Physics Glossary
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Acceleration. The rate at which velocity changes. The SI units of acceleration are m/s/s or m/s2.
Alpha decay: If a nucleus contains too many protons, it is unstable and emits an alpha particle, which is a nucleus of helium-4.
Alpha
Particle: See alpha decay. A stable nuclear particle
that consists of two protons and two neutrons. An alpha particle is the
nucleus of helium.
Amorphous
solid: See
Crystal.
Ampère’s law: An electric current produces a magnetic field.
Amplitude: (A) The amplitude of an oscillation is the maximum displacement of the medium from its equlibrium position.
Angular momentum. A measure of the rotation of an object. It is the tendency of a rotating object to keep rotating because of its inertia. Angular momentum can be expressed as L = I w. The angular momentum of a body is conserved if the net external torque acting on the object is zero. This is the law of conservation of angular momentum.
Angular velocity. The rate at which the angular displacement changes with respect to time:
Antimatter:
Antimatter particles, called antiparticles, are identical to ordinary
matter escept that if the particle has electric charge, its antiparticle
would have the opposite charge. If a particle has no charge, like the photon,
it is its own antiparticle.
Archimedes’ Principle: An object partially or completely submerged in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object.
Atomic Mass Unit, amu. A unit of mass used in the atomic realm. 1 amu = 1.6605402 ´ 10-27 kg.
Atomic number: The total number of protons in the nucleus.
Average Speed is defined as the total distance traveled divided by the time taken to travel this distance. The units of speed are units of distance divided by units of time. In the SI system, the units are m/s.
Baryon:
A subatomic particle composed of three quarks
held together by the color force.
Beta decay: If a nucleus contains too many neutrons it is also unstable and decays by emitting a beta particle, which is an electron.
Beta
particle: See
beta decay. An electron that is emitted from the nucleus of an atom
undergoing beta decay.
Binding Energy. The total energy of the nucleus is less than the total energy of its separated nucleons. This energy difference is called binding energy.
Black body: An ideal object that absorbs all radiation incident upon it. It is also a perfect emitter of radiation.
Black hole: If a star has a mass of more than 3 solar masses, gravitational compression will make the star so dense that the escape velocity from it becomes greater than the speed of light. The star contracts to a single point, called a singularity.
Boyle’s
law: See Ideal gas law.
Buoyant
force: Upward force exerted by a fluid on a floating
or immersed object as a reaction to the force exerted by the object to displace
the fluid.
Calorie:
The amount of heat required to raise the temperature
of 1 gram of water by 1°C. It is equal to 4.186 J.
Capacitor is a device for the storage of electrical energy which consists of two oppositely charged metal plates separated by an insulator.
Centripetal acceleration. An object moving with a constant speed v in a circular path of radius r has an acceleration directed toward the center of the circle called, which has a magnitude
Chain
reaction: A reaction in which some of the products initiate
further reactions of the same kind allowing the reaction to become self-sustaining.
Charles’s
law: Another name for Gay-Lussac’s law (q.v.)
Coherent
radiation: Electromagnetic radiation such as is seen in
radio waves and laser beams, where all the radiation is of a single frequency
and all the photons are in phase (in step). The coherence length
is a measure of the quality of the coherence; the bandwidth is another.
Color
Charge: A
measure of the strength of the strong interaction.
Compound. If the atoms retain their identities while they attract each other due to the electrical attraction of their respective ions (ionic bond), the atoms are said to form a compound.
Concave mirror: A curved mirror in which the interior surface is the reflecting surface.
Conservative and nonconservative forces. When the work done by an unbalanced force acting on a body depends only on the initial and final positions of the body, the force is said to be a conservative. The gravitational force is a conservative force. If the work done depends also on the path taken by the body, the force producing this motion is said to be nonconservative.
Constructive interference: See interference.
Convex mirror: A curved mirror which has the exterior surface as the reflecting one.
coulomb: SI unit of electric charge. One coulomb is the charge of 6.25x1018 electrons or an equal number of protons
Coulomb’s
law: The force exerted by one charged object on another varies inversely
as the square of the distance separating the objects and is proportional
to the product of the magnitude of the charges. The force is along the line
joining the charges and is attractive if the charges have opposite signs
and repulsive if they have the same sign. If we call q1 and q2 the magnitudes of the two charges and
r, the distance between
their centers, we can state Coulomb’s law in a single equation:
The value of Coulomb’s constant is k = 9 ´
109 N . m2/C2
Covalent
bond: A
type of chemical attraction that depends on the fact that the presence of
two electrons in a certain region of space is energetically advantageous.
