Dictionary of Physics : A to F

A

A scan a form of ultrasound display in which the amplitude of echoes

are shown against the time base

ABS braking anti-lock braking system

absolute magnitude the magnitude of a star as it would appear at a distance of

10 parsecs

AC current electric current whose flow alternates in direction; the current

usually varies sinusoidally with time

acceleration the time rate of change of velocity

acceleration due to gravity the acceleration of a body at the Earth’s surface, acted upon by

the Earth’s gravitational attraction alone

accretion a process in which material falls onto a celestial body due to

gravity; energy is generated by the process

acoustic impedance the product of density and sound velocity in a medium

active galaxy a galaxy with an unusually energetic nuclear region that is often

associated with the ejection of material in two jets

active optics an optical system utilising rapidly moving elements that correct

for atmospheric seeing effects

A–D converter an electronic chip that converts a continuously varying voltage

into a stream of digital numbers for subsequent computer

processing

aether a hypothetical medium in which electromagnetic waves were

once thought to travel. It is now clear that electromagnetic waves

need no such medium and can travel through a vacuum.

air drag resistance to motion through air

airbag a balloon-like device that inflates explosively to cushion the

driver and front seat passenger in a car crash

Airy’s diffraction pattern the diffraction pattern produced by a plane wave passing through

a circular aperture (usually in relation to light and optical

instruments). The diffraction pattern of a bright star.

alpha particle a helium-4 nucleus emitted during a nuclear decay

alpha radiation helium nuclei ejected from a nucleus during nuclear decay

alternating current an electric current that reverses direction during its cycle

ammeter an electric meter that measures current

ampere the unit of electric current

amplitude modulation transmission of information encoded as the changing amplitude

of a carrier wave with fixed frequency

amplitude the maximum displacement of a wave disturbance

angle of dip the angle between the Earth’s magnetic field lines and the

horizontal plane

angle of incidence the angle between the incident ray and the normal to the point of

incidence

angle of polarisation the angle between the vertical and the plane of vibration of a

transverse wave

angle of reflection the angle between the reflected ray and the normal to the point

of incidence

angle of refraction the angle between the refracted ray and the normal to the surface

at the point of incidence

angular acceleration the time rate of change of angular velocity

angular displacement the change in angular position over a period of time

angular momentum the product of the moment of inertia and the angular velocity

angular velocity the time rate of angular displacement

annihilation radiation electromagnetic radiation produced when a particle and its

antiparticle interact and annihilate

anode the oxidation electrode of a cell: any positive electrode

antineutron a particle with mass similar to that of the neutron but with spin

and magnetic moment oppositely oriented

antinode a point of maximum amplitude on a standing wave

antiparticle a particle with mass identical to that of another particle but with

opposite values of other properties, e.g. charge

antiproton a particle identical to the proton but with opposite charge

aperture an opening through which radiation may pass

aphelion the furthest point from the Sun on a planet’s orbit

apogee the furthest point from the Earth on an Earth satellite’s orbit

apparent magnitude the observed brightness or magnitude of a star

armature a coil of wire around an iron core that rotates in the magnetic

field of a motor

asteroid rocky and icy solar system bodies that orbit the Sun and have

radii from a few kilometres to hundreds of kilometres across

astrophysics the branch of science in which the laws of physics are applied to

astronomical observations

atmospheric seeing the distortion of wavefronts from distant stars by the atmosphere,

which results in rapid motion and defocusing of the stellar image

atom the smallest unit of a chemical element

atomic mass unit (unified) one-twelfth of the mass of a carbon-12 atom (1.660 565 5 × 10–27 kg)

atomic number the number of protons in the nucleus of an atom

attenuation a general term for the reduction of amplitude or intensity due to

absorption or scattering in a medium

attractive force any force between two bodies which causes them to move

towards each other

aurora luminous curtains of light observed from latitudes above ±55

degrees, which are caused by high-velocity charged particles

entering the atmosphere along magnetic field lines

B

B scan a form of ultrasound display in which echoes are represented as

brightness-modulated features on a time base

back EMF the voltage/EMF induced in the windings of an electric motor,

which opposes the voltage/EMF driving the motor

ballistic flight the flight under gravity of a projectile fired from a gun rather than

undergoing continuous thrust as in a rocket

Balmer series a sequence of hydrogen spectral lines seen in absorption or

emission in the wavelength range from 350 nm to 656 nm;

these result from electron transitions between n = 2 and higher

atomic levels

bandwidth the width, or spread, of a range of frequencies used for

transmitting information8

barometer an instrument used to measure pressure

baryon a sub-atomic particle involved in strong interactions, e.g. neutron,

proton

battery a device that produces an electric current by means of a chemical

reaction

beam a bundle of light rays

beats the effect produced by the interference of two waves of similar

frequencies

beta particle an electron ejected from the nucleus of a radioactive atom

beta radiation electrons (and positrons) ejected from a nucleus during

radioactive decay

Big Bang a model of the universe in which all matter and energy were once

concentrated in a single point or primeval atom. The universe

has been expanding since its creation between 13 and 20 billion

years ago.

