. See also interference.

deuterium Isotope of hydrogen whose atom has a proton, a neutron, and an electron. The common isotope of hydrogen has only a proton and an electron; therefore, deuterium has more mass.

deuteron Nucleus of a deuterium atom; it has one proton and one neutron.

dichroic crystal Crystal that divides unpolarized light into two internal beams polarized at right angles and strongly absorbs one beam while transmitting the other.

diffraction Bending of light that passes around an obstacle or through a narrow slit, causing the light to spread and to produce light and dark fringes.

diffraction grating Series of closely spaced parallel slits or grooves that are used to separate colors of light by interference.

diffuse reflection Reflection of waves in many directions from a rough surface. See also polished.

digital audio Audio reproduction system that uses binary code to record and reproduce sound.

digital signal Signal made up of discrete quantities or signals, as opposed to an analog signal which is based on a continuous signal.

diode Electronic device that restricts current to a single direction in an electric circuit; a device that changes alternating current to direct current.

dipole See electric dipole.

direct current  (dc)  Electric current whose flow of charge is always in one direction.

dispersion Separation of light into colors arranged according to their frequency, for example by interaction with a prism or a diffraction grating.

displaced Term applied to the fluid that is moved out of the way when an object is placed in fluid. A submerged object always displaces a volume of fluid equal to its own volume.

diverging lens Lens that is thinner in the middle than at the edges, causing parallel rays of light passing through it to diverge as if from a point. See also converging lens.

Doppler effect Change in frequency of a wave of sound or light due to the motion of the source or the receiver. See also red shift and blue shift.

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echo Reflection of sound.

eddy Changing, curling paths in turbulent flow of a fluid.

efficiency In a machine, the ratio of useful energy output to total energy input, or the percentage of the work input that is converted to work output.


elastic collision Collision in which colliding objects rebound without lasting deformation or heat generation.

elastic limit Distance of stretching or compressing beyond which an elastic material will not return to its original state.

elasticity Property of a solid wherein a change in shape is experienced when a deforming force acts on it, with a return to its original shape when the deforming force is removed.

electric charge Fundamental electrical property to which the mutual attractions or repulsions between electrons or protons is attributed.

electric current Flow of electric charge that transports energy from one place to another. Measured in amperes, where one ampere is the flow of 6.25 × 1018 electrons (or protons) per second.

electric dipole Molecule in which the distribution of charge is uneven, resulting in slightly opposite charges on opposite sides of the molecule.

electric field Force field that fills the space around every electric charge or group of charges. Measured by force per charge (newtons/coulomb).

electric potential Electric potential energy (in joules) per unit of charge (in coulombs) at a location in an electric field; measured in volts and often called voltage.


electric potential energy Energy a charge has due to its location in an electric field.

electric power Rate of electrical energy transfer or the rate of doing work, which can be measured by the product of current and voltage.

power = current × voltage

electrical force Force that one charge exerts on another. When the charges are the same sign, they repel; when the charges are opposite, they attract.

electrical resistance Resistance of a material to the flow of electric charge through it; measured in ohms (symbol V).

electrically polarized  Term applied to an atom or molecule in which the charges are aligned so that one side is slightly more positive or negative than the opposite side.

electricity General term for electrical phenomena, much like gravity has to do with gravitational phenomena, or sociology with social phenomena.

electrode Terminal, for example of a battery, through which electric current can pass.

electrodynamics Study of moving electric charge, as opposed to electrostatics.

electromagnet Magnet whose magnetic properties are produced by electric current.

electromagnetic induction Phenomenon of inducing a voltage in a conductor by changing the magnetic field near the conductor. If the magnetic field within a closed loop changes in any way, a voltage is induced in the loop. The induction of voltage is actually the result of a more fundamental phenomenon: the induction of an electric field. See also Faraday’s law.

electromagnetic radiation Transfer of energy by the rapid oscillations of electromagnetic fields, which travel in the form of waves called electromagnetic waves.

electromagnetic spectrum Range of frequencies over which electromagnetic radiation can be propagated. The lowest frequencies are associated with radio waves; microwaves have a higher frequency, and then infrared waves, light, ultraviolet radiation, X rays, and gamma rays in sequence.

electromagnetic wave Energy-carrying wave emitted by vibrating charges (often electrons) that is composed of oscillating electric and magnetic fields that regenerate one another. Radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X rays, and gamma rays are all composed of electromagnetic waves.

electromotive force (emf)  Any voltage that gives rise to an electric current. A battery or a generator is a source of emf.

electron Negative particle in the shell of an atom.

electron volt  (eV)  Amount of energy equal to that an electron acquires in accelerating through a potential difference of 1 volt.

electrostatics Study of electric charges at rest, as opposed to electrodynamics.

element Substance composed of atoms that all have the same atomic number and, therefore, the same chemical properties.

elementary particles Subatomic particles. The basic building blocks of all matter, consisting of two classes of particles, the quarks and the leptons.

ellipse Closed curve of oval shape wherein the sum of the distances from any point on the curve to two internal focal points is a constant.

emf Abbreviation for electromotive force.

emission spectrum Distribution of wavelengths in the light from a luminous source.

energy That which that can change the condition of matter. Commonly defined as the ability to do work; actually only describable by examples. It is not a material substance.

entropy A measure of the disorder of a system. Whenever energy freely transforms from one form to another, the direction of transformation is toward a state of greater disorder and therefore toward one of greater entropy.

equilibrium In general, a state of balance. For mechanical equilibrium, the state in which no net forces and no net torques act. In liquids, the state in which evaporation equals condensation. More generally, the state in which no net change of energy occurs.

equilibrium rule SF=0. On an object or system of objects in mechanical equilibrium, the sum of forces equal zero. Also, Si= 0; the sum of the torques equal zero.

escape velocity  Velocity that a projectile, space probe, etc., must reach to escape the gravitational influence of the Earth or celestial body to which it is attracted.

ether Hypothetical invisible medium that was formerly thought to be required for the propagation of electromagnetic waves, and thought to fill space throughout the universe.

eV Abbreviation for electron volt.

evaporation Change of phase from liquid to gas that takes place at the surface of a liquid. The opposite of condensation.

excitation Process of boosting one or more electrons in an atom or molecule from a lower to a higher energy level. An atom in an excited state will usually decay (de-excite) rapidly to a lower state by the emission of radiation. The frequency and energy of emitted radiation are related by

E= hf

excited See excitation.

eyepiece Lens of a telescope closest to the eye; which enlarges the real image formed by the first lens.

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fact Close agreement by competent observers of a series of observations of the same phenomena.

Fahrenheit scale Temperature scale in common use in the United States. The number 32 is assigned to the melt-freeze point of water, and the number 212 to the boil-condense point of water at standard pressure (one atmosphere, at sea level).

