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Gaussian units

Gaussian units, also known as the Gaussian cgs system, constitute a measurement framework primarily employed in to formulate electromagnetic phenomena within the centimeter-gram-second (cgs) base units. In this system, the permittivity of free space is effectively set such that simplifies to F = \frac{q_1 q_2}{r^2}, where force is in dynes, charges in statcoulombs (esu), and distance in centimeters, eliminating the \frac{1}{4\pi\epsilon_0} factor present in the system. This approach derives from the electrostatic unit (esu) subsystem of cgs, where charge is defined dimensionally as \sqrt{\mathrm{dyne} \cdot \mathrm{cm}^2} = \mathrm{g}^{1/2} \mathrm{cm}^{3/2} \mathrm{s}^{-1}, ensuring no separate base unit for charge beyond mechanical ones. A defining feature of Gaussian units is their symmetry between electric and magnetic quantities, achieved by incorporating the c explicitly in , such as \nabla \times \mathbf{B} = \frac{4\pi}{c} \mathbf{J} + \frac{1}{c} \frac{\partial \mathbf{E}}{\partial t}, which contrasts with the SI form \nabla \times \mathbf{B} = \mu_0 \mathbf{J} + \mu_0 \epsilon_0 \frac{\partial \mathbf{E}}{\partial t} that includes permeability \mu_0 and \epsilon_0. Consequently, the \mathbf{E} and \mathbf{B} share identical dimensions (both in statvolts per centimeter or gauss), and the law becomes \mathbf{F} = q (\mathbf{E} + \frac{\mathbf{v} \times \mathbf{B}}{c}), highlighting relativistic symmetries absent in SI units. This unification avoids the introduction of arbitrary constants like \epsilon_0 and \mu_0, rendering equations more aesthetically balanced and convenient for theoretical derivations in areas like quantum electrodynamics and . Historically, Gaussian units emerged as a blend of cgs electrostatic (esu) and electromagnetic (emu) units, proposed by in the and later refined by figures like James Clerk Maxwell, to reconcile and magnetostatics without velocity-dependent factors in basic laws. Key derived units include the for potential (1 ≈ 299.8 volts), the gauss for magnetic flux density (1 gauss = 10^{-4} ), and the erg for (1 erg = 10^{-7} joules), with charge conversion yielding 1 ≈ 2.998 × 10^9 esu. While Gaussian units excel in theoretical contexts due to their minimalism—facilitating insights into electromagnetic wave propagation where c appears naturally—they are less common in engineering and experimental settings, where the system's practicality prevails.

Introduction to Gaussian Units

Definition and Core Principles

Gaussian units form a cornerstone of the centimeter-gram-second (CGS) system specifically tailored for electromagnetism, where electrical and magnetic quantities are derived directly from the mechanical base units of length (centimeter), mass (gram), and time (second), without introducing independent electrical base units like the ampere in other systems. In this framework, the unit of electric charge is defined such that the electrostatic force between two point charges follows a simple form with a proportionality constant of unity, eschewing the rationalized $1/(4\pi\epsilon_0) factor found in other conventions. This non-rationalized approach introduces factors of $4\pi into the divergence equations of Maxwell's equations, such as Gauss's law for electricity, which contributes to the system's hallmark symmetry between electric and magnetic fields. Central to Gaussian units are the electrostatic unit (esu), also known as the statcoulomb, for charges in electrostatic contexts, and the electromagnetic unit (emu) for magnetic phenomena, with the speed of light c serving as a bridging constant to ensure consistency between the two subsystems. Dimensionally, charge carries the dimensions [Q] = [M^{1/2} L^{3/2} T^{-1}], reflecting its derivation from force and distance: for instance, one statcoulomb equals g^{1/2} \cdot cm^{3/2} / s. The electric field is measured in statvolts per centimeter, while the magnetic field uses the gauss, and notably, these units share identical dimensions (g^{1/2} cm^{-1/2} s^{-1}), underscoring the system's elegant unification of electric and magnetic phenomena. This structure renders Gaussian units particularly natural for , as the explicit appearance of c in electrodynamic equations highlights relativistic invariance and simplifies derivations in and . For example, the law incorporates c to balance electric and magnetic contributions, emphasizing the fundamental equivalence of \mathbf{E} and \mathbf{B} fields in . However, the system's mixed scales—such as the being approximately 300 volts—make it less intuitive for practical applications, where standardized units facilitate measurements and device design.

