Dictionary of Physics : A to F

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
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
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 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,
battery a device that produces an electric current by means of a chemical
beam a bundle of light rays
beats the effect produced by the interference of two waves of similar
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
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
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


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
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
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
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
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
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.


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
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
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


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
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
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

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
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


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