s in a
gas,
people in crowds, or
stars in the
night sky.
In models In physics, the concept of particles can be used to make simplifying assumptions when
modelling nature. For example, to calculate the landing location and speed of a
baseball thrown in the air, the baseball can be
idealized as a rigid smooth
sphere, ignoring its
rotation,
buoyancy, and
friction, reducing the problem to the
ballistics in
classical mechanics.
Size are so large that
stars can be considered particles relative to them. The term "particle" is usually applied differently to three classes 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
powder,
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. These particles are studied in
chemistry, as well as
atomic and
molecular physics. The smallest 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
electrons – as well as other types of particles which can only be produced in
particle accelerators or
cosmic rays. These particles are studied in
particle physics. Because of their extremely small size, the study of microscopic and subatomic particles falls in the realm of
quantum mechanics. They will exhibit phenomena demonstrated in the
particle in a box model, including
wave–particle duality, and whether particles can be considered
distinct or identical is an important question in many situations.
Particle physics Mass In the context of particle physics, a massive particle is a particle with
rest mass greater than zero while a
massless particle has zero rest mass. Only a handful of elementary particles are massless: the
photon, the
gluon, and
graviton. Every other known particle is massive as a result of the
Higgs mechanism. This also explains why ordinary matter, which is made from atoms, has mass. The mass of atoms is overwhelmingly concentrated in their nuclei, made of protons and neutrons. Electrons contribute a bit to the mass of atoms, but only a very small amount. Both the proton and neutron have over 1800 times more mass than electrons. Since protons and neutrons are made of quarks and gluons, the mass of matter can be ultimately traced to quarks and gluons and their interactions.
Composition is composed of three
quarks and held together with the
strong interaction. 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. 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. 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 be composite particles after all, and merely appear to be elementary to scientists now. While composite particles can very often be considered
point-like, although having internal structure, elementary particles have so far been found to have no structure.
Stability Both elementary (such as
muons) and composite particles (such as
uranium nuclei), are known to undergo
particle decay. Those that do not are called stable particles, such as
electrons and
helium-4 nuclei. 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.
Statistical mechanics In physical systems with vast numbers of interacting particles analysis of individual motions is impractical. For example, a single gram of O2 contains 2\times 10^{22} molecules. For these cases, statistical approaches have been developed which allow prediction of average quantities like the energy from the properties of the particles. The approach provides a theoretical basis for
thermodynamics, the
ideal gas laws, and understanding
brownian motion.
Astrophysics Astrophysics uses the idea of "particles" on many levels.
Particle radiation is emitted by stars,
cosmic rays are high energy particles (primarily bare atomic nuclei), components in
planetary rings, components of
solar wind,
meteors,
hydrogen nuclei inside the
Sun, dust particles aggregating into
planets and
interstellar gas collapsing to form a
star. Self-gravitating systems of point particles representing stars are used to model
galaxy formation.
N-body simulation In
computational physics,
N-body simulations (also called
N-particle simulations) are simulations of
dynamical systems of particles under the influence of certain conditions, such as being subject to
gravity. These simulations are common in
cosmology and
computational fluid dynamics.
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. == Atmospheric science ==