, rules, principles, effects, paradoxes, limits, constant
, experiments, & thought-experiments in physics
Pupil : Morozov Michael
Ampere's law (A.M. Ampere)
The line integral of the magnetic flux around a closed curve isproportional to the algebraic sum of electric currents flowingthrough that closed curve. This was later modified to add a second term when it wasincorporated into Maxwell's equations.
Weak anthropic principle
. The conditions necessary for the development of intelligent life will be met only in certain regions that are limited in space and time. That is, the region of the Universe in which we live is not necessarily representative of a purely random set of initial conditions; only those favorable to intelligent life would actually develop creatures who wonder what the initial conditions of the Universe were.
Strong anthropic principle.
A more forceful argument that the weak principle: It states, rather straightforwardly, that if the laws of the Universe were not conducive to the development
of intelligent creatures to ask about the initial conditions of the Universe, intelligent life would never have evolved to ask the question in the first place. In other words, the laws of the Universe are the way they are because if they weren't, you would not be able to ask such a question.
Arago spot (D.F.J. Arago)
A bright spot that appears in the shadow of a uniform disc beingbacklit by monochromatic light emanating from a point source.
A body that is submerged in a fluid is buoyed up by a force equalin magnitude to the weight of the fluid that is displaced, anddirected upward along a line through the center of gravity of thedisplaced fluid.
A weight-and-pulley system devised to measure the acceleration dueto gravity at Earth's surface by measuring the net acceleration ofa set of weights of known mass around a frictionless pulley.
Avogadro constant; L; NA
(Count A. Avogadro; 1811)
The number of atoms or molecules in a sample of an idea gas whichis at standard temperature and pressure. It is equal to about
Avogadro's hypothesis (Count A. Avogadro; 1811)
Equal volumes of all gases at the same temperature and pressurecontain equal numbers of molecules. It is, in fact, only true forideal gases.
Balmer series (J. Balmer; 1885)
An equation which describes the emission spectrum of hydrogen whenan electron is jumping to the second orbital; four of the linesare in the visible spectrum, and the remainder are in theultraviolet.
The theory, predicted by several grand-unified theories, that aclass of subatomic particles called baryons (of which the nucleons-- protons and neutrons -- are members) are not ultimately stablebut indeed decay. Present theory and experimentation demonstratethat if protons are indeed unstable, they decay with a halflife ofat least 10
An equation which states that an irrotational fluid flowingthrough a pipe flows at a rate which is inversely proportional tothe cross-sectional area of the pipe. That is, if the pipeconstricts, the fluid flows faster; if it widens, the fluid flowsslower.
BCS theory (J. Bardeen, L.N. Cooper, J.R. Schrieffer; 1957)
A theory put forth to explain both superconductivity andsuperfluidity. It suggests that in the superconducting (orsuperfluid) state electrons form Cooper pairs, where two electronsact as a single unit. It takes a nonzero amount of energy tobreak such pairs, and the imperfections in the superconductingsolid (which would normally lead to resistance) are incapable ofbreaking the pairs, so no dissipation occurs and there is noresistance.
Biot-Savart law (J.B. Biot, F. Savart)
A law which describes the contributions to a magnetic field by anelectric current. It is analogous to Coulomb's law forelectrostatics.
The radiation -- the radiance at particular frequencies all acrossthe spectrum -- produced by a blackbody -- that is, a perfectradiator (and absorber) of heat. Physicists had difficultyexplaining it until Planck introduced his quantum of action.
A mathematical formula which generates, with a fair amount ofaccuracy, the semimajor axes of the planets in order out from theSun. Write down the sequence 0, 3, 6, 12, 24, . . . and then add4 to each term. Then divide each term by 10. This is intended togive you the positions of the planets measured in astronomicalunits.
Bode's law had no theoretical justification when it was firstintroduced; it did, however, agree with the soon-to-be-discoveredplanet Uranus' orbit (19.2 au actual; 19.7 au predicted).Similarly, it predicted a missing planet betwen Mars and Jupiter,and shortly thereafter the asteroids were found in very similarorbits (2.8 au actual for Ceres; 2.8 au predicted). However, theseries seems to skip over Neptune's orbit.
Bohr magneton (N. Bohr)
The quantum of magnetic moment.
Bohr radius (N. Bohr)
The distance corresponding the mean distance of an electron fromthe nucleus in the ground state.
Boltzmann constant; k (L. Boltzmann)
A constant which describes the relationship between temperatureand kinetic energy for molecules in an ideal gas. It is equal to1.
Boyle's law (R. Boyle; 1662); Mariotte's law (E. Mariotte; 1676)
The product of the pressure and the volume of an ideal gas atconstant temperature is a constant.
Brackett series (Brackett)
The series which describes the emission spectrum of hydrogen whenthe electron is jumping to the fourth orbital. All of the linesare in the infrared portion of the spectrum.
