Centimeter-gram-second system of units (original) (raw)

cgs is short for centimetre-gram-second; it is a system of physical units which preceded the now standard SI system (which is based on the units meter-kilogram-second, hence the nonstandard but occasionally used name mks). The cgs system is still in use, particularly by physicists; this is largely because many electromagnetic formulas are simpler in cgs units, but also because much of the important physics literature uses these units, and in some cases because they are more convenient for a particular domain.

Electromagnetic Units

While for most units the difference between cgs and SI is a mere power of 10, the differences in electromagnetic units are considerable, so much so that formulas for physical laws need to be changed 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. In one variant of the cgs system, esu, or electrostatic units, 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.

There are actually about half a dozen systems of electromagnetic units in use, most based on the cgs system. These include emu, or electromagnetic units (chosen such that Biot-Savart's Law has no constant of proportionality), Gaussian, and Heaviside-Lorentz units. Further complicating matters is the fact that both physicists and engineers use hybrid units, such as volts per centimeter for electric field.

Units

The units of cgs (specifically esu) are as follows:

• length: centimetre. 1 cm = 0.01 m
• mass: gram. 1 g = 0.001 kg
• time: second
• force: dyne = g�cm�s-2 = 10-5 N
• energy: erg = g�cm2�s-2 = 10-7 J
• power: g�cm2�s-3 = 10-7 W
• pressure: bayre = g�cm-1�s-2 = 0.1 Pa
• viscosity: poise = g�cm-1�s-1 = 0.1 Pa�s
• charge: esu, franklin or statcoulomb = √ (g�cm3�s-2) = 3.336 � 10-10 C
• electric potential: statvolt = erg/esu = 299.8 V
• electric field: statvolt/cm = dyne/esu
• magnetic field: 1 gauss = 1 oersted = 1 statvolt/cm = 1 dyn/esu = 10-4 T
• magnetic flux: 1 maxwell = 1 gauss�cm2 = 10-8 Wb
• resistance: s/cm
• resistivity: s
• capacitance: cm = 1.113 � 10-12 F
• inductance: s2/cm = 8.988 � 1011 H

The mantissas 2998, 3336, 1113, and 8988 are derived from the speed of light and are more precisely 299792458, 333564095198152, 1112650056, and 89875517873681764.

A centimeter of capacitance is the capacitance between a sphere of radius 1 cm in vacuum and infinity. The capacitance between two spheres of radii R and r is