## B- E Gas Laws & Magnetics

See ideal gas laws.

Brackett series (Brackett)

The series which describes the emission spectrum of hydrogen when the electron is jumping to the fourth orbital. All of the lines are in the infrared portion of the spectrum.

See tardon.

Bragg’s law (Sir W.L. Bragg; 1912)

When a beam of x-rays strikes a crystal surface in which the layers of atoms or ions are regularly separated, the maximum intensity of the reflected ray occurs when the complement of the angle of incidence, theta, the wavelength of the x-rays, lambda, and the distance betwen layers of atoms or ions, d, are related by the equation

2 d sin theta = n lambda,

where n is an integer.

Brewster’s law (D. Brewster)

The extent of the polarization of light reflected from a transparent surface is a maximum when the reflected ray is at right angles to the refracted ray.
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Brownian motion (R. Brown; 1827)

Dalton’s law of partial pressures (J. Dalton)

The total pressure of a mixture of ideal gases is equal to the sum of the partial pressures of its components; that is, the sum of the pressures that each component would exert if it were present alone 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 of electrons; diffraction patterns were observed by an electron beam penetrating 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 inversely proportional to its momentum, where the constant of proportionality is the Planck constant.

determinism principle

The principle that if one knows the state to an infinite accuracy of a system at one point in time, one would be able to predict the state of that system with infinite accuracy at any other time, past or future. For example, if one were to know all of the positions and velocities of all the particles in a closed system, then determinism would imply that one could then predict the positions and velocities of those particles at any other time. This principle has been disfavored due to the advent of quantum mechanics, where probabilities take an important part in the actions of the subatomic world, and the uncertainty principle implies that one cannot know both the position and velocity of a particle to arbitrary precision.

Dirac constant; Planck constant, modified form; hbar

A sometimes more convenient form of the Planck constant, defined as

hbar = h/(2 pi).

Doppler effect (C.J. Doppler)

Waves emitted by a moving object as received by an 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.

Compare cosmological redshift.

Drake equation (F. Drake; 1961)

A method of estimating the number of intelligent, technological species (i.e., able to communicate with other species) in existence in our Galaxy.

N = R fp ne fl fi ft L.

N is the number of species described above at any given moment in our Galaxy. The parameters it is computed from are as follows:

R

the rate of star formation in our Galaxy (in stars per year);

fp

the fraction of stars which have planets;

ne

the number of habitable planets per system with planets;

fl

the fraction of habitable planets upon which life arises;

fi

the fraction of these planets upon which life develops intelligence;

ft

the fraction of these planets where the intelligence develops into a technological civilization capable of communication; and

L

the mean lifetime of such a technological civilization.

Of these quantities, only the first — R — is known with anything like any reliability; it is on the order of 10 stars per year. The others, most notably the fractions, are almost entirely pure speculation at this point. Calculations made by respectable astronomers differ by something like ten orders of magnitude in the final estimation of the number of species out there.

Dulong-Petit law (P. Dulong, A.T. Petit; 1819)

The molar heat capacity is approximately equal to the three times the ideal gas constant:

C = 3 R.

Eddington limit (Sir A. Eddington)

The theoretical limit at which the photon pressure would exceed the gravitational attraction of a light-emitting body. That is, a body emitting radiation at greater than the Eddington limit would break up from its own photon pressure.

Edwards-Casimir quantum vacuum drive

A hypothetical drive exploiting the peculiarities of quantum mechanics by restricting allowed wavelengths of virtual photons on one side of the drive (the bow of the ship); the pressure generated from the unrestricted virtual photons toward the aft generates a net force and propels the drive.

See Casimir effect.

The special relativistic “paradox” involving a rapidly rotating disc. Since any radial segment of the disc is perpendicular to the direction of motion, there should be no length contraction of the radius; however, since the circumference of the disc is parallel to the direction of motion, it should contract.

Einstein field equation

The cornerstone of Einstein’s general theory of relativity, relating the gravitational tensor G to the stress-energy tensor T by the simple equation

G = 8 pi T.

Einstein-Podolsky-Rosen effect; EPR effect

Consider the following quantum mechanical thought-experiment: Take a particle which is at rest and has spin zero. It spontaneously decays into two fermions (spin 1/2 particles), which stream away in opposite directions at high speed. Due to the law of conservation of spin, we know that one is a spin +1/2 and the other is spin -1/2. Which one is which? According to quantum mechanics, neither takes on a definite state until it is observed (the wavefunction is collapsed).

The EPR effect demonstrates that if one of the particles is detected, and its spin is then measured, then the other particle — no matter where it is in the Universe — instantaneously is forced to choose as well and take on the role of the other particle. This illustrates that certain kinds of quantum information travel instantaneously; not everything is limited by the speed of light.

However, it can be easily demonstrated that this effect does not make faster-than-light communication or travel possible.

electric constant

See permeability of free space.

Eotvos law of capillarity (Baron L. von Eotvos; c. 1870)

The surface tension gamma of a liquid is related to its temperature T, the liquid’s critical temperature, T*, and its density rho by

gamma ~= 2.12 (T* – T)/rho3/2.

EPR effect

See Einstein-Podolsky-Rosen effect.

epsilon_0

See permittivity of free space.

equivalence principle

The basic postulate of A. Einstein’s general theory of relativity, which posits that an acceleration is fundamentally indistinguishable from a gravitational field. In other words, if you are in an elevator which is utterly sealed and protected from the outside, so that you cannot “peek outside,” then if you feel a force (weight), it is fundamentally impossible for you to say whether the elevator is present in a gravitational field, or whether the elevator has rockets attached to it and is accelerating “upward.”

Although that in practical situations — say, sitting in a closed room — it would be possible to determine whether the acceleration felt was due to uniform thrust or due to gravitation (say, by measuring the gradient of the field; if nonzero, it would indicate a gravitational field rather than thrust); however, such differences could be made arbitrarily small. The idea behind the equivalence principle is that it acts around the vicinity of a point, rather than over macroscopic distances. It would be impossible to say whether or not a given (arbitrary) acceleration field was caused by thrust or gravitation by the use of physics alone.

The equivalence principle predicts intere