Faraday’s law Induced voltage in a coil is proportional to the product of the number of loops and the rate at which the magnetic field changes within those loops. In general, an electric field is induced in any region of space in which a magnetic field is changing with time. The magnitude of the induced electric field is proportional to the rate at which the magnetic field changes. See also Maxwell’s counterpart to Faraday’s law.

voltage induced ~ number of loops × equation

Fermat’s principle of least time Light takes the path that requires the least time when it goes from one place to another.

field See force field.

field lines See magnetic field lines.

flotation See principle of flotation.

fluid Anything that flows; in particular, any liquid or gas.

fluorescence Property of certain substances to absorb radiation of one frequency and to re-emit radiation of a lower frequency.

FM Abbreviation for frequency modulation.

focal length Distance between the center of a lens and either focal point; the distance from a mirror to its focal point.

focal plane Plane, perpendicular to the principal axis, that passes through a focal point of a lens or mirror. For a converging lens or a concave mirror, any incident parallel rays of light converge to a point somewhere on a focal plane. For a diverging lens or a convex mirror, the rays appear to come from a point on a focal plane.

focal point For a converging lens or a concave mirror, the point at which rays of light parallel to the principal axis converge. For a diverging lens or a convex mirror, the point from which such rays appear to come.

focus (pl. foci) (a) For an ellipse, one of the two points for which the sum of the distances to any point on the ellipse is a constant. A satellite orbiting the Earth moves in an ellipse that has the Earth at one focus. (b) For optics, a focal point.

force Any influence that tends to accelerate an object; a push or pull; measured in newtons. Force is a vector quantity.

force field That which exists in the space surrounding a mass, electric charge, or magnet, so that another mass, electric charge, or magnet introduced into this region will experience a force. Examples of force fields are gravitational fields, electric fields, and magnetic fields.

forced vibration Vibration of an object caused by the vibrations of a nearby object. The sounding board in a musical instrument amplifies the sound through forced vibration.

Fourier analysis Mathematical method that disassembles any periodic wave form into a combination of simple sine waves.

fovea Area of the retina that is in the center of the field of view; region of most distinct vision.

frame of reference Vantage point (usually a set of coordinate axes) with respect to which position and motion may be described.

Fraunhofer lines Dark lines visible in the spectrum of the sun or a star.

free fall Motion under the influence of gravity only.

free radical Unbonded, electrically neutral, very chemically active atom or molecular fragment.

freezing Change in phase from liquid to solid; the opposite of melting.

frequency For a vibrating body or medium, the number of vibrations per unit time. For a wave, the number of crests that pass a particular point per unit time. Frequency is measured in hertz.

frequency modulation  (FM) Type of modulation in which the frequency of the carrier wave is varied above and below its normal frequency by an amount that is proportional to the amplitude of the impressed signal. In this case, the amplitude of the modulated carrier wave remains constant.

friction Force that acts to resist the relative motion (or attempted motion) of objects or materials that are in contact.

fuel cell A device that converts chemical energy to electrical energy, but unlike a battery is continually fed with fuel, usually hydrogen.

fulcrum Pivot point of a lever.

fundamental frequency See partial tone.

fuse Device in an electric circuit that breaks the circuit when the current gets high enough to risk causing a fire.

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g (a) Abbreviation for gram. (b) When in lower-case italic g, the symbol for the acceleration due to gravity (at the Earth’s surface, 9.8 m/s2). (c) When in lower case bold g  the gravitational field vector (at the Earth’s surface, 9.8 N/kg). (d) When in upper-case italic G, the symbol for the universal gravitation constant (6.67 × 10–11 N×m2/kg2).

galvanometer Instrument used to detect electric current. With the proper combination of resistors, it can be converted to an ammeter or a voltmeter. An ammeter is calibrated to measure electric current. A voltmeter is calibrated to measure electric potential.

gamma ray High-frequency electromagnetic radiation emitted by atomic nuclei.

gas Phase of matter beyond the liquid phase, wherein molecules fill whatever space is available to them, taking no definite shape.

general theory of relativity Einstein’s generalization of special relativity, which deals with accelerated motion and features a geometric theory of gravitation.

generator Machine that produces electric current, usually by rotating a coil within a stationary magnetic field.

geodesic Shortest path between points on any surface.

geosynchronous orbit A satellite orbit in which the satellite orbits the Earth once each day. When moving westward, the satellite remains at a fixed point (about 42,000 km) above the Earth’s surface.

global warming See greenhouse effect.

gram (g)  A metric unit of mass. It is one thousandth of a kilogram.

gravitation Attraction between objects due to mass. See also law of universal gravitation and universal gravitational constant.

gravitational field Force field that exists in the space around every mass or group of masses; measured in newtons per kilogram.

gravitational potential energy Energy that a body possesses because of its position in a gravitational field. On Earth, potential energy (PE) equals mass (m) times the acceleration due to gravity (g) times height (h) from a reference level such as the Earth’s surface.

PE = mgh

gravitational red shift Shift in wavelength toward the red end of the spectrum experienced by light leaving the surface of a massive object, as predicted by the general theory of relativity.

gravitational wave Gravitational disturbance that propagates through spacetime made by a moving mass. (Undetected at this writing.)

graviton Quantum of gravity, similar in concept to the photon as a quantum of light. (Undetected at this writing.)

greenhouse effect Warming effect caused by short-wavelength radiant energy from the sun that easily enters the atmosphere and is absorbed by the Earth, but when radiated at longer wavelengths cannot easily escape the Earth’s atmosphere.

grounding Allowing charges to move freely along a connection from a conductor to the ground.

group Elements in the same column of the periodic table.

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h (a) Abbreviation for hour (though hr. is often used). (b) When in italic h, the symbol for Planck’s constant.

hadron Elementary particle that can participate in strong nuclear force interactions.

half-life Time required for half the atoms of a radioactive isotope of an element to decay. This term is also used to describe decay processes in general.

harmonic See partial tone.

heat The energy that flows from one object to another by virtue of a difference in temperature. Measured in calories or joules.

heat capacity See specific heat capacity.

heat engine A device that uses heat as input and supplies mechanical work as output, or that uses work as input and moves heat “uphill” from a cooler to a warmer place.

heat of fusion Amount of energy that must be added to a kilogram of a solid (already at its melting point) to melt it.

heat of vaporization Amount of energy that must be added to a kilogram of a liquid (already at its boiling point) to vaporize it.

heat pump A device that transfers heat out of a cool environment and into a warm environment.

heat waves See infrared waves.

heavy water Water (H2O) that contains the heavy hydrogen isotope deuterium.

hertz (Hz)  SI unit of frequency. One hertz is one vibration per second.

hologram Two-dimensional microscopic interference pattern that shows three-dimensional optical images.

Hooke’s law Distance of stretch or squeeze (extension or compression) of an elastic material is directly proportional to the applied force. Where Dx is the change in length and k is the spring constant,

F = kDx

humidity Measure of the amount of water vapor in the air. Absolute humidity is the mass of water per volume of air. Relative humidity is absolute humidity at that temperature divided by the maximum possible, usually given as a percent.

Huygens’ principle Light waves spreading out from a light source can be regarded as a superposition of tiny secondary wavelets.

hypothesis Educated guess; a reasonable explanation of an observation or experimental result that is not fully accepted as factual until tested over and over again by experiment.

Hz Abbreviation for hertz.