Bragg's law (Sir W.L. Bragg; 1912)
When a beam of x-rays strikes a crystal surface in which thelayers of atoms or ions are regularly separated, the maximumintensity of the reflected ray occurs when the sine of thecompliment of the angle of incidence is equal to an integermultiplied by the wavelength of x-rays divided by twice thedistance between layers of atoms or ions.
Brewster's law (D. Brewster)
The extent of the polarization of light reflected from atransparent surface is a maximum when the reflected ray is atright angles to the refracted ray.
Brownian motion (R. Brown; 1827)
The continuous random motion of solid microscopic particles whensuspended in a fluid medium due to the consequence of continuousbombardment by atoms and molecules.
Carnot's theorem (S. Carnot)
The theorem which states that no engine operating between twotemperatures can be more efficient than a reversible engine.
A pseudoforce -- a fictitious force resulting from being in a non-inertial frame of reference -- that occurs when one is moving inuniform circular motion. One feels a "force" outward from thecenter of motion.
Chandrasekhar limit (S. Chandrasekhar; 1930)
A limit which mandates that no white dwarf (a collapsed,degenerate star) can be more massive than about 1.2 solar masses.Anything more massive must inevitably collapse into a neutronstar.
Charles' law (J.A.C. Charles; c. 1787)
The volume of an ideal gas at constant pressure is proportional tothe thermodynamic temperature of that gas.
Cherenkov radiation (P.A. Cherenkov)
Radiation emitted by a massive particle which is moving fasterthan light in the medium through which it is travelling. Noparticle can travel faster than light in vacuum, but the speed oflight in other media, such as water, glass, etc., are considerablylower. Cherenkov radiation is the electromagnetic analogue of thesonic boom, though Cherenkov radiation is a shockwave set up inthe electromagnetic field.
Complementarity principle (N. Bohr)
The principle that a given system cannot exhibit both wave-likebehavior and particle-like behavior at the same time. That is,certain experiments will reveal the wave-like nature of a system,and certain experiments will reveal the particle-like nature of asystem, but no experiment will reveal both simultaneously.
Compton effect (A.H. Compton; 1923)
An effect that demonstrates that photons (the quantum ofelectromagnetic radiation) have momentum. A photon fired at astationary particle, such as an electron, will impart momentum tothe electron and, since its energy has been decreased, willexperience a corresponding decrease in frequency.
Coriolis pseudoforce (G. de Coriolis; 1835)
A pseudoforce -- a fictitious force, like the centrifugal "force"-- which arises because the rotation of the Earth varies atdifferent latitutdes (maximum at the equator, zero at the poles).
The principle that when a new, more specialized theory is putforth, it must reduce to the more general (and usually simpler)theory under normal circumstances. There are correspondenceprinciples for general relativity to special relativity andspecial relativity to Newtonian mechanics, but the most widelyknown correspondence principle (and generally what is meant whenone says "correspondence principle") is that of quantum mechanicsto classical mechanics.
Cosmic background radiation; primal glow
The background of radiation mostly in the frequency range 3
Hz discovered in space in 1965. It is believedto be the cosmologically redshifted radiation released by the BigBang itself. Presently it has an energy density in empty space ofabout
An effect where light emitted from a distant source appearsredshifted because of the expansion of space itself. Compare withthe Doppler effect.
The primary law for electrostatics, analogous to Newton's law ofuniversal gravitation. It states that the force between two pointcharges is proportional to the algebraic product of theirrespective charges as well as proportional to the inverse squareof the distance between them.
Curie-Weiss law (P. Curie, P.-E. Weiss)
A more general form of Curie's law, which states that thesusceptibility of a paramagnetic substance is inverselyproportional to the thermodynamic temperature of the substanceless the Weiss constant, a characteristic of that substance.
Curie's law (P. Curie)
The susceptibility of a paramagnetic substance is inverselyproportional to the thermodynamic temperature of the substance.The constant of proportionality is called the Curie constant.
Dalton's law of partial pressures (J. Dalton)
The total pressure of a mixture of ideal gases is equal to the sumof the partial pressures of its components; that is, the sum ofthe pressures that each component would exert if it were presentalone and occuped the same volume as the mixture.
Davisson-Germer experiment (C.J. Davisson, L.H. Germer; 1927)
An experiment that conclusively confirmed the wave nature ofelectrons; diffraction patterns were observed by an electron beampenetrating into a nickel target.
De Broglie wavelength (L. de Broglie; 1924)
The prediction that particles also have wave characteristics,where the effective wavelength of a particle would be inverselyproportional to its momentum, where the constant ofproportionality is the Planck constant.
Doppler effect (C.J. Doppler)
Waves emitted by a moving observer will be blueshifted(compressed) if approaching, redshifted (elongated) if receding.It occurs both in sound as well as electromagnetic phenomena,although it takes on different forms in each.