Historical Development

The foundations of Gaussian units trace back to early explorations of electricity and magnetism in the 18th century, where concepts of electric charge were first articulated. Charles François de Cisternay du Fay identified two distinct types of electricity—vitreous and resinous—in 1733 through experiments with rubbed glass and amber, laying groundwork for understanding charge interactions. Benjamin Franklin advanced this in the 1740s and 1750s by proposing a single-fluid theory, designating excess and deficiency of electrical fluid as positive and negative charges, which facilitated quantitative studies of electrostatics. These ideas set the stage for absolute measurement systems in the 19th century. In the 1830s, and Wilhelm Weber developed the centimeter-gram-second (CGS) system for absolute measurements of magnetic and electromagnetic phenomena, aiming to express these quantities solely in mechanical units without arbitrary constants. Gauss proposed in 1832 a method to measure the using torsion balance experiments, defining the unit of magnetic intensity in terms of centimeter, gram, and second, which became the basis for electromagnetic units. Weber collaborated with Gauss in the 1840s to extend this to electric currents and potentials, establishing the electromagnetic unit (emu) system as the first quantitative framework linking and to mechanics. The Gaussian units are named after Gauss for his pioneering magnetic measurements, though he did not directly formulate the full electromagnetic variant. By the mid-19th century, James Clerk Maxwell incorporated these non-rationalized CGS-based units into his unified theory of electromagnetism. In his 1861 paper and 1873 Treatise on Electricity and Magnetism, Maxwell employed absolute electromagnetic units derived from Weber's work, treating electric and magnetic quantities symmetrically while predicting electromagnetic waves propagating at the speed of light. The electrostatic unit (esu) and electromagnetic unit (emu) were formally defined in the 1873 report of a committee established in 1861 by the British Association for the Advancement of Science (BAAS), distinguishing electrostatic from magnetostatic measurements within the CGS framework. J.J. Thomson further utilized esu and emu in the 1880s and 1890s, notably measuring the electron's charge in esu during his cathode-ray experiments, which helped integrate atomic-scale phenomena into the system. The 1881 International Electrical Congress in marked a pivotal event, adopting the CGS electromagnetic units () as a standard for international comparisons and distinguishing esu for , though practical units like the were also proposed. In the late 19th and early 20th centuries, Gaussian units—combining esu and emu via the —gained traction, formalized by in 1888 to symmetrize . In the physics community around , Henry Augustus Rowland championed their adoption through his precise electromagnetic measurements and advocacy for absolute units, influencing U.S. standards and education. In the 1930s, the (IEC), in coordination with metrology bodies like the International Committee for Weights and Measures (CIPM), codified key Gaussian units such as the gauss for magnetic flux density and for strength, though the system was not officially endorsed as a primary international standard. This persistence stemmed from the units' elegance in theoretical contexts, where they highlight symmetries in without extraneous factors like 4π. Following the 1960 adoption of the (SI) by the General Conference on Weights and Measures, Gaussian units declined in practical and engineering applications but endured in , , and due to their conceptual clarity.