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ideal efficiency Upper limit of efficiency for all heat engines; it depends on the temperature difference between input and exhaust.


impulse Product of force and the time interval during which the force acts. Impulse produces change in momentum.

impulse = Ft = D (mu)

incandescence State of glowing while at a high temperature, caused by electrons bouncing around over dimensions larger than the size of an atom, emitting radiant energy in the process. The peak frequency of radiant energy is proportional to the absolute temperature of the heated substance:


incoherent light Light containing waves with a jumble of frequencies, phases, and possibly directions. See also coherent light and laser.

index of refraction  (n) Ratio of the speed of light in a vacuum to the speed of light in another material.


induced (a) Term applied to electric charge that has been redistributed on an object due to the presence of a charged object nearby. (b) Term applied to a voltage, electric field, or magnetic field that is created due to a change in or motion through a magnetic field or electric field.

induction Charging of an object without direct contact. See also electromagnetic induction.

inelastic Term applied to a material that does not return to its original shape after it has been stretched or compressed.

inelastic collision Collision in which the colliding objects become distorted and/or generate heat during the collision, and possibly stick together.

inertia Sluggishness or apparent resistance of an object to change its state of motion. Mass is the measure of inertia.

inertial frame of reference Unaccelerated vantage point in which Newton’s laws hold exactly.

infrared Electromagnetic waves of frequencies lower than the red of visible light.

infrared waves Electromagnetic waves that have a lower frequency than visible red light.

infrasonic Term applied to sound frequencies below 20 hertz, the normal lower limit of human hearing.

in parallel Term applied to portions of an electric circuit that are connected at two points and provide alternative paths for the current between those two points.

in phase Term applied to two or more waves whose crests (and troughs) arrive at a place at the same time, so that their effects reinforce each other.

in series Term applied to portions of an electric circuit that are connected in a row so that the current that goes through one must go through all of them.

instantaneous speed Speed at any instant.

insulator (a) Material that is a poor conductor of heat and that delays the transfer of heat. (b) Material that is a poor conductor of electricity.

intensity Power per square meter carried by a sound wave, often measured in decibels.

interaction Mutual action between objects where each object exerts an equal and opposite force on the other.

interference Result of superposing different waves, often of the same wavelength. Constructive interference results from crest-to-crest reinforcement; destructive interference results from crest-to-trough cancellation. The interference of selected wavelengths of light produces colors known as interference colors. See also constructive interference, destructive interference, interference pattern, and standing wave.

interference pattern Pattern formed by the overlapping of two or more waves that arrive in a region at the same time.

interferometer Device that uses the interference of light waves to measure very small distances with high accuracy. Michelson and Morley used an interferometer in their famous experiments with light.

internal energy The total energy stored in the atoms and molecules within a substance. Changes in internal energy are of principal concern in thermodynamics.

inverse-square law Law relating the intensity of an effect to the inverse square of the distance from the cause. Gravity, electric, magnetic, light, sound, and radiation phenomena follow the inverse-square law.


inversely When two values change in opposite directions, so that if one increases and the other decreases by the same amount, they are said to be inversely proportional to each other.

ion Atom (or group of atoms bound together) with a net electric charge, which is due to the loss or gain of electrons. A positive ion has a net positive charge. A negative ion has a net negative charge.

ionization Process of adding or removing electrons to or from the atomic nucleus.

iridescence Phenomenon whereby interference of light waves of mixed frequencies reflected from the top and bottom of thin films produces an assortment of colors.

iris Colored part of the eye that surrounds the black opening through which light passes. The iris regulates the amount of light entering the eye.

isotopes Atoms whose nuclei have the same number of protons but different numbers of neutrons.

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J Abbreviation for joule.

joule (J)  SI unit of work and of all other forms of energy. One joule of work is done when a force of one newton is exerted on an object moved one meter in the direction of the force.

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K (a) Abbreviation for kelvin. (b) When in lower case k, the abbreviation for the prefix kilo-. (c) When in lower case italics k, the symbol for the electrical proportionality constant in Coulomb’s law. It is approximately 9 × 109 N?m2/C2.(d) When in lower case italics k, the symbol for the spring constant in Hooke’s law.

kcal Abbreviation for kilocalorie.

KE Abbreviation for kinetic energy.

kelvin SI unit of temperature. A temperature measured in kelvins (symbol K) indicates the number of units above absolute zero. Divisions on the Kelvin scale and Celsius scale are the same size, so a change in temperature of one kelvin equals a change in temperature of one Celsius degree.

Kelvin scale Temperature scale, measured in kelvins K, whose zero (called absolute zero) is the temperature at which it is impossible to extract any more internal energy from a material. 0 K = –273.15°C. There are no negative temperatures on the Kelvin scale.

Kepler’s laws

Law 1: The path of each planet around the Sun is an ellipse with the Sun at one focus.

Law 2: The line from the Sun to any planet sweeps out equal areas of space in equal time intervals.

Law 3: The square of the orbital period of a planet is directly proportional to the cube of the average distance of the planet from the Sun (T2 ~ r3 for all planets).

kg Abbreviation for kilogram.

kilo  -  Prefix that means thousand, as in kilowatt or kilogram.

kilocalorie (kcal)  Unit of heat. One kilocalorie equals 1000 calories, or the amount of heat required to raise the temperature of one kilogram of water by 1°C. Equal to one food Calorie.

kilogram (kg)  Fundamental SI unit of mass. It is equal to 1000 grams. One kilogram is very nearly the amount of mass in one liter of water at 4°C.

kilometer (km)  One thousand meters.

kilowatt (kW)  One thousand watts.

kilowatt-hour  (kWh)  Amount of energy consumed in 1 hour at the rate of 1 kilowatt.

kinetic energy  (KE)  Energy of motion, equal (nonrelativistically) to half the mass multiplied by the speed squared.

KE= equationmu2

km Abbreviation for kilometer.

kPa Abbreviation for kilopascal. See pascal.

kWh Abbreviation for kilowatt-hour.

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L Abbreviation for liter. (In some textbooks, lowercase l is used.)

laser Optical instrument that produces a beam of coherent light—that is, light with all waves of the same frequency, phase, and direction. The word is an acronym for light amplification by stimulated emission of radiation.

latent heat of fusion The amount of energy required to change a unit mass of a substance from solid to liquid (and vice versa).

latent heat of vaporization The amount of energy required to change a unit mass of a substance from liquid to gas (and vice versa).

law General hypothesis or statement about the relationship of natural quantities that has been tested over and over again and has not been contradicted. Also known as a principle.

law of conservation of momentum In the absence of an external force, the momentum of a system remains unchanged. Hence, the momentum before an event involving only internal forces is equal to the momentum after the event:

            mu(before event) = mu(after event)

law of inertia See Newton’s laws of motion, Law 1.

law of reflection The angle of incidence for a wave that strikes a surface is equal to the angle of reflection. This is true for both partially and totally reflected waves. See also angle of incidence and angle of reflection.