In a covalent bond, atoms are bound together by sharing electrons.
Critical
angle: When light is passing from a medium of higher
index of refraction to one of low index of refraction, the critical angle
is the angle of incidence for which the angle of refraction is 90° (i.e., the emergent beam travels along the
interface)
Crystal. The forces that bind the atoms together in a solid are strong enough for the solid to maintain its shape. If the atoms arrange themselves in a pattern that is repeated through the substance, the solid is called a crystal. Solids that do not form these patterns are amorphous.
de Broglie wavelength: The wavelength associated with a particle, equal to the ratio of Planck’s constant to the momentum of the particle.
decibel: The unit of sound level, a measure of relative sound levels.
Density of a substance is its mass per unit volume. The SI unit is the kilogram per cubic meter (kg/m3)
Destructive
interference: See interference
Dewpoint:
See Humidity.
Diffraction: The spreading out of waves when passing through a small hole.
Diode:
A device that acts like a switch in an electric
circuit, permitting the flow of current in only one direction.
Doppler effect: The change in frequency perceived by a listener who is in motion relative to a source of sound.
Efficiency: The ratio of useful work performed by a machine to the total amount of energy required to operate it.
Elastic Collision: A collision in which the kinetic energy is conserved.
Elastic potential energy of a spring of force constant k stretched a distance x is 
Electric
charge: A property of subatomic particles that is responsible
for electric and magnetic phenomena. The fundamental charge is the charge
of one electron or one proton and has a magnitude of 1.602 x 10-19 C.
Electric
current is the rate at which charge flows in a conductor. If during
a time t an amount of charge q flows past a particular point in a conductor,
the electric current i is
The unit of current is the ampere,
A, which is equal to one coulomb per second.
Electric
field: Property of space around an electric charge.
The electrostatic force per unit charge.
Electric
potential difference, V, between
two points is the change in the electric potential energy of a charge qo carried from one point to the other
divided by the magnitude of that charge; that is, 
The SI unit of potential difference
is the volt, V. From the definition, 1 volt is 
Electromagnetic
wave: Propagation of oscillating electric and magnetic
fields through space.
Electron: A fundamental particle, one of the main constituents
of matter. The electron has an electric charge of 1.6 x 10-19 C
and a mass of 9.1094 x 10-31 kg or 5.486 x 10-4 amu.
Electron-Volt (eV): Unit of energy used when dealing with atoms or electrons. The transformation to joules is 1 eV = 1.602 ´ 10-19 J.
Electrostatic
force: See Coulomb’s law.
Electroweak
force: Unification of electromagnetism and the weak nuclear force. The triplet
of massive bosons W+, W-, and Z, along with the massless
photons, are the mediators of this force.
Energy. An abstract concept that is best illustrated as the capacity to do work or the result of doing work.
Entropy: From a Greek word that means transformation, entropy is a measure of the disorder of a system.
Escape velocity: The escape velocity of an object on the surface of the earth is the minimum velocity that we must impart to the object so that is escapes the gravitational grasp of the earth.
Event horizon: See Schwartzchild radius. A sphere around a black hole with a radius equal to the Schwartzchild radius. No particle inside this sphere can escape the gravitational attraction of the black hole.
Faraday’s law of induction. The induced voltage in a circuit is proportional to the rate of change of the magnetic field.
Field:
The concept of field is used to specify a quantity for all points in a particular
region of space. The electric field describes the property of the space
around an electrically charged object. A charged body distorts
the space around it in such a way that any other charged body placed in
this space feels a force that is given by Coulomb’s law. The electric
field strength E is the Coulomb
force felt by a test charge qo that is placed in the field divided by
the magnitude of this test charge qo,
The direction of the electric field
vector is the direction of the force on a positive test charge.
Fission:
See Nuclear fission.
Fluids. Substances in which the binding forces are weaker than in solids, so that the atoms or molecules do not occupy fixed positions and move at random. Liquids and gases are fluids.
Focal point: The point at which all of the rays gathered by a lens, curved mirror, or optical instrument pass.
Frame
of reference: See reference frame.
