Gaussian units
Gaussian units constitute a metric system of physical units. This system is the most common of the several electromagnetic unit systems based on cgs units. It is also called the Gaussian unit system, Gaussian-cgs units, or often just cgs units. The term "cgs units" is ambiguous and therefore to be avoided if possible: there are several variants of cgs with conflicting definitions of electromagnetic quantities and units.
SI units predominate in most fields, and continue to increase in popularity at the expense of Gaussian units. Alternative unit systems also exist. Conversions between quantities in the Gaussian unit system and the SI unit system are not as straightforward as direct unit conversions because the quantities themselves are defined differently in the different systems, which has the effect that the equations expressing physical laws of electromagnetism change depending on what system of units is being used. As an example, quantities that are dimensionless in one system may have dimension in another.
History
Gaussian units existed before the CGS system. The British Association report of 1873 that proposed the CGS contains gaussian units derived from the foot–grain–second and metre–gram–second as well. There are also references to foot–pound–second gaussian units.Alternative unit systems
The Gaussian unit system is just one of several electromagnetic unit systems within CGS. Others include "electrostatic units", "electromagnetic units", and Lorentz–Heaviside units.Some other unit systems are called "natural units", a category that includes Hartree atomic units, Planck units, and others.
SI units are by far the most common system of units today. In engineering and practical areas, SI is nearly universal and has been for decades. In technical, scientific literature, Gaussian units were predominant until recent decades, but are now getting progressively less so. The 8th SI Brochure acknowledges that the CGS-Gaussian unit system has advantages in classical and relativistic electrodynamics, but the 9th SI Brochure makes no mention of CGS systems.
Natural units may be used in more theoretical and abstract fields of physics, particularly particle physics and string theory.
Major differences between Gaussian and SI units
"Rationalized" unit systems
One difference between Gaussian and SI units is in the factors of 4π in various formulas. SI electromagnetic units are called "rationalized", because Maxwell's equations have no explicit factors of 4π in the formulae. On the other hand, the inverse-square force laws - Coulomb's law and the Biot–Savart law - do have a factor of 4π attached to the r. In unrationalized Gaussian units the situation is reversed: two of Maxwell's equations have factors of 4π in the formulas, while both of the inverse-square force laws, Coulomb's law and the Biot–Savart law, have no factor of 4π attached to r in the denominator.Unit of charge
A major difference between Gaussian and SI units is in the definition of the unit of charge. In SI, a separate base unit is associated with electromagnetic phenomena, with the consequence that something like electrical charge is a unique dimension of physical quantity and is not expressed purely in terms of the mechanical units. On the other hand, in the Gaussian system, the unit of electrical charge can be written entirely as a dimensional combination of the mechanical units, as:For example, Coulomb's law in Gaussian units has no constant:
where F is the repulsive force between two electrical charges, Q and Q are the two charges in question, and r is the distance separating them. If Q and Q are expressed in statC and r in cm, then F will come out expressed in dyne.
The same law in SI units is:
where ε0 is the vacuum permittivity, a quantity with dimension, namely 2 2 −1 −3. Without ε0, the two sides would not have consistent dimensions in SI, whereas the quantity ε0 does not appear in Gaussian equations. This is an example of how some dimensional physical constants can be eliminated from the expressions of physical law simply by the judicious choice of units. In SI, 1/ε0, converts or scales flux density, D, to electric field, E, while in rationalized Gaussian units, electric flux density is the same quantity as electric field strength in free space.
In Gaussian units, the speed of light c appears explicitly in electromagnetic formulas like Maxwell's equations, whereas in SI it appears only via the product.
Units for magnetism
In Gaussian units, unlike SI units, the electric field E and the magnetic field B have the same dimension. This amounts to a factor of c between how B is defined in the two unit systems, on top of the other differences. For example, in a planar light wave in vacuum, in Gaussian units, while in SI units.Polarization, magnetization
There are further differences between Gaussian and SI units in how quantities related to polarization and magnetization are defined. For one thing, in Gaussian units, all of the following quantities have the same dimension: E, D, P, B, H, and M. Another important point is that the electric and magnetic susceptibility of a material is dimensionless in both Gaussian and SI units, but a given material will have a different numerical susceptibility in the two systems.List of equations
This section has a list of the basic formulae of electromagnetism, given in both Gaussian and SI units. Most symbol names are not given; for complete explanations and definitions, please click to the appropriate dedicated article for each equation. A simple conversion scheme for use when tables are not available may be found inRef.
All formulas except otherwise noted are from Ref.
Maxwell's equations
Here are Maxwell's equations, both in macroscopic and microscopic forms. Only the "differential form" of the equations is given, not the "integral form"; to get the integral forms apply the divergence theorem or Kelvin–Stokes theorem.| Name | Gaussian units | SI units |
| Gauss's law | ||
| Gauss's law | ||
| Gauss's law for magnetism: | ||
| Maxwell–Faraday equation : | ||
| Ampère–Maxwell equation : | ||
| Ampère–Maxwell equation : |
Other basic laws
| Name | Gaussian units | SI units |
| Lorentz force | ||
| Coulomb's law | ||
| Electric field of stationary point charge | ||
| Biot–Savart law | ||
| Poynting vector |
Dielectric and magnetic materials
Below are the expressions for the various fields in a dielectric medium. It is assumed here for simplicity that the medium is homogeneous, linear, isotropic, and nondispersive, so that the permittivity is a simple constant.where
- E and D are the electric field and displacement field, respectively;
- P is the polarization density;
- is the permittivity;
- is the permittivity of vacuum ;
- is the electric susceptibility
Next, here are the expressions for the various fields in a magnetic medium. Again, it is assumed that the medium is homogeneous, linear, isotropic, and nondispersive, so that the permeability is a simple constant.
