Dictionary of Physics : S to Z

S

scalar a physical quantity that has magnitude but not direction

scintillation flashes of light arising from the impact of high-energy particles

(alpha particles) on a crystalline material

second the unit of time

secondary the output winding of a transformer

semiconductor a material in which the resistance decreases as its temperature

rises; its resistivity is higher than that of a conductor but lower

than that of a resistor

semiconductor, n-type a semiconductor in which 1 atom in a hundred thousand has

been replaced by an element with 1 more electron than the base

material

semiconductor, p-type a semiconductor in which 1 atom in a hundred thousand has

been replaced by an element with 1 less electron than the base

material

series circuit a circuit in which the components form a single conducting path

shell burning a phase in a star’s life during which the fusion processes occur in

a hot shell around an inert core that has not yet reached a high

enough temperature for the next fusion process to start

shock wave a wavefront produced by a source moving faster than the wave

speed (e.g. speed of sound)

significant figures the digits in a number that are known with certainty plus the first

digit that is uncertain

simple harmonic motion motion in which the force and acceleration are directed towards

the equilibrium position

Snell’s law the relation between the angles of incidence and refraction and

the refractive index when a light ray travels from one material

into another

solar cycle solar activity, as measured by sunspots, has a period of 11 years

solar system the system of planets, asteroids and comets orbiting the Sun

solar wind the stream of particles ejected from the Sun

solenoid a long coil of conducting wire

solstice the two days in the year when the Sun rises at its most northerly

and southerly points on the eastern horizon, i.e. midsummer and

midwinter

sonar the use of sound waves to detect and estimate the range of

submerged objects

sound waves longitudinal waves of compression and rarefaction

spark chamber a means of imaging particle tracks using the ionisation of gas

between stacks of high voltage plates

specific impulse the ratio of the thrust to the rate of weight ejected;

I = thrust/(g Äm/Ät)

spectra, absorption continuum spectra showing dark absorption lines at wavelengths

where energy has been absorbed by a cool intervening gas

spectra, emission spectra in which bright emission lines are observed from highly

excited gas atoms

spectral type stellar classification based on the absorption features seen in the

blue in stellar spectra: OBAFGKMRNS

spectrograph an instrument for observing and recording spectra

spectrometer an instrument which splits light into its component wavelengths.

spectroscopic parallax the derivation of a star’s distance by using its spectral type to

estimate its absolute magnitude

speed distance travelled per unit time

spiral galaxy a galaxy that shows a nuclear bulge and a spiral disc structure

split ring commutator a commutator which is split into two segments allowing the

generation of DC power

SQUID superconducting quantum interface device: a device capable of

measuring extremely small magnetic fields

standing wave the resultant wave from two wave trains of equal amplitude,

wavelength and frequency travelling in opposite directions

through the same medium

star cluster, globular a cluster of stars which formed together and now appear to form

a compact spherical system

star cluster, open a cluster of stars which formed together and now appear as a

loosely bound system, e.g. the Pleiades

stator the fixed part of an electrical motor or generator

Stephan’s law the relationship between total power radiated by a black body

and its temperature and surface area L = sAT 4

stratosphere the layer of the Earth’s atmosphere at an altitude of between

12 km and 50 km

stroboscope a device that produces regular flashes of light

strong force the interaction which binds quarks to form neutrons and

protons; the strong force acts only between quarks

Sun our nearest star

sunspot a dark region that is cooler than the surrounding photosphere

supercluster a cluster of galaxy clusters

superconductor a conductor with almost zero resistance at low temperatures

supergiant a star with a radius several hundred times that of main sequence

stars with similar temperature

 supernova the final explosive stage of evolution of a massive star that has a

core of iron

superposition the combination of two or more waves to produce a resultant

displacement

synchrotron radiation electromagnetic radiation emitted by electrons moving at close to

the speed of light in strong magnetic fields

synchrotron a machine for accelerating particles to high energies

temperature a physical quantity of matter related to the average kinetic energy

of its particles

T

terminal velocity the constant velocity reached by a body falling through a fluid

when the frictional resistance equals the gravitational force

tesla the metric unit of magnetic field strength

thermionic devices, valves an electronic switch in which electron flow from a hot filament to

the anode is controlled by an intermediate, charged grid

thermosphere the region of the Earth’s atmosphere immediately above the

mesosphere

three degree background background radiation from the Big Bang now cooled to 3 K and

observable at millimetre wavelengths

thrust force developed by rocket motors

time a fundamental unit of measurement related to our perception of

change. Time passes!