binary star, astrometric a visual binary in which the orbit has been accurately determined

by photographic or other techniques

binary star, eclipsing an orbiting pair of stars whose orbital plane is aligned so that the

stars pass in front of each other producing regular eclipses

binary star, spectroscopic an orbiting pair of stars that are too close to be resolved but

which can be detected by the continuously shifting wavelengths

of the spectral features, due to the Doppler effect produced as the

stars move around their orbits

binary star, visual an orbiting pair of stars that can be resolved in a telescope

binding energy the difference between the energy of a complete nucleus and the

energy of its component nucleons (neutrons and protons) when

separated

binding energy the energy required to break up a nucleus

black body radiation radiation emitted by an ideal black body. The spectrum is

described by Planck’s law.

black body a body that completely absorbs any electromagnetic radiation

falling upon it

black hole a region of space–time from which light and matter cannot escape

Bohr atom Bohr’s model of the atom in which electrons orbit the nucleus in

a limited number of circular orbits

bremsstrahlung electromagnetic radiation produced when an electron swings by

an ion without recombining. Often known as free-free radiation.

Brewster’s angle the angle of incidence at which a reflected light ray is completely

polarised

bright nebula a cloud of glowing, ionised gas excited by young hot stars, e.g. the

Orion Nebula

brown dwarf a white dwarf that has cooled to a temperature below about 6000 K

bubble chamber an instrument used to make the path of ionising radiation visible

as a sequence of bubbles

C

canal rays rays of positive ions formed in a cathode ray tube

capacitor a device that stores electric charge

CAT scanner computer aided tomography; a modern X-ray machine that

produces 3-D images using computer reconstruction

cathode ray oscilloscope an instrument that displays the wave shape of a voltage9

cathode ray the charged particle which moves from the cathode to the anode

of a vacuum tube; this is now called the electron

cathode rays electrons ejected from the cathode in a Crookes’ tube

cathode the reduction or negative electrode of a cell: any negative

electrode

CD-ROM a plastic disk in which digital information is contained as etched

pits on a continuous, spiral track

centre of mass/centre of the average position of a system’s mass, or the point through

gravity which a body’s weight can be thought to act

centripetal acceleration acceleration directed towards the centre of a circular path

centripetal force the force that produces a centripetal acceleration

Cepheid variable a star whose brightness varies with a period of a day to a few tens

of days as a result of its atmosphere expanding and contracting

chain reaction a reaction in which the product of the initial reaction is the

essential reactant which causes more reactions

characteristic X-rays line emission at X-ray wavelengths that results from the ejection

of electrons from the innermost shells of heavy elements and the

subsequent transition of an outer shell electron into the vacant

space

charge a fundamental property of all matter

chromosphere the transition layer of the Sun’s atmosphere, which lies between

the photosphere and the corona

circuit breaker an electrical switch that opens when the current in the circuit

exceeds a predetermined value

circular motion motion of a body in a circular path

cloud chamber an instrument used to make the path of ionising radiation visible

as trails of liquid droplets

CNO cycle the nuclear process that converts hydrogen nuclei to helium

nuclei in the core of stars more massive than about 5Mo

coherence the property of wave trains with identical wavelength and phase

coherent fibres a bundle of fibres in which the relative position of the fibres at

the input face match those at the output face, thus enabling an

image to be transmitted

collision, elastic a collision that occurs without loss of kinetic energy

colour index the difference between two magnitudes of the same star obtained

through different colour filters, e.g. B – V or U – B

comet a small core of ice and rock which orbits between the outer

planets and the Sun

commutator a ring that carries current to the rotating parts of a motor or in a

generator

component one of the vectors that can be combined to give a single vector

compression the region of a longitudinal wave in which the vibrating particles

are closer together than when in their equilibrium position

Compton effect the scattering of X-rays from electrons in atoms, in which both

behave as elastic particles

concave lens a lens that causes a parallel beam to diverge

concave mirror a mirror that causes a parallel beam to diverge

conduction band an energy band in a solid in which electrons are free to move

when an external electric field is applied20

conductor a substance which passes an electric current

confinement time the period of time that ions can be kept at a sufficiently high

temperature and density for fusion to occur in a fusion reactor

constellation a group of stars, to which a shape and name have been given

constructive interference the superposition of two waves, which are exactly in step, to

