Exchange force - Wikipedia, the free encyclopedia 5/24/10 11:52 AM

Exchange force

From Wikipedia, the free encyclopedia

In physics the term exchange force has been used to describe two distinct concepts which should not be confused.

Contents

1 Exchange of force carriers in particle physics 1.1 History 2 Exchange interaction and quantum state symmetry 3 See also 4 References 5 External links

Exchange of force carriers in particle physics

Main article: Force carrier

The preferred meaning of exchange force is in particle physics, where it denotes a force produced by the exchange of force carrier particles, such as the electromagnetic force produced by the exchange of photons between electrons and the strong force produced by the exchange of gluons between quarks.[1][2] The idea of an exchange force implies a continuous exchange of particles which accompany the interaction and transmit the force, a process that receives its operational justification through the Heisenberg uncertainty principle.[3]

History

One of the earliest uses of the term interaction was in a discussion by Niels Bohr in 1913 of the interaction between the negative electron and the positive nucleus.[4] Exchange forces were introduced by Werner Heisenberg (1932) and Ettore Majorana (1933) in order to account for the saturation of binding energy and of nuclear density.[5][6] This was done in analogy to the quantum mechanical theory of covalent bonds, such as exist between two hydrogen atoms in the hydrogen molecule wherein the chemical force is attractive if the wave function is symmetric under exchange of coordinates of the electrons and is repulsive if the wave function is anti-symmetric in this respect.[7]

Exchange interaction and quantum state symmetry

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Main article: Exchange interaction

As another, entirely distinct, meaning of exchange force, it is sometimes used [8] as a synonym for the exchange interaction, between electrons which arises from a combination of the identity of particles, exchange symmetry, and the electrostatic force.

To illustrate the concept of exchange interaction, any two electrons, for example, in the universe are considered indistinguishable particles, and so according to quantum mechanics in 3 dimensions, every particle must behave as a boson or a fermion. In the former case, two (or more) particles can occupy the same quantum state and this results in a lack of exchange interaction between them; in the latter case, the particles can not occupy the same state according to the Pauli exclusion principle. From Quantum field theory, the spin- statistics theorem demands that all particles with half-integer spin behave as fermions and all particles with integer spin behave as bosons. Thus, it so happens that all electrons are fermions, since they have spin 1/2.

As a mathematical consequence, fermions exhibit strong repulsion when their wave functions overlap, but bosons do not. This repulsion is what the exchange interaction models. Fermi repulsion results in "stiffness" of fermions. That is why atomic matter, is "stiff" or "rigid" to touch. Where wave functions of electrons overlap, Pauli repulsion takes place. The same is true for protons and neutrons where due to their larger mass, the rigidity of baryons is much larger than that of electrons.

See also

Exchange symmetry Fundamental interaction References

1. ^ Gribbin, John (2000). Encyclopedia of Particle Physics. Simon & Schuster. ISBN 0684863154. 2. ^ Exchange Forces (http://hyperphysics.phy-astr.gsu.edu/hbase/forces/exchg.html) , HyperPhysics, Georgia State University, accessed June 2, 2007. 3. ^ Falkoff, David, L. (1950). “Exchange Forces”, American Journal of Physics 18, 30. 4. ^ Niels Bohr (1913). "On the Constitution of Atoms and Molecules (Part 1 of 3) (http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Bohr/Bohr-1913a.html) ". Philosophical Magazine 26: 1–25. 5. ^ Heisenberg, Werner. (1932). "Uber den Bau der Atomkerne," Zeitschrift fur Physik 77, I. 6. ^ Majoranan, Ettore. (1933). "Uber die Kerntheorie," Ibid 82, 137. 7. ^ Jammer, Max (1957). Concepts of Force. Dover Publications, Inc.. ISBN 048640689X. 8. ^ For example, pp. 87–88, Driving Force: the natural magic of magnets, James D. Livingston, Harvard University Press, 1996. ISBN 0674216458. External links

Exchange Interaction (http://www.fkf.mpg.de/keimer/Lecture/Magnetism/Magnetism_3.pdf) (PDF) Exchange Interaction and Energy (http://wpage.unina.it/mdaquino/PhD_thesis/main/node7.html)

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Exchange Interaction and Exchange Anisotropy (http://www.cmp.liv.ac.uk/frink/thesis/thesis/node68.html) Retrieved from "http://en.wikipedia.org/wiki/Exchange_force" Categories: Physics | Pauli exclusion principle

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