law of universal gravitation For any pair of particles, each particle attracts the other particle with a force that is directly proportional to the product of the masses of the particles, and inversely proportional to the square of the distance between them (or their centers of mass if spherical objects), where F is the force, m is the mass, d is distance, and G is the gravitation constant:

F ~ equation or F = G equation

least time See Fermat’s principle of least time.

length contraction Shrinkage of space, and therefore of matter, in a frame of reference moving at relativistic speeds.

lens Piece of glass or other transparent material that can bring light to a focus.

lepton Class of elementary particles that are not involved with the nuclear force. It includes the electron and its neutrino, the muon and its neutrino, and the tau and its neutrino.

lever Simple machine made of a bar that turns about a fixed point called the fulcrum.

lever arm Perpendicular distance between an axis and the line of action of a force that tends to produce rotation about that axis.

lift In application of Bernoulli’s principle, the net upward force produced by the difference between upward and downward pressures. When lift equals weight, horizontal flight is possible.

light Visible part of the electromagnetic spectrum.

light-year The distance light travels in a vacuum in one year: 9.46 × 1012 km.

line spectrum Pattern of distinct lines of color, corresponding to particular wavelengths, that are seen in a spectroscope when a hot gas is viewed. Each element has a unique pattern of lines.

linear momentum Product of the mass and the velocity of an object. Also called momentum. (This definition applies at speeds much less than the speed of light.)

linear motion Motion along a straight-line path.

linear speed Path distance moved per unit of time. Also called simply speed.

liquid Phase of matter between the solid and gaseous phases in which the matter possesses a definite volume but no definite shape: it takes on the shape of its container.

liter (L)  Metric unit of volume. A liter is equal to 1000 cm3.

logarithmic Exponential.

longitudinal wave Wave in which the individual particles of a medium vibrate back and forth in the direction in which the wave travels - for example, sound.

Lorentz contraction See length contraction.

loudness Physiological sensation directly related to sound intensity or volume. Relative loudness, or sound level, is mea-sured in decibels.

lunar eclipse Event wherein the full moon passes into the shadow of the Earth.

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m (a) Abbreviation for meter. (b) When in italic m, the abbreviation for mass.

Mach number Ratio of the speed of an object to the speed of sound. For example, an aircraft traveling at the speed of sound is rated Mach 1.0; traveling at twice the speed of sound, Mach 2.0.

machine Device for increasing (or decreasing) a force or simply changing the direction of a force.

magnet  Any object that has magnetic properties, that is the ability to attract objects made of iron or other magnetic substances. See also electromagnetism and magnetic force.

magnetic declination Discrepancy between the orientation of a compass pointing toward magnetic north and the true geographic north.

magnetic domain Microscopic cluster of atoms with their magnetic fields aligned.

magnetic field Region of magnetic influence around a magnetic pole or a moving charged particle.

magnetic field lines Lines showing the shape of a magnetic field. A compass placed on such a line will turn so that the needle is aligned with it.

magnetic force (a) Between magnets, it is the attraction of unlike magnetic poles for each other and the repulsion between like magnetic poles. (b) Between a magnetic field and a moving charged particle, it is a deflecting force due to the motion of the particle: The deflecting force is perpendicular to the magnetic field lines and the direction of motion. This force is greatest when the charged particle moves perpendicular to the field lines and is smallest (zero) when it moves parallel to the field lines.

magnetic monopole Hypothetical particle having a single north or a single south magnetic pole, analogous to a positive or negative electric charge.

magnetic pole One of the regions on a magnet that produces magnetic forces.

magnetic pole reversal When the magnetic field of an astronomical body reverses its poles, that is, the location where the north magnetic pole existed becomes the south magnetic pole, and the south magnetic pole becomes the north magnetic pole.

magnetism Property of being able to attract objects made of iron, steel, or magnetite. See also electromagnetism and magnetic force.

magnetohydrodynamic generator  (MHD generator)  Device generating electric power by interaction of a plasma and a magnetic field.

maser Instrument that produces a beam of microwaves. The word is an acronym for microwave amplification by stimulated emission of radiation.

mass (m)  Quantity of matter in an object; the measurement of the inertia or sluggishness that an object exhibits in response to any effort made to start it, stop it, or change in any way its state of motion; a form of energy.

mass spectrometer Device that magnetically separates charged ions according to their mass.

mass-energy equivalence Relationship between mass and energy as given by the equation

E = mc2

where c is the speed of light.

matter waves See de Broglie matter waves.

Maxwell’s counterpart to Faraday’s law A magnetic field is created in any region of space in which an electric field is changing with time. The magnitude of the induced magnetic field is proportional to the rate at which the electric field changes. The direction of the induced magnetic field is at right angles to the changing electric field.

mechanical advantage Ratio of output force to input force for a machine.

mechanical energy Energy due to the position or the movement of something; potential or kinetic energy (or a combination of both).

mechanical equilibrium State of an object or system of objects for which any impressed forces cancel to zero and no acceleration occurs and when no net torque exists. That is, SF=0, and St=0.

mega -  Prefix that means million, as in megahertz or megajoule.

melting Change in phase from solid to liquid; the opposite of freezing. Melting is a different process from dissolving, in which an added solid mixes with a liquid and the solid dissociates.

meson Elementary particle with an atomic weight of zero; can participate in the strong interaction.

metastable state State of excitation of an atom that is characterized by a prolonged delay before de-excitation.

meter (m)  Standard SI unit of length (3.28 feet).

MeV Abbreviation for million electron volts, a unit of energy, or equivalently a unit of mass.

MHD Abbreviation for magnetohydrodynamic.

mi Abbreviation for mile.

microscope Optical instrument that forms enlarged images of very small objects.

microwaves Electromagnetic waves with frequencies greater than radio waves but less than infrared waves.

min Abbreviation for minute.

mirage False image that appears in the distance and is due to the refraction of light in the Earth’s atmosphere.

mixture Substances mixed together without combining chemically.

MJ Abbreviation for megajoules, million joules.

model Representation of an idea created to make the idea more understandable.

modulation Impressing a signal wave system on a higher frequency carrier wave, amplitude modulation (AM) for amplitude signals and frequency modulation (FM) for frequency signals.

molecule Two or more atoms of the same or different elements bonded to form a larger particle.

momentum Inertia in motion. The product of the mass and the velocity of an object (provided the speed is much less than the speed of light). Has magnitude and direction and therefore is a vector quantity. Also called linear momentum, and abbreviated p.

            p = mu

monochromatic light Light made of only one color and therefore waves of only one wavelength and frequency.

muon Elementary particle in the class of elementary particles called leptons. It is short-lived with a mass that is 207 times that of an electron; may be positively or negatively charged.

music Scientifically speaking, sound with periodic tones, which appear on an oscilloscope as a regular pattern.