produce a wave of the same frequency but greater amplitude

contact potential the EMF that occurs when two dissimilar metals are joined by a

conductor

control rod a metal rod that absorbs excess neutrons in a reactor core, thus

controlling the rate of fission

convection cell a region of hot air that is less dense than the surrounding cooler

air and therefore rises

convex lens a lens that causes a parallel beam to converge

convex mirror a mirror that causes a parallel beam to converge

coolant any liquid or gas used to carry heat energy away from a heat

source, e.g. liquid sodium in a reactor core

corona an electrical discharge around a sharp, high-voltage conductor

cosmogenies the study of the origin of stars, planets and their moons

cosmology the study of the universe and its origins

coulomb the SI unit of charge

Coulomb’s law the law stating that the magnitude and direction of the

electrostatic force between two charges depends directly on the

sign and size of the charges and inversely on the distance

between them

couple two parallel forces of equal magnitude acting in opposite

directions and at different points

crest a region of upward displacement in a transverse wave

critical angle the angle of incidence in an optically more-dense medium that

results in an angle of refraction of 90°

critical mass the mass of nuclear material above which runaway fission will

occur

crumple zone a region in a vehicle that absorbs energy in a crash by distorting

without affecting the integrity of the passenger compartment

current see electric current

cyclotron a particle accelerator in which charged particles are accelerated

in a spiral path between the poles of a magnet. Acceleration

occurs as a result of a radio frequency being applied to two ‘dees’

within which the particles move.

D

damping the reduction in wave amplitude due to energy loss

DC direct current: current which flows in one direction only

de Broglie waves waves associated with particles of a given momentum whose

wavelength is given by ë = h/p, where h is Planck’s constant and p

is the particles, momentum

decibel a logarithmic measure of the intensity of a sound wave

density the amount of mass per unit volume

destructive interference the superposition of two waves that are exactly out of step to

produce cancellation of the disturbance

dielectric an electric insulator

differentiation during the formation of planets, the settling out of heavy

elements to form the core and light elements to form the crust

diffraction grating an optical surface with thousands of equally spaced, parallel

grooves

diffraction pattern the intensity pattern produced by interference when waves travel

through an aperture or around an obstruction

diffraction the spreading of a wave disturbance at an edge or slit

digital data the representation of numerical data in the form of discrete

signals, e.g. 0 and 1

dimensions the combination of the fundamental quantities of mass, length

and time for a physical quantity

dipole aerial a wire or rod antenna a half wavelength long

direct current (DC) a current in which the movement of charge is in one direction

dispersion the process of separating light into its component wavelengths

displacement the change in position in a particular direction

distance the length of path travelled between two points

distribution box the junction box at which the mains electricity enters the house

and feeds lighting and power circuits

domain a tiny magnetic region in which the atoms have aligned magnetic

fields

doping the addition of impurities to a semiconductor to alter its

conducting properties

Doppler effect a change in frequency of a wave when the source and observer

are in relative motion

Doppler ultrasound the use of the Doppler shift to provide ultrasound images that

include velocity information

drift velocity the average velocity of electrons in a current-carrying conductor

dwarf a low-luminosity star on the main sequence

dynamics an explanation of why objects move as they do

E

earth leakage device a circuit breaker which cuts out if current flows from the live to

neutral circuits

echo a sound that is reflected back to its source

eddy currents a loop of induced current set up in a piece of metal by relative

motion between the metal and a magnetic field

elastic collision the impact of two or more bodies without loss of kinetic energy

or momentum

electric cell a chemical generator of EMF

electric current the rate of flow of electric charge

electric field map a map representing the electric field in a given region

electric field strength the vector quantity representing the electrostatic force on a

1 coulomb positive test charge

electric field a region where a force acts on an electric charge brought into it

electrical load a component in a circuit where electrical energy is dissipated, e.g.