Dulong-Petit law (P. Dulong, A.T. Petit; 1819)
The molar heat capacity is approximately equal to the three timesthe gas constant.
Consider the following quantum mechanical thought-experiment:Take a particle which is at rest and has spin zero. Itspontaneously decays into two fermions (spin
0.5 particles), whichstream away in opposite directions at high speed. Due to the lawof conservation of spin, we know that one is a spin +
0.5 and theother is spin -
0.5. Which one is which? According to quantummechanics, neither takes on a definite state until it is observed(the wavefunction is collapsed).
The EPR effect demonstrates that if one of the particles isdetected, and its spin is then measured, then the other particle-- no matter where it is in the Universe -- instantaneously isforced to choose as well and take on the role of the otherparticle. This illustrates that certain kinds of quantuminformation travel instantaneously; not everything is limited bythe speed of light.
However, it can be easily demonstrated that this effect doesnot make faster-than-light communication possible.
The basic postulate of A. Einstein's general theory of relativity,which posits that an acceleration is fundamentallyindistinguishable from a gravitational field. In other words, ifyou are in an elevator which is utterly sealed and protected fromthe outside, so that you cannot "peek outside," then if you feel aforce (weight), it is fundamentally impossible for you to saywhether the elevator is present in a gravitational field, orwhether the elevator has rockets attached to it and isaccelerating "upward."
The equivalence principle predicts interesting generalrelativistic effects because not only are the twoindistinguishable to human observers, but also to the Universe aswell, in a way -- any effect that takes place when an observer isaccelerating should also take place in a gravitational field, andvice versa.
The region around a rotating black hole, between the event horizonand the static limit, where rotational energy can be extractedfrom the black hole.
The radius of surrounding a black hole at which a particle wouldneed an escape velocity of lightspeed to escape; that is, thepoint of no return for a black hole.
Faraday constant; F (M. Faraday)
The electric charge carried by one mole of electrons (or singly-ionized ions). It is equal to the product of the Avogadroconstant and the (absolute value of the) charge on an electron; itis
Faraday's law (M. Faraday)
The line integral of the electric flux around a closed curve isproportional to the instantaneous time rate of change of themagnetic flux through a surface bounded by that closed curve.
Faraday's laws of electrolysis (M. Faraday)
The amount of chemical change during electrolysis is proportional to the charge passed.
2. The charge required to deposit or liberate a mass is proportional to the charge of the ion, the mass, and inversely proprtional to the relative ionic mass. The constant of proportionality is the Faraday constant.
Faraday's laws of electromagnetic induction (M. Faraday)
An electromotive force is induced in a conductor when the magnetic field surrounding it changes.
The magnitude of the electromotive force is proportional to the rate of change of the field.
3. The sense of the induced electromotive force depends on the direction of the rate of the change of the field.
Fermat's principle; principle of least time (P. de Fermat)
The principle, put forth by P. de Fermat, states that the pathtaken by a ray of light between any two points in a system isalways the path that takes the least time.
E. Fermi's conjecture, simplified with the phrase, "Where arethey?" questioning that if the Galaxy is filled with intelligentand technological civilizations, why haven't they come to us yet?There are several possible answers to this question, but since weonly have the vaguest idea what the right conditions for life andintelligence in our Galaxy, it and Fermi's paradox are no morethan speculation.
Gauss' law (K.F. Gauss)
The electric flux through a closed surface is proportional to thealgebraic sum of electric charges contained within that closedsurface.
Gauss' law for magnetic fields (K.F. Gauss)
The magnetic flux through a closed surface is zero; no magneticcharges exist.
A paradox proposed to discount time travel and show why itviolates causality. Say that your grandfather builds a timemachine. In the present, you use his time machine to go back intime a few decades to a point before he married his wife (yourgrandmother). You meet him to talk about things, and an argumentensues (presumably he doesn't believe that you're hisgrandson/granddaughter), and you accidentally kill him.
If he died before he met your grandmother and never hadchildren, then your parents could certainly never have met (one ofthem didn't exist!) and could never have given birth to you. Inaddition, if he didn't live to build his time machine, what areyou doing here in the past alive and with a time machine, if youwere never born and it was never built?
When charged particles flow through a tube which has both anelectric field and a magnetic field (perpendicular to the electricfield) present in it, only certain velocities of the chargedparticles are preferred, and will make it undeviated through thetube; the rest will be deflected into the sides. This effect isexploited in such devices as the mass spectrometer and in theThompson experiment. This is called the Hall effect.
Hawking radiation (S.W. Hawking; 1973)
The theory that black holes emit radiation like any other hotbody. Virtual particle-antiparticle pairs are constantly beingcreated in supposedly empty space. Every once in a while, onewill be created in the vicinity of a black hole's event horizon.One of these particles might be catpured by the black hole,forever trapped, while the other might escape the black hole'sgravity. The trapped particle, which would have negative energy(by definition), would reduce the mass of the black hole, and theparticle which escaped would have positive energy. Thus, from adistant, one would see the black hole's mass decrease and aparticle escape the vicinity; it would appear as if the black holewere emitting radiation. The rate of emission has a negativerelationship with the mass of the black hole; massive black holesemit radiation relatively slowly, while smaller black holes emitradiation -- and thus decrease their mass -- more rapidly.
