Dictionary of Physics : M to R


magnet, hard magnetic materials that do not lose their magnetism easily when
the source of magnetism is removed, e.g. steel
magnet, soft magnetic materials that are easy to magnetise but rapidly lose
their magnetism; these are often used in electromagnets, e.g. iron
magnetic field a region in which a magnetic force can be detected
magnetic flux the lines of flux in a region of a magnetic field
magnetic force a force due to the motion of a charged particle in a magnetic field
magnetic levitation the use of the force between two opposing magnets to lift a body
magnetic moment the small magnetic field produced by a spinning nucleus
magnetic poles the points (north, south) of maximum magnetic flux
magnetic resonance imaging the proton distribution in the body using strong
imaging (MRI) magnetic fields and radiofrequency signals
magnetopause the region separating the extended solar magnetic field (solar
wind) from the Earth’s magnetosphere
magnetosphere the region close to a planet that is dominated by its own
magnetic field rather than that of the Sun
main sequence that part of the Hertzsprung–Russell diagram in which stars are
burning hydrogen to helium in their cores
mass defect the difference between the mass of a nucleus and the sum of the
masses of its uncombined protons and neutrons
mass number the sum of the numbers of protons and neutrons in an atom
mass spectrograph an instrument that determines the mass of charged particles
mass unit, atomic one-twelfth of the mass of a carbon-12 atom (1.6605655 × 10–27 kg)
mass a fundamental physical quantity; the amount of matter
medical physics the application of physics to medical procedures, usually in
reference to therapy or diagnosis
medium a region where a wave is propagated
meson a sub-atomic particle with zero, positive or negative charges, and
a mass between that of a proton and an electron
metre the standard unit of length in the metric system
mesosphere the middle layer of the atmosphere between 50 and 85 km
Michelson-Morley the experiment which showed that an aether did not exist
microwaves electromagnetic waves with wavelengths between 1 mm and
30 cm
Milky Way Galaxy our own galaxy
mobile electrons electrons that are free to move and carry current within a
conductor or semiconductor
moderator a material used to slow down neutrons (e.g. graphite, heavy
moment a measure of the turning effect of a force about a pivot point
moment of inertia a measure of the way a body resists rotational acceleration by a
torque; the rotational equivalent of inertial mass
momentum the product of the mass and velocity of a moving object
monochromatic light light composed of a single wavelength
Morse code a system used in signalling and telegraphy which uses sequences
of short pulses or dots and longer dashes
motion, circular motion of a body in a circular path8
motor, AC a motor which is powered by AC current
motor,DC a motor which is powered by DC current
motor, induction an AC motor in which currents in the primary winding (stator)
induce currents and magnetic fields in the moving rotor,
producing rotation
M-scan a form of ultrasound display in which a sequence of sequential B
scans are displayed in a stack to show how structures vary with
mutual induction two conducting coils are arranged so that a change of current in
one induces an EMF in the other


neutral point a point in a magnetic, electric or gravitational field at which a
body experiences no net force
neutrino a fundamental particle that interacts weakly with matter
neutron star a small, high-density object composed of neutrons; it is the final
stage of a star of intermediate mass
neutron a neutral subatomic particle of mass 1.6749 × 10–27 kg
newton the metric unit of force
Newton’s laws of motion laws that describe how bodies interact through forces and which
link acceleration, force and mass
node a point of zero amplitude on a standing wave
normal (to a surface) a line drawn perpendicular to a surface at the point of incidence
normal force the reaction force that acts on a body at rest on a surface
north geographic pole the point in the northern hemisphere through which the Earth’s
axis of rotation passes
north magnetic pole a point on the Earth’s surface toward which the north-seeking
pole on a compass points
nova a ‘new star’ which suddenly becomes visible to the naked eye.
Usually an evolved binary star containing a white dwarf, which
brightens by ten million times due to the transfer of material
between the white dwarf and its companion.
nuclear magnetic a method of imaging protons in the body using strong magnetic
resonance (NMR) fields and radiofrequencies to alter their nuclear spin
nuclear precession the way the spin axis of a nucleus rotates when placed in a strong
magnetic field
nuclear radiation any high energy particles or electromagnetic radiation which is
produced when a nucleus decays
nuclear spin a characteristic associated with the angular momentum of a
nucleon a proton or neutron in the nucleus of an atom
nucleus the positively charged dense part of the atom composed of
protons and neutrons
nuclide a distinct atomic species of given mass and atomic number