Unit Systems in Electromagnetism

CGS Framework and Variants

The centimeter-gram-second (CGS) system forms the foundational framework for , establishing base units of length (, ), mass (, ), and time (second, ) to derive all other quantities mechanically without introducing arbitrary constants like the in the system. This absolute approach ensures that units emerge directly from mechanical ones, promoting consistency in . The force unit, the , is defined as $1 dyne = 1 g \cdot cm/s^2, and electromagnetic quantities are subsequently defined via the law, \mathbf{F} = q (\mathbf{E} + \mathbf{v} \times \mathbf{B}/c), where c is the . Within the CGS framework, electromagnetic variants adapt these base units to specific domains, with the electrostatic (ESU) system prioritizing dielectrics through Coulomb's law in the form F = q_1 q_2 / r^2, setting the force constant to unity and defining the statcoulomb as the charge unit. The electromagnetic (EMU) system, suited for magnetostatics, similarly sets Ampère's law constant to unity, using the biot (also known as the abampere) as the unit for current, where one emu of charge corresponds to the amount producing one emu of current in one second. Gaussian units serve as a hybrid, employing ESU for charge and current while incorporating the speed of light c = 3 \times 10^{10} cm/s as a conversion factor to unify electric and magnetic fields, such that $1 emu of charge = c esu of charge, enabling symmetric treatment in Maxwell's equations. Other notable CGS variants include Heaviside-Lorentz units, a rationalized form of Gaussian units that eliminates factors of $4\pi from field equations, making it prevalent in (QED) for cleaner formulations. Practical CGS units adapt meter-kilogram-second (MKS)-like scales to centimeters, incorporating everyday electrical measures such as the volt and but expressed in CGS dimensions for engineering applications. In contemporary , Gaussian units persist as a legacy standard in certain , such as the Runko multiphysics toolbox, where they facilitate normalized, unitless implementations of the Vlasov-Maxwell equations for efficient (PIC) modeling of relativistic plasmas. This usage underscores their enduring utility in high-energy simulations despite the dominance of in broader .

Comparison with SI and Other Systems

The (SI), based on the meter-kilogram-second- (MKSA) framework, designates the as a fundamental base unit for and incorporates rationalization by absorbing factors of $4\pi into the definitions of and permeability, thereby optimizing the system for engineering and practical measurements. A primary structural difference lies in the choice of base units: Gaussian units rely on just three—centimeter for length, gram for mass, and second for time—without dedicated bases for or charge, which renders the vacuum permittivity \epsilon_0 and permeability \mu_0 dimensionless and equal to unity. In SI, seven base units are employed, including the , leading to \epsilon_0 \approx 8.85 \times 10^{-12} F/m and \mu_0 = 4\pi \times 10^{-7} H/m as physical constants with specific dimensions. Regarding rationalization, Gaussian units maintain an "irrational" structure by preserving the $4\pi factor in vacuum electromagnetic laws, such as \nabla \cdot \mathbf{E} = 4\pi \rho, which enhances theoretical symmetry and clarity in deriving fundamental relations. rationalization, by contrast, eliminates these $4\pi terms from integral forms like Ampère's law, streamlining analysis and computations but obscuring relativistic symmetries where explicit factors complicate the transition to covariant formulations. Gaussian units also relate closely to other specialized systems, such as , which set \hbar = e = m_e = 1 and adopt Gaussian electromagnetic conventions to simplify quantum calculations for atomic-scale phenomena. In where c = \hbar = 1, Gaussian conventions prove advantageous in by yielding naturally dimensionless coupling constants, like the \alpha = e^2 / (\hbar c), without extraneous dimensional factors that arise in . A distinctive feature of Gaussian units is the explicit appearance of the c in linking electric and magnetic field magnitudes, as in electromagnetic plane waves where |\mathbf{B}| = |\mathbf{E}| / c, which underscores the relativistic interconnection of fields and facilitates covariant extensions in . While pure Gaussian units see limited use in contemporary literature, closely related Heaviside-Lorentz variants—rationalized forms omitting the $4\pi—appear in key texts like Peskin and Schroeder's An Introduction to .

