Tuesday, March 1, 2016

MKS, CGS AND SI SYSTEMS

MKS is the system of units based on measuring lengths in meters, mass in kilograms, and time in seconds. MKS is generally used in engineering and beginning physics, where the so-called cgs system (based on the centimeter, gram, and second) is commonly used in theoretic physics. The most familiar units of electricity and magnetism (ohm, farad, coulomb, etc.) are MKS units.
 
CGS is the system of units based on measuring lengths in centimeters, mass in grams, and time in seconds. It is a metric system, although not the flavor of the metric system used most commonly. It was introduced by the British Association for the Advancement of Science in 1874, and was immediately adopted by many working scientists.

There are several flavors of the cgs system: "cgs electrostatic," "cgs electromagnetic," and "cgs Gaussian." None of these are part of the SI system, except for units such as the centimeter defined in both systems (Taylor 1995, p. 11). The cgs Gaussian system is nonetheless commonly used in theoretical physics, while the MKS system (based on the meter, kilogram, and second) is commonly used in engineering and physics instruction.
unitsymbolMKS (abbrev.)cgs (abbrev.)
accelerationam sGal
capacitanceCFarad (F)cm
chargeqCoulomb (C)esu
currentIAmpere (A)esu s-1
electric fieldEV m-1statvolt cm-1
electric potentialVVolt (V)statvolt
energyworkEWJoule (J)erg
forceFNewton (N)dyne
inductanceLHenry (H)cm-1 s
lengthldmeter (m)centimeter (cm)
magnetic fieldBTesla (T)Gauss (G)
magnetic fluxWeber (w)Gauss cm
massmkilogram (kg)gram (g)
momentumpkg m s-1g cm s-1
powerPWatt (W)erg s-1
pressurePPascal (Pa)bar
resistanceROhm ()cm-1 s
temperatureTKelvin (K)Kelvin (K)
timetsecond (s)second (s)
velocityvm s-1cm s-1

"SI" stands for "System International" and is the set of physical units agreed upon by international convention. The SI units are sometimes also known as MKS units, where MKS stands for "meter, kilogram, and second." In 1939, the CCE recommended the adoption of a system of units based on the meter, kilogram, second, and ampere. This proposal was approved by the Comité International des Poids et Mesures (CIPM) in 1946. Following an international inquiry by the Bureau International des Poids et Mesures (BIPM), which began in 1948, in 1954 the 10th Conférence Générale des Poids et Mesures (CGPM) approved the introduction of the ampere, kelvin, and candela as base units for electric current, thermodynamic temperature, and luminous intensity, respectively. However, the ampere is scheduled to be phased out as a base unit in the near future in favor of the ohm, which can be measured extremely accurately using the quantum Hall effect. In turn, the volt will probably replace the ohm further in the future when measurements using Josephson junctions increase in precision.

The name International System of Units (SI) was given to the system by the 11th CGPM in 1960. At the 14th CGPM in 1971, the current version of the SI was completed by adding the mole as base unit for amount of substance, bringing the total number of base units to seven. The seven fundamental units are summarized in the following table.

physical quantitysymbolunit abbreviationunit name
lengthlmmeter
massmkgkilogram
timetssecond
currentIAAmpere
temperatureTKKelvin
luminous intensitycdcandela
amount of substancenmolmole
The derived SI units consist of combinations of the seven base units, and are summarized in the following table.
quantitysymbolSI symbolSI unit
areaAsquare meter
volumeVcubic meter
plane angleradradian
solid anglesterradsteradian
frequencyfHzHertz
velocityvmeters per second
accelerationameters per second squared
forceFNNewton
pressureP or pPaPascal
powerPWWatt
energyEJJoule
voltageVVVolt
resistanceROhm
conductanceGSSiemens
chargeQCCoulomb
capacitanceCFFarad
magnetic fluxWbWeber
magnetic flux densityBTTesla
inductanceLHHenry
luminous fluxFlmlumen
illuminationElxlux
activityABqBecquerel
energy doseGyGray
equivalent doseSvSievert
In 1960, the 11th CGPM adopted a first series of prefixes and symbols of prefixes to form the names and symbols of decimal multiples and submultiples of SI units. Over the years, the list has been extended as summarized in the following table.
factorprefixsymbol
1024yotta-Y
1021zetta-Z
1018exa-E
1015peta-P
1012tera-T
109giga-G
106mega-M
103kilo-k
102hecto-h
101deca-da
deci-d
centi-c
milli-m
micro-
nano-n
pico-p
femto-f
atto-a
zepto-z
yocto-y

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