ohm the metric unit of resistance, 1 volt per ampere
Ohm’s law the ratio of the voltage across a resistor to the current flowing
through it is a constant
Olbers’ paradox if the universe is infinite, the sky should be completely covered by
stars and should be dazzling bright at night
optical density an optically dense material is one with a large refractive index
optical path difference the difference in distances travelled by two rays or wavefronts
before they interfere
optical path length the distance travelled by a light ray or wavefront
oscillation vibration
oscilloscope equipment using a cathode ray tube to display rapidly varying
ozone layer an atmospheric layer in which a high concentration of ozone
absorbs solar ultraviolet radiation


pair production the production of a particle and antiparticle from a photon with
sufficient energy
parallax changes in the projected position of a nearby object on a distant
background (ruler), brought about by movement of the observer
parallel circuit an electric circuit that has two or more components providing
separate conducting paths for the current
parsec the distance at which the radius of the Earth’s orbit subtends 1
arc second
pendulum a body suspended so that it can swing to and fro about an axis
perigee the closest point to the Earth on an Earth satellite’s orbit
perihelion the closest point to the Sun on a planet’s orbit
period the time for a complete cycle
periodic motion any repeated pattern of motion
period–luminosity diagram the relationship between absolute magnitude (or luminosity) and
period of Cepheid variables
permanent magnet a material which maintains its magnetism when the source
inducing it is removed
PET scanner an imaging system which uses tomographic techniques to image
the distribution of beta-plus emitting radiopharmaceuticals in
the body
photodiode a semiconductor diode used in detecting light
photoelectric effect the emission of electrons by a metal when illuminated by light in
the ultraviolet to visible range
photoelectrons electrons emitted by a metal that has been illuminated by light in
the ultraviolet to visible range
photometry the measurement of the apparent magnitudes of stars
photomultiplier a detector in which emission from a photocathode followed by
amplification of secondary electrons in a dynode chain produces
a measurable current
photon a quantum or packet of light energy
photosphere the visible surface layer of the Sun, which has a temperature of
6000 K
physical laws concise, general statements about how nature is observed to
piezoelectric effect the expansion and contraction of a crystalline material when an
AC voltage is applied
pitch the tone associated with a sound of definite frequency
Planck’s constant (h) a fundamental constant in nature that relates energy to the
frequency of a photon
Planck’s law the basis of quantum theory: ‘the energy of electromagnetic
waves comes in indivisible packets or quanta. Each must be
absorbed or radiated as a whole’; E = hf
Planck’s radiation law an expression for the spectrum emitted by a black body
plane of polarisation the plane of vibration of a transverse wave
planet the following bodies which orbit the Sun: Mercury, Venus, Earth,
Mars, Jupiter, Saturn, Uranus, Neptune, Pluto
planetary nebula a shell of ionised gas ejected from a star in its late stages of
plasma an extremely hot gas consisting solely of positive ions and
negative electrons
polarised light light with vibrations in a single plane, perpendicular to the ray
polarising film a plastic material that will plane-polarise light
polychromatic light light composed of a broad range of wavelengths (colours)
positron a particle with the same mass as an electron but with positive
potential difference the work done per unit charge as the charge is moved between
two points in an electric field
potential energy energy that is stored in a system and can be released
potential energy, energy that is due to a body’s position in a gravitational field
potential energy, spring energy that is stored in the compression of a spring or elastic
power the rate of doing work; units watts (W) or joules per second (J s–1)
precession motion that results from the application of a torque to a rotating
pressure force per unit area
primary the input winding of a transformer
principle quantum number an integer number (n = 1, 2, 3 …) upon which the allowed energy
levels in an atom depend
proper motion an astronomical body’s own motion across the sky
proto-galaxy a galaxy in the process of just forming
proton a positively charged subatomic particle of mass 1.6726 × 10–27 kg
proton–proton cycle the nuclear process that converts hydrogen into helium in the
cores of stars with masses lower than about 5Mo
protostar a star which is in the process of gravitational collapse, but has not
yet begun to fuse hydrogen to helium
pulsar a rapidly spinning, compact star with strong magnetic fields,
which produces regular pulsed radio emission with periods from
a few seconds to milliseconds
pulse a single disturbance in a medium