Frequency: (f) The number of wavelengths that pass a particular point per second; the units of frequency are hertz, Hz:
f-stop: The ratio of the focal length of the lens to diameter of the aperture.
Fundamental charge: The most fundamental unit of charge is the charge of one electron or one proton. The, e, has a value e = 1.6 ´ 10-19 C. The electric charge on an charged object occurs always in multiples of the fundamental charge.
Fundamental forces: There are four fundamental forces in nature: gravitational, electromagnetic, strong, and weak. The strong force—a short-range force—holds the nucleus together. The weak force, also a short-range force, is responsible for radioactive b-decay processes.
Fundamental Units. The fundamental quantities in mechanics are length, mass, and time. The corresponding fundamental units in the SI system are the meter, the kilogram and the second.
Fusion:
See Nuclear fusion.
Galilean Principle of Relativity: The laws of mechanics are the same in all inertial frames of reference. This means that there is no special or absolute reference frame. Thus, there is no absolute standard of rest; uniform motion has to be referred to an inertial frame.
Gamma decay: If a nucleus is left in an excited state after an alpha or beta process, it will decay to the ground state by emitting one or more photons, called gamma rays.
Gamma
rays: Electromagnetic radiation with frequencies greater
than about 3x1019 Hz and wavelengths smaller then about 10-11
m.
Gauge symmetry. Physical theories that remain invariant under changes taking place everywhere in the universe are said to obey a global gauge symmetry. When the changes are different at every point in space, the theory is said to obey a local gauge symmetry. To maintain a local symmetry, in which different changes take place at different points or to different objects, a compensating change must take place.
Gay-Lussac’s
law: See Ideal gas law.
Gravitational
force: See Law of Universal Gravitation.
Gravitational
potential energy is the energy that a body possesses by virtue of its
separation from the earth. For an object of mass m
situated at a height h above the
ground, the gravitational potential energy is 
Gravitational red shift: Einstein showed that, according to general relativity, time runs more slowly in a gravitational field.
Gravity: See Law of Universal Gravitation.
Hadrons: Particles that participate in the strong interaction. Hadrons are not fundamental particles; they have a definite extension. Hadrons that decay into a proton and another stable particle are baryons. The remaining hadrons are the mesons.
Half-life is the time required for half the nuclei in a given radioactive sample to decay.
Heat:
The thermal energy transferred from a warmer
object to a cooler object. The SI unit of heat is the joule; another unit
of heat, defined during the times of the caloric theory, is the calorie,
cal , which is the amount of heat required to raise the temperature of 1
gram of water by 1ºC. The relation between calories and joules is 1 cal
= 4.186 J.
Heat
capacity. The heat required to increase the temperature of a mass m of the substance an amount DT. The specific heat capacity, c, or heat capacity per unit mass, is the heat required to raise the temperature
of a unit mass of a substance by one degree. If Q is the amount of heat required; m, the mass of the substance; and DT, the change in temperature,
the specific heat capacity is 
The units of specific heat in the SI system
are J/kg ºC, but kcal/kg ºC are more commonly used.
Heat of fusion: See latent
heat.
Heat of vaporization: See
latent heat.
Heisenberg’s Uncertainty Principle states that it is not possible to measure the exact position and the exact momentum of a particle simultaneously.
Hertz: The SI unit of frequency.
One cycle per second.
Higgs field: See
Higgs mechanism. A quantum field with very special properties
that allows it to give mass to the elementary particles when they interact
with this field.
Higgs
mechanism: See Higgs field. The Higgs mechanism is a special set of circumstances
such that the state in which the Higgs field has its lowest energy is one
of broken symmetry.
Hologram,
holography: The recording of an image by recording the
standing wave pattern caused by the interaction of two coherent beams of
radiation, one having been modified by interaction with the object.
Humidity:
Measure of the amount of water present in the air at any given time. Absolute humidity (AH) is the total mass of water vapor present in the air per unit volume,
generally given in g/m3.
Humidity at saturation, HS, is
the mass per unit volume of water vapor required to saturate the air. Relative
humidity, RH, is the ratio of the absolute humidity to the humidity at saturation:
The temperature at which the air
saturates is called the dew point.
Ideal gas law: Combining Boyle’s law and Gay-Lussac’s law into one single expression, we obtain what is known as the P V = constant ´ T.
