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been replaced by an element with 1 more electron than the base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been replaced by an element with 1 less electron than the base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

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