where
- B and H are the magnetic fields
- M is magnetization
- is magnetic permeability
- is the permeability of vacuum ;
- is the magnetic susceptibility
Vector and scalar potentials
The electric and magnetic fields can be written in terms of a vector potential A and a scalar potential φ:| Name | Gaussian units | SI units |
| Electric field | ||
| Magnetic B field |
Electromagnetic unit names
| Quantity | Symbol | SI unit | Gaussian unit | Conversion factor |
| electric charge | q | C | Fr | |
| electric current | I | A | Fr/s | |
| electric potential | φ V | V | statV | |
| electric field | E | V/m | statV/cm | |
| electric displacement field | D | C/m2 | Fr/cm2 | |
| magnetic B field | B | T | G | |
| magnetic H field | H | A/m | Oe | |
| magnetic dipole moment | m | A⋅m2 | erg/G | |
| magnetic flux | Φm | Wb | G⋅cm2 | |
| resistance | R | Ω | s/cm | |
| resistivity | ρ | Ω⋅m | s | |
| capacitance | C | F | cm | |
| inductance | L | H | s2/cm |
The conversion factors are written both symbolically and numerically. The numerical conversion factors can be derived from the symbolic conversion factors by dimensional analysis. For example, the top row says, a relation which can be verified with dimensional analysis, by expanding and C in SI base units, and expanding Fr in Gaussian base units.
It is surprising to think of measuring capacitance in centimetres. One useful example is that a centimetre of capacitance is the capacitance between a sphere of radius 1 cm in vacuum and infinity.
Another surprising unit is measuring resistivity in units of seconds. A physical example is: Take a parallel-plate capacitor, which has a "leaky" dielectric with permittivity 1 but a finite resistivity. After charging it up, the capacitor will discharge itself over time, due to current leaking through the dielectric. If the resistivity of the dielectric is "X" seconds, the half-life of the discharge is ~0.05X seconds. This result is independent of the size, shape, and charge of the capacitor, and therefore this example illuminates the fundamental connection between resistivity and time units.
Dimensionally equivalent units
A number of the units defined by the table have different names but are in fact dimensionally equivalent – i.e., they have the same expression in terms of the base units cm, g, s. The different names help avoid ambiguities and misunderstandings as to what physical quantity is being measured. In particular, all of the following quantities are dimensionally equivalent in Gaussian units, but they are nevertheless given different unit names as follows:| Quantity | In Gaussian base units | Gaussian unit of measure |
| E | cm−1/2⋅g1/2⋅s−1 | statV/cm |
| D | cm−1/2⋅g1/2⋅s−1 | statC/cm2 |
| P | cm−1/2⋅g1/2⋅s−1 | statC/cm2 |
| B | cm−1/2⋅g1/2⋅s−1 | G |
| H | cm−1/2 g1/2⋅s−1 | Oe |
| M | cm−1/2⋅g1/2⋅s−1 | dyn/Mx |
General rules to translate a formula
Any formula can be converted between Gaussian and SI units by using the symbolic conversion factors from Table 1 above.For example, the electric field of a stationary point charge has the SI formula
where r is distance, and the "SI" subscripts indicate that the electric field and charge are defined using SI definitions. If we want the formula to instead use the Gaussian definitions of electric field and charge, we look up how these are related using Table 1, which says:
Therefore, after substituting and simplifying, we get the Gaussian-units formula:
which is the correct Gaussian-units formula, as mentioned in a previous section.
For convenience, the table below has a compilation of the symbolic conversion factors from Table 1. To convert any formula from Gaussian units to SI units using this table, replace each symbol in the Gaussian column by the corresponding expression in the SI column. This will reproduce any of the specific formulas given in the list above, such as Maxwell's equations, as well as any other formula not listed. For some examples of how to use this table, see:
| Name | Gaussian units | SI units |
| electric field, electric potential | ||
| electric displacement field | ||
| charge, charge density, current, current density, polarization density, electric dipole moment | ||
| magnetic B field, magnetic flux, magnetic vector potential | ||
| magnetic H field | ||
| magnetic moment, magnetization | ||
| permittivity, permeability | ||
| electric susceptibility, magnetic susceptibility | ||
| conductivity, conductance, capacitance | ||
| resistivity, resistance, inductance |
| Name | SI units | Gaussian units |
| electric field, electric potential | ||
| electric displacement field | ||
| charge, charge density, current, current density, polarization density, electric dipole moment | ||
| magnetic B field, magnetic flux, magnetic vector potential | ||
| magnetic H field | ||
| magnetic moment, magnetization | ||
| permittivity, permeability | ||
| electric susceptibility, magnetic susceptibility | ||
| conductivity, conductance, capacitance | ||
| resistivity, resistance, inductance |
Once all occurrences of the product have been replaced by, there should be no remaining quantities in the equation with an SI electromagnetic dimension remaining.