Key Units and Quantities

Electric Quantities: Charge, Field, and Potential

In the Gaussian system of units, the fundamental is measured in , also known as electrostatic units (esu), where the is defined such that two point charges of one each, separated by one centimeter in vacuum, exert a repulsive force of exactly one on each other. This definition arises from the centimeter-gram-second (CGS) framework, ensuring that the electrostatic force law takes the simple form F = \frac{q_1 q_2}{r^2}, with force in , charges in , and distance in centimeters, without any additional constants like the of free space. The corresponding potential energy between two such charges is then U = \frac{q_1 q_2}{r}, reflecting the unit choice that sets the \epsilon_0 = 1 (dimensionless) to simplify electrostatic expressions, though this requires careful handling of dimensional consistency in broader contexts. For conversion to the (SI), one equals approximately $3.336 \times 10^{-10} coulombs, a factor derived from the c \approx 3 \times 10^{10} cm/s linking electrostatic and electromagnetic units. In the electromagnetic variant (emu), the unit of charge is the abcoulomb, where one abcoulomb equals c statcoulombs or exactly 10 coulombs, providing a bridge to magnetic quantities through the factor c. The electric field \mathbf{E} in Gaussian units has dimensions of force per charge, expressed as dynes per or equivalently per centimeter, since the field is the force on a unit charge: \mathbf{E} = \frac{\mathbf{F}}{q}. One per centimeter corresponds to approximately $3 \times 10^4 volts per meter in units, highlighting the scale where Gaussian units suit microscopic or atomic-scale phenomena due to the smaller charge . The \mathbf{D}, which accounts for free charges, follows \mathbf{D} = \mathbf{E} + 4\pi \mathbf{P} in esu, maintaining consistency with the unit system where \mathbf{P} has the same dimensions as \mathbf{D}. Electric potential V is measured in statvolts, defined such that the potential difference is the work per unit charge, with one statvolt equaling approximately 300 volts in SI. The relation \mathbf{E} = -\nabla V holds directly, and the potential due to a point charge is V = \frac{q}{r}, again benefiting from \epsilon_0 = 1 to avoid prefactors. This setup prioritizes elegance in theoretical electrostatics, as originally intended in the CGS system for simplifying Maxwell's equations.

Magnetic Quantities: Fields, Moments, and Permeability

In Gaussian units, the \mathbf{B}, often called the , is measured in gauss (G), where 1 G = $10^{-4} (T). This field arises in the law, which describes the force on a moving in electromagnetic fields: \mathbf{F} = q \left( \mathbf{E} + \frac{1}{c} \mathbf{v} \times \mathbf{B} \right), with c being the in cm/s, ensuring symmetry between electric and magnetic contributions in the cgs framework. The auxiliary magnetic field \mathbf{H}, or magnetic field strength, is measured in oersteds (Oe), where in vacuum 1 Oe corresponds numerically to 1 G, though the formal conversion is 1 Oe \approx 79.58 A/m. In materials, the relation is \mathbf{B} = \mathbf{H} + 4\pi \mathbf{M}, where \mathbf{B} and \mathbf{H} share the same dimensions in vacuum (\mathbf{B} = \mathbf{H}), contrasting with the SI system where \mathbf{B} (in T) and \mathbf{H} (in A/m) differ dimensionally; the $1/c factor in the Lorentz force maintains electromagnetic symmetry. Magnetization \mathbf{[M](/page/M)}, representing the per , is expressed in electromagnetic s per cubic centimeter (/cm³), equivalent to gauss in . The \boldsymbol{\mu} for a is in , defined as 1 = 1 erg/, reflecting the (erg) in a field (); for example, the Bohr magneton is approximately $9.27 \times 10^{-21} . Permeability in Gaussian units is dimensionless, with the vacuum value \mu_0 = 1; for linear materials, \mu = B/H \approx 1 + 4\pi \chi_m, where \chi_m is the , highlighting the system's simplicity by absorbing the into unity and scaling material responses via the $4\pi factor.