Ideal gas: Any gas in which the cohesive forces between molecules are negligible and the collisions between molecules are perfectly elastic. Collisions between the molecules of an ideal gas conserve both momentum and kinetic energy. If an ideal gas is kept at constant temperature, the pressure is inversely proportional to its volume; that is, the product of the pressure and the volume is a constant. This is Boyle’s law. The constant is the same for all gases. If the pressure of an ideal gas is kept constant, a change in volume is proportional to the change in absolute temperature. This is Gay-Lussac’s law, also known as Charles’ law
Image is formed where light rays intersect or where they appear to originate. A real image is one formed by light rays actually intersecting whereas a virtual image is an image formed by light rays which appear to come from a point.
Index of refraction, n, of the substance is the ratio of the speed of light in vacuum to that in the substance.
Inelastic
collision: A collision is which kinetic energy is not conserved.
Momentum is conserved.
Inertia: The tendency of an object to resist any change
in its state of motion.
Inertial reference frame: A reference frame in which the law of inertia holds. Thus, inertial reference frames move at constant velocities.
Infrared radiation: Electromagnetic radiation in the region of the spectrum between about 4 x 1014 and 1011 Hz and wavelengths between about 7.5 x 10-7 m and 3 x 10-3 m.
Instantaneous speed is the speed that an object has at any given instant.
Intensity of a wave is the rate at which the wave transports energy per unit of area.
Interference: When two or more waves overlap, their amplitudes add up algebraically; at some places the waves add up to produce bigger amplitudes and at other places, the waves combine to produce smaller amplitudes. When the displacement is enhanced, the waves undergo constructive interference, and when the displacement is diminished, the waves undergo destructive interference.
Internal energy: The internal energy, U, of a system is the sum of all forms of energy, thermal energy and potential energy. Thermal energy is the sum of all the random kinetic energies of the atoms and molecules in a substance and potential energy is the energy stored in the molecules, atoms, and nuclei of a substance.
Invariance:
If a system remains unchanged after some operation
is performed on it, we say the system is invariant under that operation.
Inverse
square law: A mathematical expression in which a value
for a quantity varies inversely with the square of the distance.
Ion: An atom or molecule with a net electric charge.
Ionic
bond: Bonding due to the electrical attraction between
oppositely charged ions.
Ionizing radiation is produced by a particle or a photon with enough energy to knock off an electron from an atom.
Isobaric
process is one that occurs at constant pressure. In this case, the work
done is W = P DV.
Isotope:
One
of several forms of an element having the same number of protons but a different
number of neutrons.
Joule (J). The SI unit of energy: 1 J = 1 N ´ 1 m. As an illustration, lifting a baseball from the ground to your chest requires about 1 joule of energy.
Kepler’s Laws of planetary motion. Kepler, in the early 1600s, discovered the three laws of planetary motion that bear his name:
I. Law of orbits: Each planet moves around the sun in an elliptical orbit, with the sun at one focus.
II. Law of areas: A planet moves around the sun at a rate such that the line from the sun to the planet sweeps out equal areas in equal intervals of time.
III. Harmonic law or law of periods: The squares of the periods of any two planets are proportional to the cubes of their average distances from the sun.
Kinetic energy. The energy that an object has by virtue of its motion. It is equal to one-half the product of the mass, m, and the square of the speed, v2:
Laser (light amplification by stimulated emission of radiation) produces a narrow beam of single-wavelength, coherent light by stimulated emission of photons.
Latent heat: The heat absorbed or released by one kilogram of a substance during a phase transition. If the transition is from solid to liquid or vice versa, this latent heat is called latent heat of fusion, Lf. If the transition involves the liquid and gas phases of a substance, it is called the latent heat of vaporization, Lv. For water, Lf = 80 kcal/kg = 335 kJ/kg, Lv = 540 kcal/kg = 2,259 kJ/kg. The heat required to melt a solid of mass m is Q = mLf; the heat required to vaporize a liquid of mass m is Q = mLv.
Law
of reflection: The angle at which a light ray bounce
s off a surface or angle of reflection is equal to the angle at which the ray
strikes the same surface or angle of incidence. The incident ray, the reflected
ray and the normal are all in the same plane.
Law of refraction or Snell’s law says that the angle of refraction is in a constant relationship to the angle of incidence.
Law
of Universal Gravitation. Any two objects of mass M
and m, separated by a distance
r, anywhere
in the universe, will attract each other with a force given by
The universal constant G has a value of 6.67´10-11 N.m2/kg2.