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N Abbreviation for newton.

nanometer Metric unit of length that is 10–9 meter (one billionth of a meter).

natural frequency Frequency at which an elastic object naturally tends to vibrate if it is disturbed and the disturbing force is removed.

neap tide Tide that occurs when the moon is halfway between a new moon and a full moon, in either direction. The tides due to the sun and the moon partly cancel, so that the high tides are lower than average and the low tides are not as low as average. See also spring tide.

net force Combination of all the forces that act on an object.

neutrino Elementary particle in the class of elementary particles called leptons. It is uncharged and almost massless; three kinds - electron, muon, and tau neutrinos, are the most common high-speed particles in the universe; more than a billion pass unhindered through each person every second.

neutron Electrically neutral particle that is one of the two kinds of nucleons that compose an atomic nucleus.

neutron star Star that has undergone a gravitational collapse in which electrons are compressed into protons to form neutrons.

newton (N)  SI unit of force. One newton is the force applied to a one-kilogram mass that will produce an acceleration of one meter per second per second.

Newton’s law of cooling The rate of cooling of an object - whether by conduction, convection, or radiation - is approximately proportional to the temperature difference between the object and its surroundings.

Newton’s laws of motion

Law 1: Every body continues in its state of rest, or of motion in a straight line at constant speed, unless it is compelled to change that state by a net force exerted upon it. Also known as the law of inertia.

Law 2: >The acceleration produced by a net force on a body is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the body.

Law 3: Whenever one body exerts a force on a second body, the second body exerts an equal and opposite force on the first.

node Any part of a standing wave that remains stationary; a region of minimal or zero energy.

noise Scientifically speaking, sound that corresponds to an irregular vibration of the eardrum produced by some irregular vibration, which appears on an oscilloscope as an irregular pattern.

nonlinear motion Any motion not along a straight-line path.

normal At right angles to, or perpendicular to. A normal force acts at right angles to the surface on which it acts. In optics, a normal defines the line perpendicular to a surface about which angles of light rays are measured.

normal force Component of support force perpendicular to a supporting surface. For an object resting on a horizontal surface, it is the upward force that balances the weight of the object.

northern lights See aurora borealis.

nuclear fission Splitting of an atomic nucleus, particularly that of a heavy element such as uranium-235, into two lighter elements, accompanied by the release of much energy.

nuclear force Attractive force within a nucleus that holds neutrons and protons together. Part of the nuclear force is called the strong interaction. The strong interaction is an attractive force that acts between protons, neutrons, and mesons (another nuclear particle); however, it acts only over very short distances (10–15 meter). The weak interaction is the nuclear force responsible for beta (electron) emission.

nuclear fusion Combining of nuclei of light atoms, such as hydrogen, into heavier nuclei, accompanied by the release of much energy. See also thermonuclear fusion.

nuclear reactor Apparatus in which controlled nuclear fission or nuclear fusion reactions take place.

nucleon Principal building block of the nucleus. A neutron or a proton; the collective name for either or both.

nucleus (pl. nuclei) Positively charged center of an atom, which contains protons and neutrons and has almost all the mass of the entire atom but only a tiny fraction of the volume.

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objective lens In an optical device using compound lenses, the lens closest to the object observed.

octave In music, the eighth full tone above or below a given tone. The tone an octave above has twice as many vibrations per second as the original tone; the tone an octave below has half as many vibrations per second.

ohm (V) SI unit of electrical resistance. One ohm is the resistance of a device that draws a current of one ampere when a voltage of one volt is impressed across it.

Ohm’s law Current in a circuit is directly proportional to the voltage impressed across the circuit, and is inversely proportional to the resistance of the circuit.


opaque Term applied to materials that absorb light without re-emission, and consequently do not allow light through them.

optical fiber Transparent fiber, usually of glass or plastic, that can transmit light down its length by means of total internal reflection.

oscillation Same as vibration: a repeating to-and-fro motion about an equilibrium position. Both oscillation and vibration refer to periodic motion, that is, motion that repeats.

oscillatory motion To-and-fro vibratory motion, such as that of a pendulum.

out of phase Term applied to two waves for which the crest of one wave arrives coincident with a trough of the second wave. Their effects tend to cancel each other.

overtone Musical term where the first overtone is the second harmnic. See also partial tone.

oxidize Chemical process in which an element or molecule loses one or more electrons.

ozone Gas, found in a thin layer in the upper atmosphere, composed of molecules of three oxygen atoms. Atmospheric oxygen gas is composed of molecules of two oxygen atoms.

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Pa Abbreviation for the SI unit pascal.

parabola Curved path followed by a projectile acting under the influence of gravity only.

parallax Apparent displacement of an object when viewed by an observer from two different positions; often used to calculate the distance of stars.

parallel circuit Electric circuit with two or more devices connected in such a way that the same voltage acts across each one and any single one completes the circuit independently of the others. See also in parallel.

partial tone One of the many tones that make up one musical sound. Each partial tone (or partial) has only one frequency. The lowest partial of a musical sound is called the fundamental frequency. Any partial whose frequency is a multiple of the fundamental frequency is called a harmonic. The fundamental frequency is also called the first harmonic. The second harmonic has twice the frequency of the fundamental; the third harmonic, three times the frequency, and so on.

pascal (Pa)  SI unit of pressure. One pascal of pressure exerts a normal force of one newton per square meter. A kilopascal (kPa) is 1000 pascals.

Pascal’s principle Changes in pressure at any point in an enclosed fluid at rest are transmitted undiminished to all points in the fluid and act in all directions.

PE Abbreviation for potential energy.

penumbra Partial shadow that appears where some of the light is blocked and other light can fall. See also umbra.

percussion In musical instruments, the striking of one object against another.

perigee Point in an elliptical orbit closest to the focus about which orbiting takes place. See also apogee.

period In general, the time required to complete a single cycle. (a) For orbital motion, the time required for a complete orbit. (b) For vibrations or waves, the time required for one complete cycle, equal to 1/frequency.

periodic table Chart that lists elements by atomic number and by electron arrangements, so that elements with similar chemical properties are in the same column (group). See Figure 11.15, pages 214–215.

perturbation Deviation of an orbiting object (e.g., a planet) from its path around a center of force (e.g., the sun) caused by the action of an additional center of force (e.g., another planet).

phase (a) One of the four main forms of matter: solid, liquid, gas, and plasma. Often called state. (b) The part of a cycle that a wave has advanced at any moment. See also in phase and out of phase.

phosphor Powdery material, such as that used on the inner surface of a fluorescent light tube, that absorbs ultraviolet photons, then gives off visible light.

phosphorescence Type of light emission that is the same as fluorescence except for a delay between excitation and de-excitation, which provides an afterglow. The delay is caused by atoms being excited to energy levels that do not decay rapidly. The afterglow may last from fractions of a second to hours, or even days, depending on such factors as the type of material and temperature.

photoelectric effect Ejection of electrons from certain metals when exposed to certain frequencies of light.

photon Localized corpuscle of electromagnetic radiation whose energy is proportional to its radiation frequency: E~ f, or E=hf, where h is Planck’s constant.

pigment Fine particles that selectively absorb light of certain frequencies and selectively transmit others.

pitch Term that refers to our subjective impression about the “highness” or “lowness” of a tone, which is related to the frequency of the tone. A high-frequency vibrating source produces a sound of high pitch; a low-frequency vibrating source produces a sound of low pitch.