Material Response: Polarization and Magnetization

In Gaussian units, the \mathbf{P} describes the per volume within a material and carries units of statcoulombs per square centimeter (esu/cm²). This arises from the alignment of molecular dipoles in response to an applied , contributing to the material's overall response. The \mathbf{D} incorporates both the free-space and the material's through the \mathbf{D} = \mathbf{E} + 4\pi \mathbf{P}, where \mathbf{E} is the electric field. In linear isotropic dielectrics, \mathbf{P} = \chi_e \mathbf{E}, with \chi_e denoting the electric susceptibility (dimensionless), yielding the dielectric constant \kappa = 1 + 4\pi \chi_e. This formulation simplifies the treatment of bound charges, as the divergence of \mathbf{D} relates directly to free charge density without additional constants. Polarization induces bound charges within the : the volume bound charge density is \rho_b = -\nabla \cdot \mathbf{P}, while the surface bound charge density is \sigma_b = \mathbf{P} \cdot \hat{n}, where \hat{n} is the outward unit normal. These expressions derive from the physical picture of as a collection of aligned dipoles, leading to effective positive and negative charge separations. The total effective in includes both free and bound contributions, but the Gaussian system's structure allows bound effects to be isolated cleanly in \mathbf{P}. Analogously, for magnetic materials, the \mathbf{M} quantifies the moment per unit volume and has units of electromagnetic units per cubic centimeter (/cm³). The \mathbf{B} relates to the \mathbf{H} and \mathbf{M} via \mathbf{B} = \mathbf{H} + 4\pi \mathbf{M}. This reflects the contribution of atomic currents and spins to the total magnetic field inside the material. In linear media, \mathbf{M} = \chi_m \mathbf{H}, where the \chi_m is dimensionless, resulting in the \mu = 1 + 4\pi \chi_m. For ferromagnetic materials, the magnetization M_s—the maximum achievable \mathbf{M}—is commonly expressed in emu/cm³, providing a key parameter for characterizing material strength. Magnetization similarly produces bound magnetic effects, modeled via fictitious magnetic poles: the volume bound pole density is \rho_m = -\nabla \cdot \mathbf{M}, and the surface bound pole density is \sigma_m = \mathbf{M} \cdot \hat{n}. These poles arise from the divergence of , analogous to electric bound charges, and facilitate the description of demagnetizing fields in non-uniformly magnetized samples. The presence of $4\pi factors in these relations stems from the non-rationalized nature of Gaussian units, which eschews the $4\pi adjustment in integral forms of Ampère's and Gauss's laws found in rationalized systems like ; this eliminates vacuum \epsilon_0 and permeability \mu_0 (setting them to ) but introduces $4\pi in material constitutive equations. Consequently, volume and integrals over material distributions often simplify compared to , avoiding explicit factors of \epsilon_0 or \mu_0 in derivations. Despite the global shift toward , Gaussian units persist in specialized fields like nanomagnetics simulations as of 2025, where codes such as OOMMF employ emu/cm³ for M_s and related quantities to align with historical micromagnetic literature.