quantum a discrete unit of energy; a small building block of matter, e.g. a
quantum of charge
quark a fundamental particle which is thought to form the building
blocks of protons, neutrons and other hadrons
quasar unresolved, starlike sources with large redshifts and hence at
large distances. These are now known to be galaxies with nuclear
regions hundreds of thousands of times brighter than normal.


radial velocity the velocity of a star towards or away from the observer
radian the unit of angular measurement, 2ð radians = 360°
radio frequency signal an electromagnetic wave with frequencies in the radio waveband
radio waves electromagnetic waves produced by an alternating current in a
radioactive decay when unstable nuclei split into less massive daughter nuclei and
energetic particles
radioactive decay series a sequence of radioactive decays in which a long-lived nucleus
(e.g. 232Th) decays by intermediate stages to a second long-lived
nucleus (208Pb)
radioactivity the spontaneous breakdown of a nucleus with emission of
particles and rays
radiography the use of X-rays to image body tissue
radioisotope an isotope of an element that is radioactive
radiopharmaceutical a radioisotope used in medical procedures
radius of gyration the radius at which all of the mass of a rotating body must be
concentrated to have the same moment of inertia as the original
rarefaction the region of a longitudinal wave in which the particles are
further apart than in their equilibrium position
ray a line drawn perpendicular to a wavefront, representing the path
of that wavefront through an optical system
recombination the neutralisation of free electrons and positive ions in an ionised
gas. The energy produced is emitted as electromagnetic waves in
the radio, infrared, visible and ultraviolet spectra.
red giant the stage in the life of a star shortly after it has left the main
sequence and its atmosphere has expanded and cooled
redshift the wavelength shift observed in the spectrum of a star or galaxy
commonly interpreted as a Doppler shift
re-entry the action of entering the Earth’s atmosphere at high velocity on
return from orbit
reference frame a set of axes against which the motion of a body is measured
reflection the return of a wave from a boundary
reflection, angle of the angle between the reflected ray and the normal to the point
of incidence
refraction the change in direction of a wave as it passes from one medium
into another
refractive index the ratio of the speed of light in a vacuum to its speed in a given
relativity the theory based on the equivalence of observations of an event
from different frames of reference, having different velocities and
relaxation time the process by which the magnetisation of a sample of nuclei
return to their original orientations in NMR
relaxation, spin-lattice the exchange of energy between the nuclei and the surrounding
material, which leads to relaxation
relaxation, spin-spin the exchange of energy between the nuclei leading to relaxation
repulsive force a force between two bodies which causes them to move apart
resistance the ratio of the voltage across an electrical component to the
current flowing through it
resistor an electrical component with a specified resistance
resolving power the resolving power of a telescope is the smallest angular
separation between two point sources at which they can be
identified as double
resonance frequency the driving frequency at which the transfer of energy into a
vibrating system is most efficient
resonant mode a system vibrating at its natural frequency
restoring force the force which acts on an object to return it to its equilibrium
position after it has been disturbed
resultant a vector equal to the sum of two or more component vectors
rheostat a variable resistor
rotational inertia the resistance of an object to changes in its rotational motion
rotor the moving part of a motor or generator
RR Lyra variable variable stars with light curves similar to those of Cepheids, but
which have shorter periods of less than a day. They are found in
globular clusters.
Rydberg constant a constant used to calculate the wavelength of spectral lines from
a hydrogen atom


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