Dictionary of Physics : M to R

M

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

experiment

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

water)

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

time

mutual induction two conducting coils are arranged so that a change of current in

one induces an EMF in the other

N

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

nucleus

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

O

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

voltages

ozone layer an atmospheric layer in which a high concentration of ozone

absorbs solar ultraviolet radiation

P

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

behave

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

evolution

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

charge

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

gravitational

potential energy, spring energy that is stored in the compression of a spring or elastic

string

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

object

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

Q

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.

R331

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

conductor

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

body

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

medium

relativity the theory based on the equivalence of observations of an event

from different frames of reference, having different velocities and

accelerations

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