Electrodynamic Laws and Equations

Maxwell's Equations

In Gaussian units, Maxwell's equations describe the fundamental behavior of electric and magnetic fields in vacuum, unifying , , and into a single framework of electrodynamics. These equations, originally formulated by James Clerk Maxwell in the 1860s using component form and quaternions, were reformulated into their modern by in 1881, with the Gaussian system providing a natural cgs-based expression that emphasizes symmetry between electric and magnetic quantities. The differential forms of Maxwell's equations in Gaussian units, assuming vacuum where the charge density ρ represents free charges and the current density J represents free currents, are: \begin{align} \nabla \cdot \mathbf{E} &= 4\pi \rho, \\ \nabla \cdot \mathbf{B} &= 0, \\ \nabla \times \mathbf{E} &= -\frac{1}{c} \frac{\partial \mathbf{B}}{\partial t}, \\ \nabla \times \mathbf{B} &= \frac{4\pi}{c} \mathbf{J} + \frac{1}{c} \frac{\partial \mathbf{E}}{\partial t}. \end{align} These forms highlight the symmetry between the E and B, as well as between ρ and current density J, with the c explicitly appearing to connect spatial and temporal derivatives, underscoring the relativistic invariance of . Unlike the system, no ε₀ or permeability μ₀ is required, as they are absorbed into the unit definitions (effectively ε₀ = 1/(4π) and μ₀ = 4π/c² in Gaussian units). The corresponding integral forms, derived via the divergence and Stokes' theorems, express the equations over closed surfaces and loops: \begin{align} \oint_S \mathbf{E} \cdot d\mathbf{A} &= 4\pi Q_\text{enc}, \\ \oint_S \mathbf{B} \cdot d\mathbf{A} &= 0, \\ \oint_C \mathbf{E} \cdot d\mathbf{l} &= -\frac{1}{c} \frac{d\Phi_B}{dt}, \\ \oint_C \mathbf{B} \cdot d\mathbf{l} &= \frac{4\pi}{c} I_\text{enc} + \frac{1}{c} \frac{d\Phi_E}{dt}, \end{align} $$where Q_enc is the enclosed free charge, I_enc is the enclosed free current, Φ_B = ∫_S B · dA is the [magnetic flux](/page/Magnetic_flux), and Φ_E = ∫_S E · dA is the [electric flux](/page/Electric_flux).[](https://terpconnect.umd.edu/~mattland/assets/notes/SI_Gaussian_unit_conversion.pdf)[](https://bohr.physics.berkeley.edu/classes/221/1112/notes/emunits.pdf) In media, these vacuum equations are extended using constitutive relations to account for material [polarization](/page/Polarization) and [magnetization](/page/Magnetization), but the forms above apply directly to free charges and currents in empty space.[](https://bohr.physics.berkeley.edu/classes/221/1112/notes/emunits.pdf) ### Fundamental Laws: [Electrostatics](/page/Electrostatics) and [Magnetostatics](/page/Magnetostatics) In Gaussian units, the [fundamental](/page/Fundamental) [law](/page/Law) governing the electrostatic [force](/page/Force) between two point charges $q_1$ and $q_2$ separated by a [distance](/page/Distance) $r$ in [vacuum](/page/Vacuum) is [Coulomb's law](/page/Coulomb's_law), expressed as $\mathbf{F} = \frac{q_1 q_2}{r^2} \hat{\mathbf{r}}$, where the force is measured in dynes, distance in centimeters, and charge in statcoulombs (esu).[](https://physics.byu.edu/faculty/colton/docs/phy441-fall19/lecture-41-gaussian-units.pdf)[](https://bohr.physics.berkeley.edu/classes/221/1112/notes/emunits.pdf) This formulation lacks the $\frac{1}{4\pi\epsilon_0}$ factor present in SI units, as the unit of charge is defined such that the proportionality constant is unity, effectively setting $\epsilon_0 = \frac{1}{4\pi}$.[](https://physics.byu.edu/faculty/colton/docs/phy441-fall19/lecture-41-gaussian-units.pdf) The electrostatic field $\mathbf{E}$ produced by a charge distribution satisfies Gauss's law, which in integral form states that the flux through a closed surface is $\oint \mathbf{E} \cdot d\mathbf{A} = 4\pi Q_{\rm enc}$, where $Q_{\rm enc}$ is the total enclosed charge.[](https://physics.byu.edu/faculty/colton/docs/phy441-fall19/lecture-41-gaussian-units.pdf) In differential form, this becomes $\nabla \cdot \mathbf{E} = 4\pi \rho$, with $\rho$ the charge density in esu/cm³ and $\mathbf{E}$ in statvolts/cm (equivalent to dynes/esu).