Laws of thermodynamics:
· The zeroth law of thermodynamics states that if two objects are each in thermal equilibrium with a third object (the thermometer), they are in thermal equilibrium with each other.
• The first law of thermodynamics is a restatement of the principle of conservation of energy and says that: In an isolated system, the total internal energy of a system remains constant, although it can change from one kind to another.
• The second law of thermodynamics was stated by Clausius as follows: Heat does not pass spontaneously from cold to hot. Entropy is a measure of the disorder of a system. In terms of entropy, the second law can be stated as follows: The entropy of the universe never decreases; all natural changes take place in the direction of increasing entropy.
· The third law of thermodynamics can be stated as follows: It is impossible to reach the absolute zero temperature in a process with a finite number of steps.
Length contraction: An observer in motion relative to an object measures the length of that object along the direction of motion to be contracted when compared to the length measured by an observer at rest relative to the object.
Leptons: Particles that interact via the weak force. All leptons are truly elementary particles, without internal structure. Leptons are classified in three generations, each containing a charged lepton and a neutrino.
Lever arm. The perpendicular distance from the center of rotation to the point of application of the force.
light year: The distance traveled by light in one year. It is equal to about 9.5 x 1012 km.
Magnetic field: Describes the property of space around a magnet. Because magnetic poles exist only in pairs, magnetic field lines do not start or end anywhere, an essential difference from electric field lines, which start on positive charges and end on negative charges.
Magnetosphere: The earth has a magnetic field that is believed to be due to motions of the metallic core which produce electric currents in the hot, electrically conductive material; these currents flow upward and are in turn carried around by the earth’s fast rotation. This magnetic field traps some matter from the solar wind.
Magnification, M, is the ratio of the size of the image to the size of the object, which is also equal to the ratio of the image distance to the object distance.
Mass
number: The total number of protons and neutrons in
a nucleus.
Maxwell’s equations.: James Clerk Maxwell developed a theory of electromagnetism, summarized into four equations. The first equation provides a relationship between an electric charge and the electric field it produces. The second equation describes how magnetic field lines always form closed loops; that is, they do not start or stop anywhere. The third equation is Faraday’s law which says that a changing magnetic field creates an electric field. The fourth equation is an extension of Ampère’s law, and states that a changing electric field creates a magnetic field.
Meniscus: The curved surface of a liquid in a container produced by the cohesive forces between the liquid molecules and the adhesive forces between the liquid and the container.
Meson: Particle with a mass that falls between the mass of the electron and the mass of the proton. Mesons are combinations of a quark and an antiquark.
Metastable state: An excited energy state of an atom with a longer life time than that of regular excited states.
meter: SI unit of length. The meter is defined as the distance traveled by light in 1/299 792 458 of a second.
Microwaves: Electromagnetic radiation in the region of the spectrum between about 3 x 108 and 3 x 1011 Hz and wavelengths between about 10-4 and 1 m.
Molecule. When atoms combine in such a way as to share some of their electrons, in a covalent bond, the new structure thus formed is called a molecule.
Moment of inertia. The moment of inertia of a body measures its resistance to change in its state of rotation. The moment of inertia could change if the orientation with respect to the axis of rotation changes.
Muon: A fundamental particle with a mass about 200 times that of the electron. A muon is a lepton that decays into an electron and neutrinos.
Newton. The SI unit of force, defined as1 N = 1 kg ´ m/s2.
Newton’s
Laws of Motion:
Newton’s First Law: Every body continues in its state of rest or
of uniform motion in a straight line unless it is compelled to change that
state by forces impressed upon it.
Newton’s Second Law: The acceleration of an object is directly proportional
to the net force acting on it and inversely proportional to its mass.
Newton’s Third Law: To every action there is always an equal and
opposite reaction. The mutual actions of two bodies upon each other are
always equal and directed to contrary parts.
Nöther’s theorem: Theorem that establishes the connection between symmetry and the laws of physics. It can be stated as follows: For every continuous symmetry in the laws of physics there exists a corresponding conservation law. A continuous symmetry is one in which the corresponding transformation can be varied continuously, as in a rotation.
Nuclear
fission occurs when a heavy nucleus splits into two smaller nuclei called
fission fragments.
Nuclear fusion takes place when two light nuclei are fused together to form a heavier nucleus.
