Editing Point particle (section)

{{Short description|Idealised model of a particle in physics}}
[[File:Collage of point particles.png|thumb|300x300px|Examples of point particles: {{em|(counterclockwise from top left)}} point mass for [[Newton's law of universal gravitation]], point particles to measure distance between two charged particles, [[Pendulum|simple pendulum]] (point mass attached to the end of a massless string), [[ideal gas]] particles devoid of interactions (no collisions, gravitational force, or [[Coulomb's law|Coulomb's force]] between particles)]]
{{Use American English|date = February 2019}}

A '''point particle''', '''ideal particle'''<ref>{{cite book | first1 = H. C. | last1 = Ohanian | first2 = J. T. | last2 = Markert | year = 2007 | title = Physics for Engineers and Scientists | volume = 1 | edition = 3rd | publisher = [[W. W. Norton & Company|Norton]] | isbn = 978-0-393-93003-0 | page = 3 }}</ref> or '''point-like particle''' (often spelled '''pointlike particle''') is an [[idealization (science philosophy)|idealization]] of [[particle]]s heavily used in [[physics]]. Its defining feature is that it lacks spatial [[extension (metaphysics)|extension]]; being dimensionless, it does not take up [[space]].<ref>{{cite book | first1 = F. E. | last1 = Udwadia | first2 = R. E. | last2 = Kalaba | year = 2007 | title = Analytical Dynamics: A New Approach | publisher = [[Cambridge University Press]] | isbn = 978-0-521-04833-0 | page = 1 }}</ref> A point particle is an appropriate representation of any object whenever its size, shape, and structure are irrelevant in a given context. For example, from far enough away, any finite-size object will look and behave as a point-like object. Point masses and point charges, discussed below, are two common cases.  When a point particle has an additive property, such as mass or charge, it is often represented mathematically by a [[Dirac delta function]]. In classical mechanics there is usually no concept of rotation of point particles about their "center".

In [[quantum mechanics]], the concept of a point particle is complicated by the [[Uncertainty principle|Heisenberg uncertainty principle]], because even an [[elementary particle]], with no internal structure, occupies a nonzero volume. For example, the [[atomic orbital|atomic orbit]] of an [[electron]] in the [[hydrogen atom]] occupies a volume of ~{{val|e=-30|u=m3}}. There is nevertheless a distinction between elementary particles such as [[electron]]s or [[quark]]s, which have no known internal structure, and [[composite particle]]s such as [[proton]]s and neutrons, whose internal structures are made up of quarks.
Elementary particles are sometimes called "point particles" in reference to their lack of internal structure, but this is in a different sense than that discussed herein.