time dilation the relativistic effect whereby the time measured by a moving and

a stationary observer differ

topography computer reconstruction of a section through an object

torque the product of a force times its lever arm

torsional wave a wave in which the particles twist backward and forward around

the direction of motion of the wave

total internal reflection when light travels from a region of high optical density to one

with a lower optical density at high angles of incidence it will be

totally reflected back into the material of high optical density

transducer (PZT) a device which converts electrical power into ultrasonic waves

transformer an electronic device that allows changes of voltage levels

transformer, step down an electrical device that converts high voltages across the

primary coils to lower voltages across the secondary coils

transformer, step up an electrical device that converts low voltages across the primary

coils to higher voltages across the secondary coils

transistor an electronic semiconductor switch which is based on p- and ntype

materials in a p-n-p configuration

transmutation the conversion of one element or nucleus into another

transverse wave a wave in which the particles move perpendicular to the direction

of the waves of propagation

troposphere the lowest layer of the Earth’s atmosphere to an altitude of 12 km

in which most of the weather systems are found

trough a region of downward displacement in a transverse wave

U

UHF ultra high frequency radio waves

ultrasound sound waves with frequencies above about 0.5 MHz

ultraviolet light electromagnetic radiation of higher frequency (shorter

wavelength) than visible light

unified field theory a theory that explains the effects of all four fundamental forces as

the manifestation of single field

units, imperial British units of measurement, e.g. pound, gallon, foot

Units, Système French system of units, now recognised internationally,

International d’Unités e.g. kilo, litre, metre

universe, closed a universe that contains sufficient matter to collapse back onto

itself

universe, flat a universe which will gradually slow its expansion until it stops

universe, open a universe which will continue to expand forever

V

valence band the energy band in a solid in which the outermost valence

electrons are found

Van Allen radiation belts two regions at between 1000 km and 5000 km and 15 000 km and

25 000 km in which charged particles from the solar wind

become trapped

Van de Graaff generator a particle accelerator that transfers charge to an insulating sphere

by means of a moving belt made of an insulating material

vector dot product the result of vector multiplication, e.g. A.B. |A| |B| cosè

vector a quantity that has both a magnitude and a direction

velocity the time rate of change of displacement

VHF very high frequency radio waves

volt the unit of potential difference; the potential energy per unit

charge

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

difference of 1 volt

Voyager two spacecraft which flew by the giant gas planets, passing

Jupiter (1979), Saturn (1981), Uranus (1986) and Neptune (1989)

W

watt the unit of power, one joule per second (J s–1)

wave equation an equation expressing the amplitude of a wave in terms of its

time and position

wave train a continuous wave disturbance

wave velocity the product of a wave’s frequency and wavelength

wave, wavelength the distance between wave crests or troughs

wave, compression a wave in which the particles vibrate along the direction in which

the wave travels; sound waves are compression waves

wavefront a line connecting all points on a wave crest that are oscillating

together

wavelength the distance between a point on a wave and the nearest point in

phase with it

weak force the fundamental force that is responsible for the change of

particles from one form to another, e.g. the decay of neutrons

into protons

weber the unit of magnetic flux

weight the gravitational force acting on a mass

Wein’s displacement law the relationship between the wavelength at which most

electromagnetic radiation is emitted and the temperature of the

emitting source

white dwarf a star of intermediate mass that has produced a planetary nebula

and can now no longer support nuclear fusion in its core, it

therefore cools by radiating energy away to space336

work function the minimum energy required to remove an electron from a

surface in the photoelectric effect

work the product of the displacement and the force in the direction of

the displacement

X

X-rays high energy electromagnetic radiation of high penetrating power

X-ray diffraction the crystalline structure of materials can be studied by looking at

the interference patterns produced by X-rays reflected from the

different crystal planes

Z

Zeeman effect the energy levels of an atom split when the atom is in a strong

magnetic field. This splitting produces a splitting in the observed

spectral lines.