a resistor

electrochemical series a sequence of metals placed in order of their contact potential

electrode a conducting element in an electric cell or electronic tube

electrolyte a substance whose solution (commonly water) conducts an

electric current322

electromagnet a solenoid with a soft iron core

electromagnetic induction the process of inducing an EMF in a conductor by a changing

magnetic flux

electromagnetic waves transverse waves such as light, which have perpendicular electric

and magnetic components

electromotive force (EMF) the energy per unit charge supplied by the source of electric

current; the potential difference (or voltage) produced by sources

of electrical energy

electron gun a hot filament source and a series of electrodes which produce a

high velocity beam of electrons in a CRO or television

electron microscope an instrument that uses electron beams to produce a magnified

image of a sample

electron volt the energy required to move an electron through a potential

difference of 1 volt

electron a negatively charged, sub-atomic particle of mass 9.1095 × 10–31 kg

and charge –1.6022 × 10–19 C

electroscope an instrument used to detect electric charge

electrostatics the science that deals with stationary electric charges

element a pure substance that cannot be broken down into simpler

substances by chemical means alone

elliptical galaxy a collection of billions of stars, gas and dust, with a well defined

ellipsoidal shape

emission spectra a spectrum of light released by an element in an excited state

endoscope an instrument for inspecting body cavities using coherent fibre

optics

endothermic a chemical process that absorbs energy

energy level one of a series of discrete energy values that characterise an atom

energy the capacity to do work

energy, elastic potential the potential energy in a stretched or compressed spring

energy, gravitational the potential energy acquired when work is done against

potential gravity

energy, internal the total of the kinetic and potential energies of a molecule or

atom

energy, kinetic energy due to motion

energy, mechanical energy which is in a form that can be used to perform useful work

energy, nuclear binding the energy required to pull apart the constituents of a nucleus

energy, potential energy that is due to position

energy, thermal the total potential and kinetic energy associated with the random

motion of the molecules of a substance

equilibrium position the point at which no net force or net torque acts on a body

equinox the two days in the year when the length of day and night are

equal. On these days the Sun rises due east and sets due west.

equivalent resistance the resistance of a single resistor that can be used to replace a

more complex network of resistors in a circuit

errors, absolute the uncertainty of an observation expressed as a number,

e.g. x = 100 and Äx = 3 is represented as 100 ± 3

errors, relative The uncertainty of an observation expressed as a fraction or

percentage, e.g. x = 100 and Äx = 3 gives a relative error of 3/100

or 3%23

escape velocity the minimum velocity needed for a body to escape a planet’s

gravitational pull; for Earth it is 11.2 km s–1

exchange particle an extremely short-lived particle that is responsible for one of the

four fundamental forces

excited state any energy level more energetic than the ground state of an atom

exothermic a chemical process that releases energy

extrinsic variable a variable star which varies due to some external cause, e.g.

eclipsing binaries

F

Faraday’s law of a relation linking the EMF induced in a circuit with the rate of

electromagnetic induction change of a magnetic field

fax (facsimile machine) a machine used to digitise an image and send it sequentially via a

phone line to a printer or computer

ferromagnetism a property of some substances (e.g. iron, cobalt, nickel, alnico

alloy) that makes then strongly attracted by magnets

fibre bundle a cable made up of many individual optical fibres

fission the splitting of a nucleus of an atom

fluorescence the absorption of one wavelength of e/m radiation by a material

followed by the emission of a different wavelength

fluorescence the emission of light from a material when it is illuminated by a

higher frequency ( X-rays) or impacted by electrons

flux, magnetic the density of magnetic field lines

force a push or a pull

force, centripetal the force that keeps an object in circular motion

force, electric a force due to a charged particle in an electric field

force, gravitational a mutual force of attraction between two masses

force, magnetic a force due to the motion of a charged particle in a magnetic field

force, nuclear binding the force binding the constituents of a nucleus

fossil fuel a carbon-based source of energy, usually an organic material, e.g.

wood, peat, coal, oil

frame of reference a system for specifying the position of objects in space

free fall an object falling under gravitational force alone

free-free radiation bremsstrahlung

frequency modulation transmission of information encoded as changes in frequency of

a fixed-amplitude carrier wave

frequency the number of vibrations/cycles per unit time; the number of

complete cycles occurring every second in a periodic

phenomenon

friction a force that resists the motion of objects that are in close contact

fundamental the lowest frequency produced by the source of a musical sound

fuse a device that melts when an excessive current passes through it

thus protecting the electrical circuits

fusion an energy-producing reaction in which light nuclei combine to

make heavier nuclei