exchange interaction (redirected from Pauli force)

exchange interaction [iks′chānj ‚int·ə′rak·shən]

(quantum mechanics)

An interaction represented by a potential involving exchange of space or spin coordinates, or both, of the particles involved; can be visualized physically in terms of exchange of particles.

Any interaction which can be looked upon as due to exchange of particles.

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Exchange interaction

A quantum-mechanical phenomenon that gives the energy of two elementary particles. Exchange effects arise for all kinds of elementary particles, but these effects were first introduced into physics in consideration of atomic structure and the energy of the electrons in an atom. In this context, they arise as a consequence of two facts: electrons are indistinguishable, and they obey the Pauli exclusion principle. See Elementary particle

Indistinguishability demands that the wave function describing two electrons either is unchanged or changes sign when the labels of the two electrons are exchanged (that is, it is either symmetric or antisymmetric to this exchange). In the case of electrons, the wave function is antisymmetric; this is a more precise statement of the Pauli exclusion principle. See Electron, Exclusion principle

When the spins of the two electrons are parallel the spatial part of the wave function is antisymmetric under exchange, and when they are opposed it is symmetric. The distribution in space of the two electrons is different in these two states, and so their mutual electrostatic energy is different. This difference in the electrostatic energy, called the exchange energy, appears as an interaction between the two electrons which depends on their relative orientation (although this dependence is incidental).

Exchange is the mechanism by which the electron spins in many magnetic materials are lined up parallel or opposed (for example, in α-iron and the ferrites). It is also the mechanism whereby the electron spins are parallel in the first excited state of the helium atom, and the spins of the two electrons are opposed in the carbon-carbon covalent bond. See Atomic structure and spectra, Electron configuration, Ferromagnetism, Quantum mechanics, Spin (quantum mechanics)