Planck’s constant  (h) Fundamental constant of quantum theory that determines the scale of the small-scale world. Planck’s constant multiplied by the frequency of radiation gives the energy of a photon of that radiation.

E = hf,  h= 6.6 × 10–34 joule-second

plane mirror Flat-surfaced mirror.

plane-polarized wave A wave confined to a single plane.

plasma Fourth phase of matter, in addition to solid, liquid, and gas. In the plasma phase, existing mainly at high temperatures, matter consists of positively charged ions and free electrons.

polarization Aligning of vibrations in a transverse wave, usually by filtering out waves of other directions. See also plane-polarized wave and dichroic crystal.

polished Describes a surface that is so smooth that the distances between successive elevations of the surface are less than about one-eighth the wavelength of the light or other incident wave of interest. The result is very little diffuse reflection.

positron Antiparticle of an electron; a positively charged electron.

postulates of special relativity

First: All laws of nature are the same in all uniformly moving frames of reference.

Second: The speed of light in free space has the same measured value regardless of the motion of the source or the motion of the observer; that is, the speed of light is invariant.

potential difference Difference in electric potential (voltage) between two points. Free charge flows when there is a difference, and will continue until both points reach a common potential.

potential energy  (PE)  Energy of position, usually related to the relative position of two things, such as a stone and the Earth (gravitational PE), or an electron and a nucleus (electric PE).

power Rate at which work is done or energy is transformed, equal to the work done or energy transformed divided by time; measured in watts.

power = equation

precession Wavering of a spinning object, such that its axis of rotation traces out a cone.

pressure Force per surface area where the force is normal (perpendicular) to the surface; measured in pascals. See also atmospheric pressure.

pressure = equation

primary colors See additive primary colors and subtractive primary colors.

principal axis Line joining the centers of curvature of the surfaces of a lens. Line joining the center of curvature and the focus of a mirror.

principle General hypothesis or statement about the relationship of natural quantities that has been tested over and over again and has not been contradicted; also known as a law.

principle of equivalence Observations made in an accelerating frame of reference are indistinguishable from observations made in a gravitational field.

principle of flotation A floating object displaces a weight of fluid equal to its own weight.

prism Triangular solid of a transparent material such as glass, that separates incident light by refraction into its component colors. These component colors are often called the spectrum.

projectile Any object that moves through the air or through space, acted on only by gravity (and air resistance, if any).

proton Positively charged particle that is one of the two kinds of nucleons in the nucleus of an atom.

pseudoscience Fake science that pretends to be real science.

pulley Wheel that acts as a lever used to change the direction of a force. A pulley or system of pulleys can also multiply forces.

pupil Opening in the eyeball through which light passes.

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quality Characteristic timbre of a musical sound, governed by the number and relative intensities of partial tones.

quantum (pl. quanta) From the Latin word quantus, meaning “how much,” a quantum is the smallest elemental unit of a quantity, the smallest discrete amount of something. One quantum of electromagnetic energy is called a photon. See also quantum mechanics and quantum theory.

quantum mechanics Branch of physics concerned with the atomic microworld based on wave functions and probabilities, introduced by Max Planck (1900) and developed by Werner Heisenberg (1925), Erwin Schrödinger (1926), and others.

quantum physics Branch of physics that is the general study of the microworld of photons, atoms, and nuclei.

quantum theory Theory that describes the microworld, where many quantities are granular (in units called quanta), rather than continuous, and where particles of light (photons) and particles of matter (such as electrons) exhibit wave as well as particle properties.

quark One of the two classes of elementary particles. (The other is the lepton.) Two of the six quarks (up and down) are the fundamental building blocks of nucleons (protons and neutrons).

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rad Unit used to measure a dose of radiation; the amount of energy (in centijoules) absorbed from ionizing radiation per kilogram of exposed material.

radiant energy  Any energy, including heat, light, and X rays, that is transmitted by radiation. It occurs in the form of electromagnetic waves.

radiation (a) Energy transmitted by electromagnetic waves. (b) The particles given off by radioactive atoms such as uranium. Do not confuse radiation with radioactivity.

radiation curve of sunlight See solar radiation curve.

radio waves Electromagnetic waves of the longest frequency.

radioactive Term applied to an atom having an unstable nucleus that can spontaneously emit a particle and become the nucleus of another element.

radioactivity Process of the atomic nucleus that results in the emission of energetic particles. See radiation.

radiotherapy Use of radiation as a treatment to kill cancer cells.

rarefaction Region of reduced pressure in a longitudinal wave.

rate How fast something happens or how much something changes per unit of time; a change in a quantity divided by the time it takes for the change to occur.

ray Thin beam of light. Also lines drawn to show light paths in optical ray diagrams.

reaction force Force that is equal in strength and opposite in direction to the action force, and one that acts simultaneously on whatever is exerting the action force. See also Newton’s third law.

real image Image formed by light rays that converge at the location of the image. A real image, unlike a virtual image, can be displayed on a screen.

red shift Decrease in the measured frequency of light (or other radiation) from a receding source; called the red shift because the decrease is toward the low-frequency, or red, end of the color spectrum. See also Doppler effect.

reflection Return of light rays from a surface in such a way that the angle at which a given ray is returned is equal to the angle at which it strikes the surface. When the reflecting surface is irregular, the light is returned in irregular directions; this is diffuse reflection. In general, the bouncing back of a particle or wave that strikes the boundary between two media.

refraction Bending of an oblique ray of light when it passes from one transparent medium to another. This is caused by a difference in the speed of light in the transparent media. In general, the change in direction of a wave as it crosses the boundary between two media in which the wave travels at different speeds.

regelation Process of melting under pressure and the subsequent refreezing when the pressure is removed.

relationship of impulse and momentum Impulse is equal to the change in the momentum of the object that the impulse acts upon. In symbol notation,

Ft = Dmu

relative Regarded in relation to something else, depending on point of view, or frame of reference. Sometimes referred to as “with respect to.”

relative humidity Ratio between how much water vapor is in the air and the maximum amount of water vapor that could be in the air at the same temperature.

relativistic Pertaining to the theory of relativity; or approaching the speed of light.

relativity See special theory of relativity, postulates of the special theory of relativity, and general theory of relativity.

rem Acronym of roentgen equivalent in man, it is a unit used to measure the effect of ionizing radiation on human beings.

resistance See electrical resistance.

resistor Device in an electric circuit designed to resist the flow of charge.

resolution (a) Method of separating a vector into its component parts. (b) Ability of an optical system to make clear or to separate the components of an object viewed.

resonance Phenomenon that occurs when the frequency of forced vibrations on an object matches the object’s natural frequency, producing a dramatic increase in amplitude.

rest energy  The “energy of being” given by the equation E = mc2.

resultant Net result of a combination of two or more vectors.

retina Layer of light-sensitive tissue at the back of the eye, composed of tiny light-sensitive antennae called rods and cones. Rods sense light and darkness. Cones sense color.

reverberation Persistence of a sound, as in an echo, due to multiple reflections.

revolution Motion of an object turning around an axis that lies outside the object.