Heisenberg uncertainty principle (W. Heisenberg; 1927)
A principle, central to quantum mechanics, which states that themomentum (mass times velocity) and the position of a particlecannot both be known to infinite accuracy; the more you know aboutone, the lest you know about the other.
It can be illustrated in a fairly clear way as follows: Tosee something (let's say an electron), we have to fire photons atit, so they bounce off and come back to us, so we can "see" it.If you choose low-frequency photons, with a low energy, they donot impart much momentum to the electron, but they give you a veryfuzzy picture, so you have a higher uncertainty in position sothat you can have a higher certainty in momentum. On the otherhand, if you were to fire very high-energy photons (x-rays orgammas) at the electron, they would give you a very clear pictureof where the electron is (high certainty in position), but wouldimpart a great deal of momentum to the electron (higheruncertainty in momentum). In a more generalized sense, the uncertainty principle tellsus that the act of observing changes the observed in fundamentalway.
Hooke's law (R. Hooke)
The stress applied to any solid is proportional to the strain itproduces within the elastic limit for that solid. The constant ofthat proportionality is the Young modulus of elasticity for thatsubstance.
Hubble constant; H0
(E.P. Hubble; 1925)
The constant which determines the relationship between thedistance to a galaxy and its velocity of recession due to theexpansion of the Universe. It is not known to great accuracy, butis believed to lie between 49 and 95
Hubble's law (E.P. Hubble; 1925)
A relationship discovered between distance and radial velocity.The further away a galaxy is away from is, the faster it isreceding away from us. The constant of proportionality isHubble's constant, H0
. The cause is interpreted as the expansionof space itself.
Huygens' construction; Huygens' principle (C. Huygens)
The mechanics propagation of a wave of light is equivalent toassuming that every point on the wavefront acts as point source ofwave emission.
Ideal gas constant; universal molar gas constant; R
The constant that appears in the ideal gas equation. It is equalto 8.314 34.
Ideal gas equation
An equation which sums up the ideal gas laws in one simpleequation. It states that the product of the pressure and thevolume of a sample of ideal gas is equal to the product of theamount of gas present, the temperature of the sample, and theideal gas constant.
Ideal gas laws
The pressure of an ideal gas is inversely proportional to the volume of the gas at constant temperature.
The volume of an ideal gas is directly proportional to the thermodynamic temperature at constant pressure.
The pressure law.
The pressure of an ideal gas is directly
proportional to the thermodynamic temperature at constant volume.
Joule-Thomson effect; Joule-Kelvin effect (J. Joule, W. Thomson)
The change in temperature that occurs when a gas expands into aregion of lower pressure.
Joule's first law.
The heat produced when an electric current flows through a resistance for a specified time is equal to
the square of the current multiplied by the resistivity
multiplied by the time.
Joule's second law.
The internal energy of an ideal gas is independent of its volume and pressure, depending only on its temperature.
Josephson effects (B.D. Josephson; 1962)
Electrical effects observed when two superconducting materials areseparated by a thin layer of insulating material.
Kepler's laws (J. Kepler)
Kepler's first law
. A planet orbits the Sun in an ellipse with the Sun at one focus.
Kepler's second law
. A ray directed from the Sun to a planet sweeps out equal areas in equal times.
Kepler's third law
. The square of the period of a planet's orbit is proportional to the cube of that planet's semimajor axis; the constant of proportionality is the same for all planets.
Kerr effect (J. Kerr; 1875)
The ability of certain substances to differently refract lightwaves whose vibrations are in different directions when thesubstance is placed in an electric field.
Kirchhoff's law of radiation (G.R. Kirchhoff)
The emissivity of a body is equal to its absorptance at the sametemperature.
Kirchhoff's rules (G.R. Kirchhoff)
The loop rule.
The sum of the potential differences encountered in a round trip around any closed loop in a circuit is zero.
The point rule.
The sum of the currents toward a branch point is equal to the sum of the currents away from the same branch
Kohlrausch's law (F. Kohlrausch)
If a salt is dissolved in water, the conductivity of the solutionis the sum of two values -- one depending on the positive ions andthe other on the negative ions.
Lambert's laws (J.H. Lambert)
Lambert's first law.
The illuminance on a surface illuminated by
light falling on it perpendicularly from a point source is
proportional to the inverse square of the distance between the
surface and the source.
Lambert's second law.
If the rays meet the surface at an angle, then the illuminance is also proportional to the cosine of the angle with the normal.
Lambert's third law.
The luminous intensity of light decreases exponentially with the distance that it travels through an absorbing medium.
