# What is Cgs: Definition and 19 Discussions

The centimetre–gram–second system of units (abbreviated CGS or cgs) is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time. All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways in which the CGS system was extended to cover electromagnetism.The CGS system has been largely supplanted by the MKS system based on the metre, kilogram, and second, which was in turn extended and replaced by the International System of Units (SI). In many fields of science and engineering, SI is the only system of units in use, but there remain certain subfields where CGS is prevalent.
In measurements of purely mechanical systems (involving units of length, mass, force, energy, pressure, and so on), the differences between CGS and SI are straightforward and rather trivial; the unit-conversion factors are all powers of 10 as 100 cm = 1 m and 1000 g = 1 kg. For example, the CGS unit of force is the dyne, which is defined as 1 g⋅cm/s2, so the SI unit of force, the newton (1 kg⋅m/s2), is equal to 100000 dynes.
On the other hand, in measurements of electromagnetic phenomena (involving units of charge, electric and magnetic fields, voltage, and so on), converting between CGS and SI is more subtle. Formulas for physical laws of electromagnetism (such as Maxwell's equations) take a form that depends on which system of units is being used. This is because the electromagnetic quantities are defined differently in SI and in CGS, whereas mechanical quantities are defined identically. Furthermore, within CGS, there are several plausible ways to define electromagnetic quantities, leading to different "sub-systems", including Gaussian units, "ESU", "EMU", and Lorentz–Heaviside units. Among these choices, Gaussian units are the most common today, and "CGS units" often used specifically refers to CGS-Gaussian units.

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1. ### A CGS units confusions in plasma physics journal papers

Hi, I was wondering of those who might write papers in this field. What is the convention of units in plasma physics papers, and is this often all screwed around and you have to already understand it to understand it? I'll give you an example, because I am looking at this paper to understand...
2. ### Dimension of the electric charge in CGS and in SI?

The Bohr magneton is (see e.g. Wikipedia) in SI units: $$\mu_B=\frac{e\hbar}{2m_e}$$ and in CGS units: $$\mu_B=\frac{e\hbar}{2m_ec}$$ So the dimension of the electric charge in SI, ##[q_{SI}]##, is related to the dimension of the electric charge in CGS, ##[q_{CGS}]##, by...
3. ### Time period of small oscillations of the point dipole

Homework Statement In an infinite flat layer of thickness 2d, volume charge density is given according to the law: ρ=(ρ°)(x)/d and (-d≤x≤d). Here, x is the axis perpendicular to the plane. In the layer, there is a thin channel in which a point dipole of mass m and dipole moment p is placed...
4. ### I Cgs or SI in quantum field theory?

I have an acquaintance who maintains that in quantum field theory, primarily the cgs system is used. OK, I know it's not really important, but I was under the impression that everyone had switched to SI. (My book on quantum field theory has very few actual quantities with units outside of GeV...
5. ### I How to Convert Cgs to SI Units for Flux and Energy Values?

Hi I have a value that is 10.7 mJy, that I need to convert to SI units. I thought it would be 1.7*10^(-2)*10^(-26) but that might be wrong? Then I have a flux value, kappa, of 2 cm^2.g^(-1) that needs to be converted to meters and kilogram. I thought it was 2*10^(-7) m2/kg, but that might be...
6. ### Superposition of Moments of Inertia about Different CGs

Hi all, You can use superposition to add moments of inertia when they're calculated about the same center of gravity (cg), but let's say you calculate the moments of inertia of several elements of a system about one cg and then use the Parallel Axis Theorem to then reference the total moments...
7. ### Maxwells equation in cgs system

hi, I have just encountered following Maxwell equation in cgs system ▽×H = (1/c)((∂D)/(∂t))+((4π)/c)j, ▽×E = -(1/c)((∂B)/(∂t)), ▽.D = 4πρ, ▽.B = 0, now , c is the speed of light in vacuum , my question is that we have studied earlier equation without this factor 'c' and 4pi in third...
8. ### Why Is the Conversion Between CGS and SI Units in Electromagnetism Confusing?

Dear friends, As you know, in CGS units we have vacuum permeability as unity (1). so H=B and we use Oersted for H. on the other hand, in SI, permeability is (4 x pi x 0.0000001) and we use Ampere per meter for H. unfortunately after trying for couple hours, I couldn't derive the algebra of...
9. ### Why c is divided while converting cgs into SI system?

In conversion of formula from cgs into SI, usually c, the velocity of light is divided. Why it is so?
10. ### Gauss's law in cgs unit system

Homework Statement Is there a constant parrallel to the ε0 permittivity in the Gauss law in c.g.s? Homework Equations Coulomb force in m.k.s: ##F=\frac{1}{4\pi\varepsilon_0}\frac{qq'}{r^2}## Coulomb force in c.g.s: ##F=\frac{qq'}{r^2}## Gauss's law in m.k.s: ##\frac{N}{A}=\varepsilon_0 E##...
11. ### Confusion of the electrical conductivity unit in CGS.

Hello. Electrical conductivity is written as \frac{n_{e}e^{2}}{m_{e}\nu_{ei}} where n_{e}, e, m_{e} and \nu_{ei} are for electron density, charge, mass and electron-ion collision frequency. According to Wikipedea, http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity Unit...
12. ### Converting Between CGS and SI Units in Electromagnetism: A Complete Guide

So I understand that converting between CGS and SI units is no easy task when you look at the general form of each equation in electromagnetism, as you can't simply relate the two systems given they have different ways of looking at what is a derived unit and what is fundamental. But once you've...
13. ### What is the CGS of current and voltage.

Si unit of current which is a fundamental physical quantity is ampere and what is its CGS unit, and for voltage..
14. ### Instantaneous Unit Conversion between CGS and SI units

In doing some research, I cam across an equation for instantaneous radiated power, with the CGS units "erg/(sec rad cm)", rad being radians (not rad the unit for radiation exposure). Try as I might, I'm not able to come up with a way to convert it to the SI units for watts. Does anyone know...
15. ### Converting Between MKS and CGS Unit Systems: Tips and Tricks?

"cgs" unit systems i'm used to mks unit systems, but i have to read a book with cgs recently. it's difficult for me to transform it between them. can anyguys give me some advises to deal with it ! thanks for responds!
16. ### Converting mks energy density to cgs

Homework Statement In the SI system, the energy density of the electric and magnetic fields is: u = \frac {\epsilon_{0} E^{2}}{2} + \frac{B^{2}}{2 \mu_{0}} From the equation above, derive an exact expression for the energy density U in the Gaussian system of units. The...
17. ### Using CGS or SI Units: When to Choose?

I have a simple question. I understand the quantitative differences between cgs and SI units, but when will I be expected to use one over the other? For example, do physicists prefer one set of units? Thanks for the help.
18. ### Using cgs units, can I set c=h-bar=1?

Using cgs units, can I set c = h-bar =1 ? or should I change some other units to do that? Thanks
19. ### The Lorentz force in the MKS and CGS measurement systems

Hi there! I have a problem deriving the formula for the Lorenz force that I found in my lecture notes in Theoretical Mechanics. The formula is: \vec{F}=q \cdot \vec{E} + \frac{q \cdot (\vec{v} \times \vec{B})}{c} Where: \vec{F} is the Lorentz force \vec{E} is the electric...