Object and Image distances: The distance from the location of the object to the mirror. Image distance is the distance from the image to the mirror. For a plane mirror, the object distance is equal to the image distance.
Ohm’s Law: The current flowing through a conductor is directly proportional to the voltage V that exists between the two ends of the conductor or I=V/R, where R is the resistance of the conductor.
Pair annihilation: See pair production. When a particle encounters its antiparticle, they annihilate each other, disappearing in a burst of photons.
Pair production: See pair annihilation. The inverse process, in which high energy photons create a positron-electron pair.
Pascal’s principle. The pressure applied to a liquid is transmitted undiminished to all points of the liquid and to the walls of the container.
Period: (T) The time required to complete one cycle of a periodic motion. Period is the inverse of frequency:
Photoelectric effect: Light of a certain frequency incident on a surface causes electrons to be emitted from the substance.
Photon. Quanta of light. Einstein’s generalization of the concept of the quantum introduced by Max Planck.
Pion: The particle that mediates the strong force. Also called the π-meson.
Polarization: Orientation of the oscillation vector of a wave or of the rotation axis of a spinning object.
Positron: An antielectron. A fundamental particle with the same mass as the electron and a positive electric charge of the same magnitude as that of the electron. A constituent of antimatter.
Potential energy: The energy that an object has by virtue of its position in a field.
Power: The rate at which work is done or energy is released.
Pressure is defined as the force per unit area. The SI unit of pressure is the newton per square meter (N/m2), which is also called the pascal (Pa). The atmospheric pressure at sea level is 101.3 kPa or 1 atm. The pressure in a liquid increases with depth.
Principle of Equivalence: it is impossible to distinguish accelerated motion from the effects of gravity. It extends the relativity principle to accelerated frames of reference. The principle of equivalence can be then stated in the alternative form: gravitational mass and inertial mass are equivalent and no experiment can ever distinguish one from the other.
Proper time: The time interval measured by an observer in his own reference frame.
Proton: One of the main constituents of matter. The proton has an electric charge of 1.602 x 10-19 C and mass of 1.6726 x 10-27 kg or 1.007276 amu.
Quanta. Bundles or packets of energy, introduced in 1900 by Max Planck to explain the behavior of the radiation emitted by a hot body. Albert Einstein generalized this revolutionary concepts and stated that light behaves both as a wave and as a bundle of quanta of energy or photons.
Quantum chromodynamics: (QCD) The theory that explains the color interactions between quarks. This theory is modeled after quantum electrodynamics or QED.
Quarks: . Quarks are believed to be—like leptons—truly fundamental particles. Hadrons are thought to be composed of quarks. There are six flavors or varieties of quarks: Up and down, strange and charm, and bottom and top. Like leptons, quarks are also grouped in generations. The strong force arises from the interaction between quarks. Quarks possess a kind of charge called color charge: red, green and blue. All hadrons are color-neutral or white.
radian:
The central angle in a circle subtended by an
arc length equal to the radius of the circle. It is equal to 57.3º.
Radio
waves: Electromagnetic radiation in the region of
the spectrum smaller than about 3 x 108 Hz and wavelengths greater
than about 1 m.
Radioactive
decay: see radioactivity.
Radioactivity:
Emission of several particles such as electrons,
photons, neutrinos, or positrons, due to the decay of unstable nuclei.
Resistance:
The ratio of the voltage drop to the current is the resistance, R, of the conductor. Current, resistance,
and voltage are related by 
The units of resistance are ohms, W:
Schwartzchild radius: See event horizon. The radius to which a given object must be reduced so that its escape velocity equals the speed of light. It is the event horizon of a black hole.
Semiconductor: Materials with electrical conductivities that are intermediate between those of conductors and insulators. The conductivity of a semiconductor is changed by the addition of small amounts of impurities to its crystal structure.
Solar wind: The stream of energetic particles emitted by the sun.
Sound: A mechanical longitudinal wave that propagates through a medium with frequencies that range from a fraction of a hertz to several megahertz. Audible waves are sound waves with frequencies between 20 and 20,000 Hz. Sound waves with frequencies below 20 Hz are called infrasonic waves and those with frequencies above 20,000 Hz are called ultrasonic waves.
Space-time:
A four-dimensional
geometry consisting of the three coordinates of space and one of time.
Specific
heat: The heat required to increase the temperature
of 1 gram of substance by one degree Celsius.