A principle which states that it doesn't explicitly take energy tocompute data, but rather it takes energy to erase
any data,since erasure is an important step in computation.
Laplace's equation (P. Laplace)
For steady-state heat conduction in one dimension, the temperaturedistribution is the solution to Laplace's equation, which statesthat the second derivative of temperature with respect todisplacement is zero.
Laue pattern (M. von Laue)
The pattern produced on a photographic film when high-frequencyelectromagnetic waves (such as x-rays) are fired at a crystallinesolid.
Laws of conservation
A law which states that, in a closed system, the total quantity ofsomething will not increase or decrease, but remain exactly thesame. For physical quantities, it states that something canneither be created nor destroyed.
The most commonly seen are the laws of conservation of mass-energy (formerly two conservation laws before A. Einstein), ofelectric charge, of linear momentum, and of angular momentum.There are several others that deal more with particle physics,such as conservation of baryon number, of strangeness, etc., whichare conserved in some fundamental interactions but not others.
Law of reflection
For a wavefront intersecting a reflecting surface, the angle ofincidence is equal to the angle of reflection.
Laws of black hole dynamics
First law of black hole dynamics.
For interactions between black holes and normal matter, the conservation laws of total energy, total momentum, angular momentum, and electric charge, hold.
Second law of black hole dynamics.
With black hole interactions, or interactions between black holes and normal matter, the sum
of the surface areas of all black holes involved can never
Laws of thermodynamics
First law of thermodynamics.
The change in internal energy of a system is the sum of the heat transferred to or from the system and the work done on or by the system.
Second law of thermodynamics.
The entropy -- a measure of the unavailability of a system's energy to do useful work -- of a
closed system tends to increase with time.
Third law of thermodynamics.
For changes involving only perfect crystalline solids at absolute zero, the change of the total entropy is zero.
Zeroth law of thermodynamics.
If two bodies are each in thermal equilibrium with a third body, then all three bodies are in thermal equilibrium with each other.
Lawson criterion (J.D. Lawson)
A condition for the release of energy from a thermonuclearreactor. It is usually stated as the minimum value for theproduct of the density of the fuel particles and the containmenttime for energy breakeven. For a half-and-half mixture ofdeuterium and tritium at ignition temperature, nG
t is between1014
Le Chatelier's principle (H. Le Chatelier; 1888)
If a system is in equilibrium, then any change imposed on thesystem tends to shift the equilibrium to reduce the effect of thatapplied change.
Lenz's law (H.F. Lenz; 1835)
An induced electric current always flows in such a direction thatit opposes the change producing it.
Loschmidt constant; Loschmidt number; NL
The number of particles per unit volume of an ideal gas atstandard temperature and pressure. It has the value 2.68719.
A substance, which filled all the empty spaces between matter,which was used to explain what medium light was "waving" in. Nowit has been discredited, as Maxwell's equations imply thatelectromagnetic radiation can propagate in a vacuum, since theyare disturbances in the electromagnetic field rather thantraditional waves in some substance, such as water waves.
The series which describes the emission spectrum of hydrogen whenelectrons are jumping to the ground state. All of the lines arein the ultraviolet.
Mach's principle (E. Mach; 1870s)
The inertia of any particular particle or particles of matter isattributable to the interaction between that piece of matter andthe rest of the Universe. Thus, a body in isolation would have noinertia.
A rotating cylinder in a moving fluid drags some of the fluidaround with it, in its direction of rotation. This increases thespeed in that region, and thus the pressure is lower.Consequently, there is a net force on the cylinder in thatdirection, perpendicular to the flow of the fluid. This is calledthe Magnus effect.
Malus's law (E.L. Malus)
The light intensity travelling through a polarizer is proportionalto the initial intensity of the light and the square of the cosineof the angle between the polarization of the light ray and thepolarization axis of the polarizer.
Maxwell's demon (J.C. Maxwell)
A thought experiment illustrating the concepts of entropy. Wehave a container of gas which is partitioned into two equal sides;each side is in thermal equilibrium with the other. The walls(and the partition) of the container are a perfect insulator. Now imagine there is a very small demon who is waiting at thepartition next to a small trap door. He can open and close thedoor with negligible work. Let's say he opens the door to allow afast-moving molecule to travel from the left side to the right, orfor a slow-moving molecule to travel from the right side to the
left, and keeps it closed for all other molecules. The net effectwould be a flow of heat -- from the left side to the right -- eventhough the container was in thermal equilibrium. This is clearlya violation of the second law of thermodynamics. So where did we go wrong? It turns out that information hasto do with entropy as well. In order to sort out the moleculesaccording to speeds, the demon would be having to keep a memory ofthem -- and it turns out that increase in entropy of the simplemaintenance of this simple memory would more than make up for thedecrease in entropy due to the heat flow.
