Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Magnetic monopole wikipedia , lookup
Aharonov–Bohm effect wikipedia , lookup
Maxwell's equations wikipedia , lookup
Time in physics wikipedia , lookup
List of unusual units of measurement wikipedia , lookup
Electric charge wikipedia , lookup
Lorentz force wikipedia , lookup
centimetre-gram-second system (CGS) is a system of physical units. It is always the same for mechanical units, but there are several variants of electric additions. It was replaced by the MKS, or metre-kilogram-second system, which in turn was replaced by the International System of Units (SI), which has the three base units of MKS plus the ampere, mole, candela and kelvin. The system goes back to a proposal made in 1832 by the German mathematician Carl Friedrich Gauss and was in 1874 extended by the British physicists James Clerk Maxwell and William Thomson with a set of electromagnetic units. The sizes (order of magnitude) of many CGS units turned out to be inconvenient for practical purposes, therefore the CGS system never gained wide general use outside the field of electrodynamics and was gradually superseded internationally starting in the 1880s but not to a significant extent until the mid-20th century by the more practical MKS (metre-kilogram-second) system, which led eventually to the modern SI standard units. CGS units in electromagnetism While for most units the difference between cgs and SI are just powers of 10, the differences in electromagnetic units are more involved; so much so that formulas for physical laws of electromagnetism are adjusted depending on what system of units one uses. In SI, electric current is defined via the magnetic force it exerts and charge is then defined as current multiplied with time. The e.m.u. system within which the abvolt, etc., were defined was itself based on the setting of the value of magnetic permeability to 1; in 1901 it was realized that, were permeability set at 10-7 rather than 1, the whole array of practical terms shown above fitted unchanged into the metric m.k.s. system without any multipliers. Thus, what began as part of a c.g.s. system became very readily part of the m.k.s. system, which was increasingly favoured, and ultimately the like-structured SI system. In one variant of the cgs system, Electrostatic units (ESU), charge is defined via the force it exerts on other charges, and current is then defined as charge per time. One consequence of this approach is that Coulomb’s law does not contain a constant of proportionality. What this means specifically is that in cgs electrostatic units, the unit of charge or statcoulomb, is defined as such a quantity of charge that the Coulomb force constant is set to 1. That is, for two point charges, each with 1 statcoulomb spaced apart by 1 centimetre, the electrostatic force between them will be, by definition, precisely one dyne. This also has the effect of eliminating a separate dimension or fundamental unit for electric charge. In cgs electrostatic units, a statcoulomb is the same as a centimetre times square root of dyne. Dimensionally in the cgs esu system, charge Q is equivalent to M1/2L3/2T-1 and not an independent dimension of physical quantity. This reduction of units is an application of the Buckingham π theorem. While the proportional constants in cgs simplify theoretical calculations, they have the disadvantage that the units in cgs are hard to define through experiment. SI on the other hand starts with a unit of current, the ampere which is easy to determine through experiment, but which requires that the constants in the electromagnetic equations take on odd forms. Ultimately, relating electromagnetic phenomena to time, length and mass relies on the forces observed on charges. There are two fundamental laws in action. The first is Coulomb's law, which describes the electrostatic force between charges . The second is Ampère's law, which describes the electrodynamic (or electromagnetic) force between currents for two long parallel wires). The proportionality constants in these two equations are related by kC / kA = c2, where c is the speed of light. The static definition of magnetic fields (Biot-Savart law) yields a third proportionality constant, α, which establishes convenient dimensions. If we wish to describe the electric displacement field and the magnetic field in a medium other than a vacuum, we need to also define the constants ε0 and μ0, which are the vacuum permittivity and permeability, respectively. These two values are related by . Then we have (generally) and . The factors λ and λ′ are rationalization constants, which are usually chosen to both be equal to 4πkCε0, which is dimensionless. If this quantity equals 1, the system is said to be rationalized. The table below shows the constant values used in some common systems: system electrostatic (esu) electromagnetic (emu) Gaussian Heaviside-Lorentz SI kC 1 c2 1 1/4π c2/b α 1 1 c c 1 ε0 1 c-2 1 1 b/(4πc2) Electrostatic units The electrostatic system of units is a system of units used to measure electrical quantities of electric charge, current, and voltage, within the centimeter gram second (or "CGS") metric system of units. In electrostatic units, electrical charge is defined via the force it exerts on other charges. Although CGS units have mostly been supplanted by the MKS or "International System of Units" (SI) units, electrostatic units are still in use in some applications, most notably physics. The main electrostatic units are: Statcoulomb or "esu" for charge Statvolt for voltage Gauss for magnetic induction Dimension Unit Definition SI electrostatic unit of charge, 1 esu = 1 statC = 1 Fr charge = 3.33564 × 10−10 C franklin, statcoulomb = √(g·cm³/s²) = 3.33564 × 10−10 electric current biot 1 esu/s C/s electric potential statvolt 1 statV = 1 erg/esu = 299.792458 V 1 statV/cm = 1 = 2.99792458 × 104 electric field dyn/esu V/m magnetic field = 1000/(4π) A/m = oersted 1 Oe strength H 79.577 A/m magnetic flux maxwell 1 Mw = 1 G·cm² = 10−8 Wb magnetic gauss 1 G = 1 Mw/cm² = 10−4 T induction B resistance 1 s/cm = 8.988 × 1011 Ω resistivity 1s = 8.988 × 109 Ω·m capacitance 1 cm = 1.113 × 10−12 F inductance statH = 8.988 × 1011 H wavenumber kayser 1 /cm = 100 /m The mantissas derived from the speed of light are more precisely 299 Mks system of units A physical system of units that expresses any given measurement using fundamental units of the meter, kilogram, and/or second (mks). Historically the mks system of units led to the International System of Units. The SI system now serves as the international standard. Therefore the exact composition of the mks system is a historical issue. As a matter of historical record the mks system encorporated fundamental units other than the meter, kilogram, and second in addition to derived units. An incomplete list of the fundamental/derived units appears below. If the mks system of units never had a governing body to rule on a standard definition then this list might depend on different conventions and different times. cycle (This dimensionless quantity became synonymous with the term "cycle per second" as an abbreviation. This circumstance confused the exact definition of the term cycle. Therefore the term "cycle per meter" became ill-defined. The cycle did not become an SI unit.) cycle per second [1] cycle per meter (This measure of wavenumber became ill-defined due to the abbrevitiation of "cycle per second" as "cycle".)