Back emf – an induced electromotive force which is directed in the opposite sense to the applied voltage

Balmer series – the wavelengths of the spectrum of hydrogen gas, where the electrons final state is the n=2 state (see equation 27.1)

Barrier penetration – the existence of a non-zero probability that a particle will appear outside a barrier that it cannot classically penetrate

Baryon – a subgroup of hadrons ( particles that interact through the strong nuclear force ) whose decay always conserves barion number

Base units – units of measurement that establish a standard (sometimes called fundamental units)

Battery – a source of electrical energy consisting of one or more cells

Beats – variations in amplitude due to the addition of two waves of different frequencies

Becquerel – the SI unit of activity, one decay per second

Bernoulli’s equation– describes the relationship of a fluid's pressure, velocity, and height for steady, non viscous flow of an incompressible fluid (see equation 10.8)

Beta minus – another name for the electron

Beta plus – a particle with the same mass as an electron but one unit (e) of positive electric charge; also called a positron

beta rays – historically, another name for electrons

Big bang – term used to describe the origin event of the universe

Binding energy – the energy necessary to separate the nucleus into protons and neutrons

Blackbody – a surface or object that is a perfect emitter of radiation and a perfect absorber of all radiation falling on it

Bohr magneton - the magnetic moment of the electron in its lowest angular momentum state

Boltzmann constant– a constant of proportionality between the average kinetic energy per molecule and the absolute temperature (see equation 12.7a)

Boyle’s law – states that at a constant temperature the product of the pressure and volume of an enclosed gas is a constant (see equation 12.1)

Bragg's law – defines the relationship between the observed angles for maximum intensity of x-ray diffraction and the spacing between the planes of atoms in the crystal (see equation 26.4)

Brewster's law – gives the condition for maximum polarization for reflected light, that occurs when the reflected ray and the refracted ray are at right angles to each other (see equation 24.11)

Brownian motion – the continual irregular movement of minute particles suspended in a fluid

Buoyant force – upward force on an object due to the displacement of a fluid by that object