Maxwell's equations (J.C. Maxwell; 1864)
Four elegant equations which describe classical electromagnetismin all its splendor. They are:
The electric flux through a closed surface is proportional to the algebraic sum of electric charges contained within that closed surface.
Gauss' law for magnetic fields.
The magnetic flux through a closed surface is zero; no magnetic charges exist.
The line integral of the electric flux around a closed curve is proportional to the instantaneous time rate of change of the magnetic flux through a surface bounded by that closed curve.
Ampere's law, modified form.
The line integral of the magnetic flux around a closed curve is proportional to the sum of two terms: first, the algebraic sum of electric currents flowing through that closed curve; and second, the instantaneous time rate of change of the electric flux through a surface bounded by that closed curve.
In addition to describing electromagnetism, his equations alsopredict that waves can propagate through the electromagneticfield, and would always propagate at the same speed -- these are
Meissner effect (W. Meissner; 1933)
The decrease of the magnetic flux within a superconducting metalwhen it is cooled below the critical temperature. That is,superconducting materials reflect magnetic fields.
Michelson-Morley experiment (A.A. Michelson, E.W. Morley; 1887)
Possibly the most famous null-experiment of all time, designed toverify the existence of the proposed "lumeniferous aether" throughwhich light waves were thought to propagate. Since the Earthmoves through this aether, a lightbeam fired in the Earth'sdirection of motion would lag behind one fired sideways, where noaether effect would be present. This difference could be detectedwith the use of an interferometer.
The experiment showed absolutely no aether shift whatsoever,where one should have been quite detectable. Thus the aetherconcept was discredited as was the constancy of the speed oflight.
Millikan oil drop experiment (R.A. Millikan)
A famous experiment designed to measure the electronic charge.Drops of oil were carried past a uniform electric field betweencharged plates. After charging the drop with x-rays, he adjustedthe electric field between the plates so that the oil drop wasexactly balanced against the force of gravity. Then the charge onthe drop would be known. Millikan did this repeatedly and foundthat all the charges he measured came in integer multiples only ofa certain smallest value, which is the charge on the electron.
Newton's law of universal gravitation (Sir I. Newton)
Two bodies attract each other with equal and opposite forces; themagnitude of this force is proportional to the product of the twomasses and is also proportional to the inverse square of thedistance between the centers of mass of the two bodies.
Newton's laws of motion (Sir I. Newton)
Newton's first law of motion.
A body continues in its state of rest or of uniform motion unless it is acted upon by an external force.
Newton's second law of motion.
For an unbalanced force acting on a body, the acceleration produces is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.
Newton's third law of motion.
In a system where no external forces are present, every action is always opposed by an equal
and opposite reaction.
Ohm's law (G. Ohm; 1827)
The ratio of the potential difference between the ends of aconductor to the current flowing through it is constant; theconstant of proportionality is called the resistance, and isdifferent for different materials.
Olbers' paradox (H. Olbers; 1826)
If the Universe is infinite, uniform, and unchanging then theentire sky at night would be bright -- about as bright as the Sun.The further you looked out into space, the more stars there wouldbe, and thus in any direction in which you looked your line-of-sight would eventually impinge upon a star. The paradox isresolved by the Big Bang theory, which puts forth that theUniverse is not infinite, non-uniform, and changing.
Pressure applied to an enclosed imcompressible static fluid istransmitted undiminished to all parts of the fluid.
The series which describes the emission spectrum of hydrogen whenthe electron is jumping to the third orbital. All of the linesare in the infrared portion of the spectrum.
Pauli exclusion principle (W. Pauli; 1925)
No two identical fermions in a system, such as electrons in anatom, can have an identical set of quantum numbers.
Peltier effect (J.C.A. Peltier; 1834)
The change in temperature produced at a junction between twodissimilar metals or semiconductors when an electric currentpasses through the junction.
permeability of free space
; magnetic constant;
The ratio of the magnetic flux density in a substance to theexternal field strength for vacuum. It is equal to 4
permittivity of free space;
The ratio of the electric displacement to the intensity of theelectric field producing it in vacuum. It is equal to 8.854
The series which describes the emission spectrum of hydrogen whenthe electron is jumping to the fifth orbital. All of the linesare in the infrared portion of the spectrum.
An effect explained by A. Einstein that demonstrate that lightseems to be made up of particles, or photons. Light can exciteelectrons (called photoelectrons) to be ejected from a metal.Light with a frequency below a certain threshold, at anyintensity, will not cause any photoelectrons to be emitted fromthe metal. Above that frequency, photoelectrons are emitted inproportion to the intensity of incident light. The reason is that a photon has energy in proportion to itswavelength, and the constant of proportionality is Planck'sconstant. Below a certain frequency -- and thus below a certainenergy -- the incident photons do not have enough energy to knockthe photoelectrons out of the metal. Above that threshold energy,called the workfunction, photons will knock the photoelectrons outof the metal, in proportion to the number of photons (theintensity of the light). At higher frequencies and energies, thephotoelectrons ejected obtain a kinetic energy corresponding tothe difference between the photon's energy and the workfunction.