Ritz combination principle For an element, the frequencies of some spectral lines are either the sum or the difference of the frequencies of two other lines in that element’s spectrum. Also called the Rydberg-Ritz combination principle

rods See retina.

rotation Spinning motion that occurs when an object rotates about an axis located within the object (usually an axis through its center of mass).

rotational inertia Reluctance or apparent resistance of an object to change its state of rotation, determined by the distribution of the mass of the object and the location of the axis of rotation or revolution.

rotational speed Number of rotations or revolutions per unit of time; often measured in rotations or revolutions per second or minute.

rotational velocity Rotational speed together with a direction for the axis of rotation or revolution.

RPM Abbreviation for rotations or revolutions per minute.

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s Abbreviation for second.

satellite Projectile or smaller celestial body that orbits a larger celestial body.

saturated Term applied to a substance, such as air, that contains the maximum amount of another substance, such as water vapor, at a given temperature and pressure.

scalar quantity Quantity in physics, such as mass, volume, and time, that can be completely specified by its magnitude, and has no direction.

scale In music, a succession of notes of frequencies that are in simple ratios to one another.

scaling Study of how size affects the relationship among weight, strength, and surface area.

scatter To absorb sound or light and re-emit it in all directions.

scattering Emission in random directions of light that encounters particles that are small compared to the wavelength of light; more often at short wavelengths (blue) than at long wavelengths (red).

Schrödinger’s wave equation Fundamental equation of quantum mechanics, which interprets the wave nature of material particles in terms of probability wave amplitudes. It is as basic to quantum mechanics as Newton’s laws of motion are to classical mechanics.

scientific method Orderly method for gaining, organizing, and applying new knowledge.

self-induction Induction of an electric field within a single coil, caused by the interaction of the loops within the same coil. This self-induced voltage is always in a direction opposing the changing voltage that produces it, and is commonly called back electromotive force or back emf.

semiconductor Device made of material not only with properties that fall between a conductor and an insulator but with resistance that changes abruptly when other conditions change, such as temperature, voltage, and electric or magnetic field.

series circuit Electric circuit with devices connected in such a way that the electric current through each of them is the same. See also in series.

shadow Shaded region that appears where light rays are blocked by an object.

shell model of the atom Model in which the electrons of an atom are pictured as grouped in concentric shells around the nucleus.

shock wave Cone-shaped wave produced by an object moving at supersonic speed through a fluid.

short circuit Disruption in an electric circuit, caused by the flow of charge along a low-resistance path between two points that should not be directly connected, thus deflecting the current from its proper path; an effective “shortening of the circuit.”

SI Abbreviation for Système International, an international system of units of metric measure accepted and used by scientists throughout the world. See Appendix A for more details.

simple harmonic motion Vibratory or periodic motion, like that of a pendulum, in which the force acting on the vibrating body is proportional to its displacement from its central equilibrium position and acts toward that position.

simultaneity Occurring at the same time. Two events that are simultaneous in one frame of reference need not be simultaneous in a frame moving relative to the first frame.

sine curve Curve whose shape represents the crests and troughs of a wave, as traced out by a pendulum that drops a trail of sand while swinging at right angles to and over a moving conveyor belt.

sine wave The simplest of waves with only one frequency and the shape of a sine curve.

sliding friction Contact force produced by the rubbing together of the surface of a moving object with the material over which it slides.

solar constant 1400 J/m2 received from the sun each second at the top of the Earth’s atmosphere, expressed in terms of power, 1.4 kW/m2.

solar eclipse Event wherein the moon blocks light from the sun and the moon’s shadow falls on part of the Earth.

solar power Energy per unit time derived from the sun. See also solar constant.

solar radiation curve Graph of brightness versus frequency (or wavelength) of sunlight.

solid Phase of matter characterized by definite volume and shape.

solidify To become solid, as in freezing or the setting of concrete.

sonic boom Loud sound resulting from the incidence of a shock wave.

sound Longitudinal wave phenomenon that consists of successive compressions and rarefactions of the medium through which the wave travels.

sound barrier The pile up of sound waves in front of an aircraft approaching or reaching the speed of sound, believed in the early days of jet aircraft to create a barrier of sound that a plane would have to break through in order to go faster than the speed of sound. The sound barrier does not exist.

spacetime Four-dimensional continuum in which all events take place and all things exist: Three dimensions are the coordinates of space and the fourth is of time.

special theory of relativity Comprehensive theory of space and time that replaces Newtonian mechanics when velocities are very large. Introduced in 1905 by Albert Einstein. See also postulates of the special theory of relativity.

specific heat capacity Quantity of heat required to raise the temperature of a unit mass of a substance by one degree Celsius (or equivalently, by one kelvin). Often simply called specific heat.

spectral lines Colored lines that form when light is passed through a slit and then through a prism or diffraction grating, usually in a spectroscope. The pattern of lines is unique for each element.

spectrometer See spectroscope.

spectroscope An optical instrument that separates light into its constituent frequencies or wavelengths in the form of spectral lines. A spectrometer is an instrument that can also measure the frequencies or wavelengths.

spectrum (pl. spectra) For sunlight and other white light, the spread of colors seen when the light is passed through a prism or diffraction grating. The colors of the spectrum, in order from lowest frequency (longest wavelength) to highest frequency (shortest wavelength) are red, orange, yellow, green, blue, indigo, violet. See also absorption spectrum, electromagnetic spectrum, emission spectrum, and prism.

speed How fast something moves; the distance an object travels per unit of time; the magnitude of velocity. See also average speed, linear speed, rotational speed, and tangential speed.


spherical aberration Distortion of an image caused when the light that passes through the edges of a lens focuses at slightly different points from the point where the light passing through the center of the lens focuses. It also occurs with spherical mirrors.

spring tide High or low tide that occurs when the sun, Earth, and moon are all lined up so that the tides due to the sun and moon coincide, making the high tides higher than average and the low tides lower than average. See also neap tide.

stable equilibrium State of an object balanced so that any small displacement or rotation raises its center of gravity.

standing wave Stationary wave pattern formed in a medium when two sets of identical waves pass through the medium in opposite directions. The wave appears not to be traveling.

static friction Force between two objects at relative rest by virtue of contact that tends to oppose sliding.

streamline Smooth path of a small region of fluid in steady flow.

strong force Force that attracts nucleons to each other within the nucleus; a force that is very strong at close distances but decreases rapidly as the distance increases. Also called strong interaction. See also nuclear force.

strong interaction See strong force.

subcritical mass See critical mass.

sublimation Direct conversion of a substance from the solid to the vapor phase, or vice versa, without passing through the liquid phase.

subtractive primary colors The three colors of light-absorbing pigments—magenta, yellow, and cyan—that when mixed in certain proportions will reflect any color in the spectrum.

superconductor Material that is a perfect conductor with zero resistance to the flow of electric charge.

supercritical mass See critical mass.

superposition principle In a situation where more than one wave occupies the same space at the same time, the displacements add at every point.

supersonic Traveling faster than the speed of sound.

support force Upward force that balances the weight of an object on a surface.

surface tension Tendency of the surface of a liquid to contract in area and thus behave like a stretched elastic membrane.

