Particle

For other uses, see Particle (disambiguation).

Particles are very often represented as dots. This figure could represent the movement of atoms in a gas, people in crowds or stars in the night sky.

In the physical sciences, a particle is a small localized object to which can be ascribed several physical properties such as volume or mass. The word is rather general in meaning, and is refined as needed by various scientific fields. Whether objects can be considered particles depends on the scale of the context; if geometrical properties such as size, shape, or structure are irrelevant, it can be considered a particle. For example, grains of sand on a beach can be considered particles because the size of one grain of sand (~1 mm) is negligible compared to the beach, and the features of individual grains of sand are usually irrelevant to the problem at hand. However, grains of sand would not be considered particles if compared to buckyballs (~1 nm).

Size

Galaxies are so large that stars can be considered particles next to them

The term "particle" is usually applied differently to three class of sizes. The term macroscopic particle, usually refers to particles much larger than atoms and molecules. These are usually abstracted as point-like particles, even though they have volumes, shapes, structures, etc. Examples of macroscopic particles would include dust, sand, pieces of debris during a car accident, or even objects as big as the stars of a galaxy. Another type, microscopic particles usually refers to particles of sizes ranging from atoms to molecules, such as carbon dioxide, nanoparticles, and colloidal particles. The smallest of particles are the subatomic particles, which refer to particles smaller than atoms. These would include particles such as the constituents of atoms – protons, neutrons, and electron – as well as other types of particles which can only be produced in particle accelerators or cosmic rays.

Composition

Proton are composed of three quarks, but quarks are not composed of other particles

It is also useful to classify particles according to composition. Composite particles refer to particles that have composition – that is particles which are made of other particles. For example, a carbon-14 atom is made of six protons, eight neutrons, and eight electrons. By contrast, elementary particles (also called fundamental particles) refer to particles that are not made of other particles. According to our current understanding of the world, only a very small number of these exist, such as the leptons, quarks or 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. While composite particles can very often be considered point-like, elementary particles are truly punctual.

See also

3

Further reading

• "What is a particle?". University of Florida, Particle Engineering Research Center. 23 July 2010.
• D.J. Griffiths (2008). Introduction to Particle Physics (2nd ed.). Wiley-VCH. ISBN 3527406018.

External links

• An Introduction to Particle Physics from York University