Speed of sound: Depends on the elastic and inertial properties of the particular medium. It also depends on the temperature of the medium. In air, the speed of sound at 20 ºC is 343 m/s. At any other temperature T in degrees Celsius, the speed of sound in air is given by v = (331 + 0.61T) m/s.
Standing waves : Two identical wave motions of equal amplitude and wavelength traveling in opposite directions. The points that do not move are called nodes. Standing waves can be seen on a stretched string fixed at both ends. The natural frequencies of vibration form a harmonic series.
Strong force: See nuclear force. Holds the nucleons together in a nucleus. It is a short-range foce, becoming negligible at distances greater than 10-15 m. The strong force acts on protons and neutrons but not on electrons, neutrinos or photons. The strong force is 137 times stronger than the electromagnetic force.
Supergravity: Theory that attempts to unify gravity with the other three forces with the use of a powerful new gauge symmetry that unites quarks and leptons with messenger particles called supersymmetry. Supergravity theories are formulated in more than four space-time dimensions.
Superstring theory: A remarkable new theory that promises to provide a unified description of all the forces of nature. According to this theory, all elementary particles are represented by strings—open or closed—no longer than 10-35 m.
Surface tension: The intermolecular forces that act on the molecules on the surface of the liquid act to make the surface of the liquid as small as possible. These forces are also responsible for the rising of the liquid in very thin tubes, a phenomenon known as capillarity.
Symmetry: If something remains unchanged after some operation is performed on it, it is said to be symmetric under that operation.
Temperature: The measure of the average random kinetic energy per molecule. Thermal energy is the total random energy of the molecules in a substance.
Thermal expansion: The change in length when a change in temperature DT has occurred is DL = a Lo DT, where a is the coefficient of linear expansion and Lo, the initial length.
Time dilation: Time in the moving reference frame always flows more slowly.
Torque. The product of the applied force times the lever arm: The ability to rotate an object depends on the applied torque.
Total internal reflection occurs when light passes from a medium with a high index of refraction to one with a low index of refraction at angles of incidence greater than the critical angle.
Transistor:
A semiconductor device that can act as a current
switch and as an electronic amplifier in a circuit.
Ultraviolet
radiation: Electromagnetic radiation in the region of
the spectrum between about 7.5 x 1014 and 3 x1018
Hz and wavelengths between about 4 x 10-7 m and 10-10
m.
Uncertainty
principle: See Heisenberg’s Uncertainty Principle.
Uniform Motion. Motion at a constant velocity.
Uniformly Accelerated Motion. Motion with constant acceleration. One important example of uniformly accelerated motion is the vertical motion of an object falling towards the ground due to the gravitational attraction of the earth. This gravitational attraction produces a constant acceleration g = 9.8 m/s2.
Universal law of gravitation: See Law of universal gravitation.
Vector
Quantities. Quantities that require a magnitude
and a direction for their complete
specification, like velocity.
Velocity. The speed and the direction of the motion of an object.
Virtual particles: Particles that exist only for the brief moment allowed by Heisenberg’s uncertainty principle.
Visible
light: Electromagnetic radiation in the region of the
spectrum between about 4 x 1014 and 7.5 x1014 Hz and
wavelengths between about 7.5 x 10-7 m and 4 x 10-7
m.
Voltage: The change in electric potential energy of a charge divided by the magnitude of that charge.
Wave is a mechanism for the transmission of energy. Mechanical waves transmit energy but the medium itself does not travel. Light, on the other hand, is a wave that does not require a medium to propagate.
Wavelength: (l) The distance between two identical points of a periodic wave.
Weak force: See strong
force. Responsible for radioactive β-decay processes, such as the
transformation within the nucleus of a neutron into a proton or a proton
into a neutron. The weak force controls many of the reactions that produce
energy in the sun and the stars. The weak force is some hundred thousand
times weaker than the strong force.
Weight. The force with which the earth attracts an object towards its center when the object is on or near the earth’s surface.
Work function: The minimum amount of energy required to release electrons
from a particular metal.
Work. When a constant force acts on an object along the direction of motion of the object, the work done on the object is equal to the force times the distance that the object moves, orWork = force ´ distance. If the applied force is at right angles to the direction of motion of the object, no work is done.
X-rays: Electromagnetic radiation in the region of the spectrum between about 3 x 1016 and 1021 Hz and wavelengths between about 10-8 m and 3 x10-13 m.