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tachyon Hypothetical particle that can travel faster than light and thus move backward in time.

tangent Line that touches a curve in one place only and is parallel to the curve at that point.

tangential speed Linear speed along a curved path.

tangential velocity Component of velocity tangent to the trajectory of a projectile.

tau The heaviest elementary particle in the class of elementary particles called leptons.

technology Method and means of solving practical problems by implementing the findings of science.

telescope Optical instrument that forms images of very distant objects.

temperature Measure of the average translational kinetic energy per molecule of a substance, measured in degrees Celsius or Fahrenheit, or in kelvins.

temperature inversion Condition wherein upward convection of air is stopped, sometimes because an upper region of the atmosphere is warmer than the region below it.

terminal speed Speed attained by an object wherein the resistive forces, often air resistance, counterbalance the driving forces, so motion is without acceleration.

terminal velocity Terminal speed together with the direction of motion (down for falling objects).

terrestrial radiation Radiant energy emitted from the Earth.

theory Synthesis of a large body of information that encompasses well-tested and verified hypotheses about aspects of the natural world.

thermal contact State of two or more objects or substances in contact such that heat can flow from one object or substance to the other.

thermal equilibrium State of two or more objects or substances in thermal contact when they have reached a common temperature.

thermal pollution Undesirable heat expelled from a heat engine or other source.

thermodynamics Study of heat and its transformation to mechanical energy, characterized by two principal laws:

First Law: >A restatement of the law of conservation of energy as it applies to systems involving changes in temperature: Whenever heat is added to a system, it transforms to an equal amount of some other form of energy.

Second Law: Heat cannot be transferred from a colder body to a hotter body without work being done by an outside agent.

thermometer Device used to measure temperature, usually in degrees Celsius, degrees Fahrenheit, or kelvins.

thermonuclear fusion Nuclear fusion brought about by extremely high temperatures; in other words, the welding together of atomic nuclei by high temperature.

thermostat Type of valve or switch that responds to changes in temperature and that is used to control the temperature of something.

time dilation Slowing down of time for an object moving at relativistic speeds.

torque Product of force and lever-arm distance, which tends to produce rotational acceleration.

torque = lever-arm distance × force

total internal reflection The 100% reflection (with no transmission) of light that strikes the boundary between two media at an angle greater than the critical angle.

transformer Device for increasing or decreasing voltage or transferring electric power from one coil of wire to another by means of electromagnetic induction.

transistor See semiconductor.

transmutation Conversion of an atomic nucleus of one element into an atomic nucleus of another element through a loss or gain in the number of protons.

transparent Term applied to materials that allow light to pass through them in straight lines.

transuranic element Element with an atomic number above 92, which is the atomic number for uranium.

transverse wave Wave with vibration at right angles to the direction the wave is traveling. Light consists of transverse waves.

tritium Unstable, radioactive isotope of hydrogen whose atom has a proton, two neutrons, and an electron.

trough One of the places in a wave where the wave is lowest or the disturbance is greatest in the opposite direction from a crest. See also crest.

turbine Paddle wheel driven by steam, water, etc., that is used to do work.

turbogenerator Generator that is powered by a turbine.

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ultrasonic Term applied to sound frequencies above 20,000 hertz, the normal upper limit of human hearing.

ultraviolet (UV)  Electromagnetic waves of frequencies higher than those of violet light.

umbra Darker part of a shadow where all the light is blocked. See also penumbra.

uncertainty principle The principle formulated by Heisenberg, stating that Planck’s constant, h, sets a limit on the accuracy of measurement at the atomic level. According to the uncertainty principle, it is not possible to measure exactly both the position and the momentum of a particle at the same time, nor the energy and the time associated with a particle simultaneously.

universal gravitational constant The proportionality constant G that measures the strength of gravity in the equation for Newton’s law of universal gravitation.


unstable equilibrium State of an object balanced so that any small displacement or rotation lowers its center of gravity.

UV Abbreviation for ultraviolet.

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V (a) In lower-case italic v; the symbol for speed or velocity. (b) In uppercase V, the abbreviation for voltage.

vacuum Absence of matter; void.

Van Allen radiation belts Two donut-shaped belts of radiation that surround the Earth.

vaporization The process of a phase change from liquid to vapor; evaporation.

vector Arrow whose length represents the magnitude of a quantity and whose direction represents the direction of the quantity.

vector quantity Quantity in physics that has both magnitude and direction. Examples are force, velocity, acceleration, torque, and electric and magnetic fields.

velocity Speed of an object and its direction of motion; a vector quantity.

vibration Oscillation; a repeating to-and-fro motion about an equilibrium position - a "wiggle in time."

virtual image Image formed by light rays that do not converge at the location of the image. Mirrors, converging lenses used as magnifying glasses, and diverging lenses all produce virtual images. The image can be seen by an observer, but cannot be projected onto a screen.

visible light Part of the electromagnetic spectrum that the human eye can see.

visible spectrum See electromagnetic spectrum.

volt (V)  SI unit of electric potential. One volt is the electric potential difference across which one coulomb of charge gains or loses one joule of energy. 1 V = 1 J/C

voltage Electrical “pressure” or a measure of electrical potential difference.


voltage source Device, such as a dry cell, battery, or generator, that provides a potential difference.

voltmeter See galvanometer.

volume Quantity of space an object occupies.

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W (a) Abbreviation for watt. (b) When in italic W, the abbreviation for work.

watt SI unit of power. One watt is expended when one joule of work is done in one second. 1 W = 1 J/s

wave A “wiggle in space and time”; a disturbance that repeats regularly in space and time and that is transmitted progressively from one place to the next with no net transport of matter.

wave front Crest, trough, or any continuous portion of a two-dimensional or three-dimensional wave in which the vibrations are all the same way at the same time.

wave speed Speed with which waves pass a particular point.

wave speed = wavelength × frequency

wave velocity Wave speed stated with the direction of travel.

wavelength Distance between successive crests, troughs, or identical parts of a wave.

weak force Also called weak interaction. The force within a nucleus that is responsible for beta (electron) emission. See nuclear force.

weak interaction See nuclear force and weak force.

weight  The force that an object exerts on a supporting surface (or, if suspended, in a supporting string) - often, but not always, due to the force of gravity.

weight density See density.

weightlessness Condition of free fall toward or around the Earth, in which an object experiences no support force (and exerts no force on a scale).

white light Light, such as sunlight, that is a combination of all the colors. Under white light, white objects appear white and colored objects appear in their individual colors.

work (W)  Product of the force on an object and the distance through which the object is moved (when force is constant and motion is in a straight line in the direction of the force); measured in joules.

work = force × distance

work-energy theorem Work done on an object is equal to the kinetic energy gained by the object.

work = change in energy or W = DKE

wormhole Hypothetical enormous distortion of space and time, similar to a black hole, but opens out again in some other part of the universe.

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X ray Electromagnetic radiation, higher in frequency than ultraviolet, emitted by atoms when the innermost orbital electrons undergo excitation.

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zero-point energy Extremely small amount of kinetic energy that molecules or atoms have even at absolute zero.