Planck constant; h
The fundamental constant equal to the ratio of the energy of aquantum of energy to its frequency. It is the quantum of action.It has the value 6.626196
Planck's radiation law
A law which more accurately described blackbody radiation becauseit assumed that electromagnetic radiation is quantized.
Poisson spot (S.D. Poisson)
See Arago spot. Poisson predicted the existence of such a spot,and actually used it to demonstrate that the wave theory of lightmust be in error.
Principle of causality
The principle that cause must always preceed effect. Moreformally, if an event A ("the cause") somehow influences an eventB ("the effect") which occurs later in time, then event B cannotin turn have an influence on event A. The principle is best illustrated with an example. Say thatevent A constitutes a murderer making the decision to kill hisvictim, and that event B is the murderer actually committing theact. The principle of causality puts forth that the act ofmurder cannot have an influence on the murderer's decision tocommit it. If the murderer were to somehow see himself committingthe act and change his mind, then a murder would have beencommitted in the future without a prior cause (he changed hismind). This represents a causality violation. Both time traveland faster-than-light travel both imply violations of causality,which is why most physicists think they are impossible, or atleast impossible in the general sense.
Principle of determinism
The principle that if one knows the state to an infinite accuracyof a system at one point in time, one would be able to predict thestate of that system with infinite accuracy at any other time,past or future. For example, if one were to know all of thepositions and velocities of all the particles in a closed system,then determinism would imply that one could then predict thepositions and velocities of those particles at any other time.This principle has been disfavored due to the advent of quantummechanics, where probabilities take an important part in theactions of the subatomic world, and the Heisenberg uncertaintyprinciple implies that one cannot know both the position andvelocity of a particle to arbitrary precision.
Rayleigh criterion; resolving power
A criterion for the how finely a set of optics may be able todistinguish. It begins with the assumption that central ring ofone image should fall on the first dark ring of the other.relativity principle; principle of relativity
A formula which describes all of the characteristics of hydrogen'sspectrum, including the Balmer, Lyman, Paschen, Brackett, andPfund series.
Schroedinger's cat (E. Schroedinger; 1935)
A thought experiment designed to illustrate the counterintuitiveand strange notions of reality that come along with quantummechanics.
A cat is sealed inside a closed box; the cat has ample air,food, and water to survive an extended period. This box isdesigned so that no information (i.e., sight, sound, etc.) canpass into or out of the box -- the cat is totally cut off fromyour observations. Also inside the box with the poor kitty(apparently Schroedinger was not too fond of felines) is a phialof a gaseous poison, and an automatic hammer to break it, floodingthe box and killing the cat. The hammer is hooked up to a Geigercounter; this counter is monitoring a radioactive sample and isdesigned to trigger the hammer -- killing the cat -- should aradioactive decay be detected. The sample is chosen so thatafter, say, one hour, there stands a fifty-fifty chance of a decayoccurring.
The question is, what is the state of the cat after that onehour has elapsed? The intuitive answer is that the cat is eitheralive or dead, but you don't know which until you look. But it
is one of them. Quantum mechanics, on the other hands, saysthat the wavefunction describing the cat is in a superposition ofstates: the cat is, in fact, fifty per cent alive and fifty percent dead; it is both. Not until one looks and "collapses thewavefunction" is the Universe forced to choose either a live cator a dead cat and not something in between.
This indicates that observation also seems to be an importantpart of the scientific process -- quite a departure from theabsolutely objective, deterministic way things used to be withNewton.
The radius that a spherical mass must be compressed to in order totransform it into a black hole; that is, the radius of compressionwhere the escape velocity at the surface would reach lightspeed.
Snell's law; law of refraction
A relation which relates the change in incidence angle of awavefront due to refraction between two different media.
Speed of light in vacuo
One of the postulates of A. Einstein's special theory ofrelativity, which puts forth that the speed of light in vacuum --often written c, and which has the value 299 792 458 m/s -- ismeasured as the same speed to all observers, regardless of theirrelative motion. That is, if I'm travelling at 0.9 c away fromyou, and fire a beam of light in that direction, both you and Iwill independently measure the speed of that beam as c. One of the results of this postulate (one of the predictionsof special relativity is that no massive particle can beaccelerated to (or beyond) lightspeed, and thus the speed of lightalso represents the ultimate cosmic speed limit. Only masslessparticles (photons, gravitons, and possibly neutrinos, should theyindeed prove to be massless) travel at lightspeed, and all otherparticles must travel at slower speeds.
An effect that causes atomic energy levels to be split becauseelectrons have intrinsic angular momentum (spin) in addition totheir extrinsic orbital angular momentum.
The distance from a rotating black hole where no observer canpossibly remain at rest (with respect to the distant stars)because of inertial frame dragging.
