Point particle
A point particle, ideal particle
In
Point mass
Point mass (pointlike mass) is the concept, for example in classical physics, of a physical object (typically matter) that has nonzero mass, and yet explicitly and specifically is (or is being thought of or modeled as) infinitesimal (infinitely small) in its volume or linear dimensions. In the theory of
Point charge
Similar to point masses, in electromagnetism physicists discuss a point charge, a point particle with a nonzero electric charge.[6] The fundamental equation of electrostatics is Coulomb's law, which describes the electric force between two point charges. Another result, Earnshaw's theorem, states that a collection of point charges cannot be maintained in a static equilibrium configuration solely by the electrostatic interaction of the charges. The electric field associated with a classical point charge increases to infinity as the distance from the point charge decreases towards zero, which suggests that the model is no longer accurate in this limit.
In quantum mechanics
In
Nevertheless, there is good reason that an elementary particle is often called a point particle. Even if an elementary particle has a delocalized wavepacket, the wavepacket can be represented as a quantum superposition of quantum states wherein the particle is exactly localized. Moreover, the interactions of the particle can be represented as a superposition of interactions of individual states which are localized. This is not true for a composite particle, which can never be represented as a superposition of exactly-localized quantum states. It is in this sense that physicists can discuss the intrinsic "size" of a particle: The size of its internal structure, not the size of its wavepacket. The "size" of an elementary particle, in this sense, is exactly zero.
For example, for the electron, experimental evidence shows that the size of an electron is less than 10−18 m.[7] This is consistent with the expected value of exactly zero. (This should not be confused with the classical electron radius, which, despite the name, is unrelated to the actual size of an electron.)
See also
- Test particle
- Brane
- Charge (physics) (general concept, not limited to electric charge)
- Standard Model of particle physics
- Wave–particle duality
Notes and references
Notes
- ISBN 978-0-393-93003-0.
- ISBN 978-0-521-04833-0.
- ^ Fowles, Grant R; Cassiday, George L. Analytical Mechanics. §6.2 Gravitational Force between a Uniform Sphere and a Particle.
- ISBN 0-520-08817-4.
- ^ I. Newton, A. Motte, J. Machin (1729), p. 270–271.Newton, I. (1729). The Mathematical Principles of Natural Philosophy. Translated by Motte, A.; Machin, J. Benjamin Motte. pp. 270–271.
- ISBN 0-521-78751-3.
- ^ "Precision pins down the electron's magnetism". 4 October 2006.
Bibliography
- C. Quigg (2009). "Particle, Elementary". Encyclopedia Americana. Grolier Online. Archived from the original on 2013-04-01. Retrieved 2009-07-04.
- S. L. Glashow (2009). "Quark". Encyclopedia Americana. Grolier Online. Archived from the original on 2013-04-01. Retrieved 2009-07-04.
- M. Alonso; E. J. Finn (1968). Fundamental University Physics Volume III: Quantum and Statistical Physics. ISBN 0-201-00262-0.
Further reading
- Cornish, F. H. J. (1965). "Classical radiation theory and point charges". .
- Jefimenko, Oleg D. (1994). "Direct calculation of the electric and magnetic fields of an electric point charge moving with constant velocity". doi:10.1119/1.17716.
External links
- Media related to Point particle at Wikimedia Commons