Boltzmann constant; k (L. Boltzmann)

Togliatti

1998

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.

Anthropic principle

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.

Archimedes' principle

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.

Atwood's machine

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; N_A (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 6.022 52 ^.10²³ mol^-1.

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.

Baryon decay

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 ³⁴ y.

Bernoulli's equation

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.

Blackbody radiation

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.

Bode's law

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)

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.