The term particle is rather general in meaning, and is refined as needed by various scientific fields. Anything that is composed of particles may be referred to as being particulate.[3] However, the noun particulate is most frequently used to refer to pollutants in the Earth's atmosphere, which are a suspension of unconnected particles, rather than a connected particle aggregation.
The concept of particles is particularly useful when modellingnature, as the full treatment of many phenomena can be complex and also involve difficult computation.[4] It can be used to make simplifying assumptions concerning the processes involved. Francis Sears and Mark Zemansky, in University Physics, give the example of calculating the landing location and speed of a baseball thrown in the air. They gradually strip the baseball of most of its properties, by first idealizing it as a rigid smooth sphere, then by neglecting rotation, buoyancy and friction, ultimately reducing the problem to the ballistics of a classicalpoint particle.[5] The treatment of large numbers of particles is the realm of statistical physics.[6]
Particles can also be classified according to composition. Composite particles refer to particles that have composition – that is particles which are made of other particles.[16] For example, a carbon-14 atom is made of six protons, eight neutrons, and six electrons. By contrast, elementary particles (also called fundamental particles) refer to particles that are not made of other particles.[17] According to our current understanding of the world, only a very small number of these exist, such as leptons, quarks, and gluons. However it is possible that some of these might turn up to be composite particles after all, and merely appear to be elementary for the moment.[18] While composite particles can very often be considered point-like, elementary particles are truly punctual.[19]
Both elementary (such as muons) and composite particles (such as uraniumnuclei), are known to undergo particle decay. Those that do not are called stable particles, such as the electron or a helium-4nucleus. The lifetime of stable particles can be either infinite or large enough to hinder attempts to observe such decays. In the latter case, those particles are called "observationally stable". In general, a particle decays from a high-energy state to a lower-energy state by emitting some form of radiation, such as the emission of photons.
N refers to the number of particles considered. As simulations with higher N are more computationally intensive, systems with large numbers of actual particles will often be approximated to a smaller number of particles, and simulation algorithms need to be optimized through various methods.[20]
Examples of a stable and of an unstable colloidal dispersion.
Colloidal particles are the components of a colloid. A colloid is a substance microscopically dispersed evenly throughout another substance.[21] Such colloidal system can be solid, liquid, or gaseous; as well as continuous or dispersed. The dispersed-phase particles have a diameter of between approximately 5 and 200 nanometers.[22] Soluble particles smaller than this will form a solution as opposed to a colloid. Colloidal systems (also called colloidal solutions or colloidal suspensions) are the subject of interface and colloid science. Suspended solids may be held in a liquid, while solid or liquid particles suspended in a gas together form an aerosol. Particles may also be suspended in the form of atmospheric particulate matter, which may constitute air pollution. Larger particles can similarly form marine debrisorspace debris. A conglomeration of discrete solid, macroscopic particles may be described as a granular material.
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F. W. Sears; M. W. Zemansky (1964). "Equilibrium of a Particle". University Physics (3rd ed.). Addison-Wesley. pp. 26–27. LCCN63015265.
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F. W. Sears; M. W. Zemansky (1964). "Equilibrium of a Particle". University Physics (3rd ed.). Addison-Wesley. p. 27. LCCN63015265. A body whose rotation is ignored as irrelevant is called a particle. A particle may be so small that it is an approximation to a point, or it may be of any size, provided that the action lines of all the forces acting on it intersect in one point.