Stefan-Boltzmann constant; sigma (Stefan, L. Boltzmann)
The constant of proportionality present in the Stefan-Boltzmannlaw. It is equal to
Stefan-Boltzmann law (Stefan, L. Boltzmann)
The radiated power (rate of emission of electromagnetic energy) ofa hot body is proportional to the emissivity, an efficiencyrating, the radiating surface area, and the fourth power of thethermodynamic temperature. The constant of proportionality is theStefan-Boltzmann constant.
Stern-Gerlach experiment (O. Stern, W. Gerlach; 1922)
An experiment that demonstrates the features of spin (intrinsicangular momentum) as a distinct entity apart from orbital angularmomentum.
The phenomena by which, at sufficiently low temperatures, aconductor can conduct charge with zero resistance.
The phenomena by which, at sufficiently low temperatures, a fluidcan flow with zero viscosity.
Superposition principle of forces
The net force on a body is equal to the sum of the forcesimpressed upon it.
Superposition principle of states
The resultant quantum mechnical wavefunction due to two or moreindividual wavefunctions is the sum of the individualwavefunctions.
Superposition principle of waves
The resultant wave function due to two or more individual wavefunctions is the sum of the individual wave functions.
Thomson experiment; Kelvin effect (Sir W. Thomson [later Lord Kelvin])
When an electric current flows through a conductor whose ends aremaintained at different temperatures, heat is released at a rateapproximately proportional to the product of the current and thetemperature gradient.
One of the most famous "paradoxes" in history, predicted by A.Einstein's special theory of relativity. Take two twins, born onthe same date on Earth. One, Albert, leaves home for a triparound the Universe at very high speeds (very close to that oflight), while the other, Henrik, stays at home at rests. Specialrelativity predicts that when Albert returns, he will find himselfmuch younger than Henrik. That is actually not the paradox. The paradox stems fromattempting to naively analyze the situation to figure out why.From Henrik's point of view (and from everyone else on Earth),Albert seems to speed off for a long time, linger around, and thenreturn. Thus he should be the younger one, which is what we see.But from Albert's point of view, it's Henrik (and the whole of the
Earth) that are travelling, not he. According to specialrelativity, if Henrik is moving relative to Albert, then Albertshould measure his clock as ticking slower -- and thus Henrik isthe one who should be younger. But this is not what happens.
So what's wrong with our analysis? The key point here is thatthe symmetry was broken. Albert did something that Henrik didnot -- Albert accelerated in turning around. Henrik did noaccelerating, as he and all the other people on the Earth canattest to (neglecting gravity). So Albert broke the symmetry, andwhen he returns, he is the younger one.
A shortcoming of the Rayleigh-Jeans formula, which attempted todescribe the radiancy of a blackbody at various frequencies of theelectromagnetic spectrum. It was clearly wrong because as thefrequency increased, the radiancy increased without bound;something quite not observed; this was dubbed the "ultravioletcatastrophe." It was later reconciled and explained by theintroduction of Planck's radiation law.
Universal constant of gravitation; G
The constant of proportionality in Newton's law of universalgravitation and which plays an analogous role in A. Einstein'sgeneral relativity. It is equal to 6.664
Van der Waals force (J.D. van der Waals)
Forces responsible for the non-ideal behavior of gases, and forthe lattice energy of molecular crystals. There are three causes:dipole-dipole interaction; dipole-induced dipole moments; anddispersion forces arising because of small instantaneous dipolesin atoms.
The principle of quantum mechanics which implies that light (and,indeed, all other subatomic particles) sometimes act like a wave,and sometime act like a particle, depending on the experiment youare performing. For instance, low frequency electromagneticradiation tends to act more like a wave than a particle; highfrequency electromagnetic radiation tends to act more like aparticle than a wave.
The ratio of the thermal conductivity of any pure metal to itselectrical conductivity is approximately constant for any giventemperature. This law holds fairly well except at lowtemperatures.
Wien's displacement law
For a blackbody, the product of the wavelength corresponding tothe maximum radiancy and the thermodynamic temperature is aconstant. As a result, as the temperature rises, the maximum ofthe radiant energy shifts toward the shorter wavelength (higherfrequency and energy) end of the spectrum.
Rules governing the formation of products during certain types oforganic reactions.
Young's experiment; double-slit experiment (T. Young; 1801)
A famous experiment which shows the wave nature of light (andindeed of other particles). Light is passed from a small sourceonto an opaque screen with two thin slits. The light is refractedthrough these slits and develops an interference pattern on theother side of the screen.
Zeeman effect; Zeeman line splitting (P. Zeeman; 1896)
The splitting of the lines in a spectrum when the source is
exposed to a magnetic field.
«Basic Postulats» by Gabrele O’Hara
«Elementary Physic For Students» by Bill Strong
«Atomic Physic» by Steve Grevesone
«Optica» by Steve Grevesone
«Thermodynamic’s Laws» by Kay Fedos