General Science: IES

Saturday, December 12, 2009

dOSE -- pHYSICS

Piezomagnetism is a phenomenon observed in some antiferromagnetic crystals. It is characterised by a linear coupling between the system's magnetic polarisation and mechanical strain. In a piezomagnetic, one may induce a spontaneous magnetic moment by applying physical stress, or a physical deformation by applying a magnetic field.
In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboringspins (on different sublattices) pointing in opposite directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism.
An electron is a subatomic particle that carries a negative electric charge. It has no known components or substructure, and therefore is believed to be anelementary particle. An electron has a mass that is approximately 1/1836 that of the proton.
In physics, subatomic particles are the particles composing nucleons and atoms. There are two types of subatomic particles: elementary particles, which are not made of other particles, and composite particles.Particle physics and nuclear physics study these particles and how they interact.
In physics, a nucleon is a collective name for two baryons: the neutron and the proton.
Baryons are the family of composite particles made of three quarks, as opposed to the mesons which are the family of composite particles made of one quark and one antiquark. Both baryons and mesons are part of the larger particle family comprising all particles made of quarks—the hadrons. The term baryon is derived from the Greek βαρύς (barys), meaning "heavy", because at the time of their naming it was believed that baryons were characterized by having greater masses than other particles.
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particlescalled hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei.
In particle physics, a hadron is a particle made of quarks held together by the strong force (similar to how molecules are held together by theelectromagnetic force). Hadrons are either mesons (made of one quark and one antiquark) or baryons (made of three quarks). Other combinations, such as tetraquarks (an "exotic" meson) and pentaquarks (an"exotic" baryon), may be possible but no evidence conclusively suggests their existence as of 2009. The best known mesons are pions and kaons, while the best known baryons are protons and neutrons.
In particle physics, a pion (short for pi meson; denoted π) is any of three subatomic particles: π⁰, π⁺ and π⁻. Pions are the lightest mesons and play an important role in explaining low-energy properties of the strong nuclear force.
In particle physics, a kaon is any one of a group of four mesons distinguished by the fact that they carry a quantum number called strangeness. In the quark model they are understood to contain a single strange quark (or antiquark).
In particle physics, strangeness S is a property of particles, expressed as a quantum number, for describing decay of particles in strong and electromagnetic reactions, which occur in a short period of time. The strangeness of a particle is defined as:
$S = -(n_s - n_{\overline{s}})$
where n_s represents the number of strange quarks (s) and n_s represents the number of strange antiquarks (s).

The linear, surface, or volume charge density is the amount of electric charge in a line, surface, or volume. It is measured in coulombs per metre (C/m), square metre (C/m²), or cubic metre (C/m³), respectively. Since there are positive as well as negative charges, the charge density can take on negative values. Like any density it can depend on position. It should not be confused with the charge carrier density. As related to chemistry, it can refer to the charge distribution over the volume of a particle, molecule, or atom. Therefore, a lithium cation will carry a higher charge density than a sodium cation due to its smaller ionic radius.

Homogeneous charge density

For the special case of a homogeneous charge density, that is one that is independent of position, equal to $ρ q,0$ the equation simplifies to:

$Q=V\cdot \rho_{q,0}$

The proof of this is simple. Start with the definition of the charge of any volume:

$Q=\int\limits_V \rho_q(\mathbf r) \,\mathrm{d}V$

Then, by definition of homogeneity, $\rho_q(\mathbf r)$ is a constant that we will denote $ρ q,0$ to differentiate between the constant and non-constant forms, and thus by the properties of an integral can be pulled outside of the integral resulting in:

$Q=\rho_{q,0} \int\limits_V \,\mathrm{d}V = \rho_{q,0} \cdot V$

so,

$Q=V \cdot \rho_{q,0}$

The equivalent proofs for linear charge density and surface charge density follow the same arguments as above.

Discrete charges

If the charge in a region consists of $N$ discrete point-like charge carriers like electrons the charge density can be expressed via the Dirac delta function, for example, the volume charge density is:

$\rho(\mathbf{r})=\sum_{i=1}^N\ q_i\delta(\mathbf{r} - \mathbf{r}_i)\,\!$ ;

where $\mathbf{r}\,\!$ is the test position， $q_i\,\!$ is the charge of the ith charge carrier, whose position is $\mathbf{r}_i\,\!$ .

If all charge carriers have the same charge $q$ (for electrons $q = - e$ ) the charge density can be expressed through the charge carrier density $n(\mathbf r)$ : Again, the equivalent equations for the linear and surface charge densities follow directly from the above relations.