~ 1.0 10-34 Js, below which there is no
information held in the system. From our macroscopic point
of view, this appears as uncertainty.adiabatic - this is a way to conduct a process. Other ways are isothermal or isobaric and so on. A process is said to be adiabatic, if the system does not exchange heat with the surroundings during the process
bit - Binary digit, amount of information associated with a choice between two equally likely outcomes. In physical units, log 2 = 0.693... units of information.
Bose-Einstein statistics - The behavior of any number of identical bosons (entities with whole number spin). The only possible states are those invariant under permutation of the identity of the particles. Probability for particles to end up in the same state is increased compared to Maxwell-Boltzmann statistics, cf. also Fermi-Dirac statistics.
byte - Eight bits.
closed system - a system that does not exchange matter with the surroundings
energy - the quantity that has to be minimized for a mechanical system to be in equilibrium; alternatively: a property of the dynamics of a mechanical system
enthalpy or Helmholtz free energy (identifier H) - for thermodynamic systems, it makes a difference if a process that changes U is carried out at constant volume or at constant pressure. For a process at constant volume and entropy, use internal energy U. For a process at constant pressure and entropy, enthalpy is a better choice
entropy - A quantity that determines the direction of processes in thermodynamical systems. See Gian's introductory article.
ergodic - A more precise formulation of the concept of randomness. With an ergodic series of data there is no memory, no correlation with past data, so each new data point adds the same amount of new information.
Fermi-Dirac statistics - The behavior of any number of identical fermions (entities with half-odd spin). The only possible states are those invariant under permutation of the identity of the particles, combined with a sign change for odd permutations. Probability for particles to end up in the same state is zero (Pauli exclusion principle), cf. Maxwell-Boltzmann and Bose-Einstein statistics.
Gibbs free energy (identifier G) - chemical processes usually take place at constant pressure and temperature. Under these circumstances, G is the best choice to calculate energy differences of a system. Also, at equilibrium in thermodynamical systems, G is a minimum
Gibbs' paradox - See James's article on it.
heat - A process, by which the internal energy of a system can be changed. At the same time, may, but must not, be changed, too
Hilbert space - Vector space with infinitely many dimensions. The dot product u · v = Sum ui vi (or Sum ui vi* for spaces on the complex numbers) is replaced by the corresponding infinite sum or integral (Hilbert spaces defined either way are identical). The (length of u)2 is defined as u · u as usual, and the space only contains those vectors whose length is finite.
information - A logarithmic measure of improbability. See Marijke's introductory article. The amount of information there is to be known about a system is what constitutes its entropy.
insulated system - a system that does not exchange matter nor energy with the surroundings
internal energy (identifier U) - when heat is added to a thermodynamical system, it is stored as internal energy. A system cannot contain heat, since heat is a process. See also heat. At equilibrium, U is not necessarily a minimum for thermodynamical systems. See also enthalpy
isobaric - this is a way to conduct a process. Other ways are isothermal or adiabatic and so on. A process is said to be isobaric, if the pressure is held constant during the process
isothermal - this is a way to conduct a process. Other ways are adiabatic or isobaric and so on. A process is said to be isothermal, if temperature is held constant during the process
Maxwell-Boltzmann statistics - behavior of many-particle systems without taking quantum mechanics into account, cf. Bose-Einstein and Fermi-Dirac statistics.
Maxwell's demon - A certain thought experiment (see article on this site's name).
Maxwellian distribution - Distribution of velocities in a gas at equilibrium at a certain temperature.
open system - a system that does exchange matter as well as energy with the surroundings
phase space - In non-quantum physics, a mathematical space with one point for every possible state of the system, having as many dimensions as there are degrees of freedom in the system. In QM, phase space is replaced by the state space formalism, usually a Hilbert space.
temperature - temperature is what you measure with a thermometer (this is kind of an operational definition). More precisely, the temperature of a system tells how much the internal energy of the system grows upon a given increase of entropy
uncertainty principle - One essential difference between quantum mechanics
(QM) and classical physics is that systems simply do not have values for all
observable quantities simultaneously. Observation can affect the system, and a
system prepared or observed to be in a certain state w.r.t. one quantity may
not be the same system as one prepared or observed to be in a certain state
w.r.t. another quantity. Certain complementary pairs of quantities
exist: time & energy, position & momentum, angle & angular
momentum, etc. (the product of whose dimensionality is always kg m²/s)
that have a lower bound to the product of the standard deviations of
measurements on such a pair, of the order of ~ 1.0 10-34 Js, below which there is no
information held in the system. From our macroscopic point
of view, this appears as uncertainty.
zero-point energy - The energy of a system in its ground (lowest energy) state. By the uncertainty principle if a system is localised in space its momentum will be spread out so its energy is nonzero.
Under construction, GVa + MvG
Last update Apr. 22, 1998
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