[](https://bohr.physics.berkeley.edu/classes/221/1112/notes/emunits.pdf) This law encapsulates the inverse-square nature of the electric field and simplifies calculations for symmetric charge distributions, such as spheres or infinite planes, without additional permittivity factors. For magnetostatics, [Gauss's law](/page/Gauss's_law) asserts the absence of magnetic monopoles, given by $\oint \mathbf{B} \cdot d\mathbf{A} = 0$ for any closed surface, or in differential form $\nabla \cdot \mathbf{B} = 0$, where $\mathbf{B}$ is the [magnetic field](/page/Magnetic_field) in gauss.[](https://physics.byu.edu/faculty/colton/docs/phy441-fall19/lecture-41-gaussian-units.pdf) This reflects the experimental observation that magnetic field lines form closed loops, with no isolated north or south poles, distinguishing [magnetism](/page/Magnetism) from [electrostatics](/page/Electrostatics) in the Gaussian framework. The [magnetic field](/page/Magnetic_field) due to a steady [current](/page/Current) is described by the Biot-Savart law: \mathbf{B}(\mathbf{r}) = \frac{1}{c} \int \frac{I d\mathbf{l}' \times (\mathbf{r} - \mathbf{r}')}{|\mathbf{r} - \mathbf{r}'|^3}, where $I$ is the [current](/page/Current) in emu (or biot), $d\mathbf{l}'$ the differential length element along the current path, $c$ the [speed of light](/page/Speed_of_light) in cm/s, and the [integral](/page/Integral) is over the current distribution.[](https://web2.ph.utexas.edu/~vadim/Classes/2016s/BSL.pdf) This expression introduces the factor $1/c$ to maintain dimensional consistency in the cgs system, linking electric currents to magnetic effects. Ampère's law in the static case relates the magnetic field to currents via $\oint \mathbf{B} \cdot d\mathbf{l} = \frac{4\pi}{c} I_{\rm enc}$, or differentially $\nabla \times \mathbf{B} = \frac{4\pi}{c} \mathbf{J}$, where $\mathbf{J}$ is the [current density](/page/Current_density) in emu/cm² and $I_{\rm enc}$ the enclosed [current](/page/Current).[](https://people.umass.edu/~prokofev/P605/Mstatics.pdf) This circuital law is particularly useful for finding $\mathbf{B}$ in symmetric configurations, such as infinite straight wires or solenoids, and underscores the role of currents as sources of magnetic fields in Gaussian units. ### Potentials and Wave Equations In Gaussian units, the electromagnetic fields can be expressed in terms of a [scalar potential](/page/Scalar_potential) $\phi$ and a [vector potential](/page/Vector_potential) $\mathbf{A}$. The [magnetic field](/page/Magnetic_field) $\mathbf{B}$ is given by $\mathbf{B} = \nabla \times \mathbf{A}$, which satisfies $\nabla \cdot \mathbf{B} = 0$ automatically through the vector identity $\nabla \cdot (\nabla \times \mathbf{A}) = 0$.[](https://bohr.physics.berkeley.edu/classes/221/1112/notes/emunits.pdf) The [electric field](/page/Electric_field) $\mathbf{E}$ is then $\mathbf{E} = -\nabla \phi - \frac{1}{c} \frac{\partial \mathbf{A}}{\partial t}$, where $c$ is the [speed of light](/page/Speed_of_light), ensuring consistency with Faraday's law $\nabla \times \mathbf{E} = -\frac{1}{c} \frac{\partial \mathbf{B}}{\partial t}$.[](https://bohr.physics.berkeley.edu/classes/221/1112/notes/emunits.pdf) These potential formulations are not unique due to gauge freedom, allowing transformations $\mathbf{A}' = \mathbf{A} + \nabla \chi$ and $\phi' = \phi - \frac{1}{c} \frac{\partial \chi}{\partial t}$ for an arbitrary scalar $\chi$, which leave $\mathbf{E}$ and $\mathbf{B}$ unchanged.[](https://bohr.physics.berkeley.edu/classes/221/1112/notes/emunits.pdf) A convenient choice is the Lorentz gauge, defined by $\nabla \cdot \mathbf{A} + \frac{1}{c} \frac{\partial \phi}{\partial t} = 0$, which simplifies the equations of motion and preserves Lorentz invariance.[](http://kirkmcd.princeton.edu/examples/maxwell.pdf) Substituting the potentials into [Gauss's law](/page/Gauss's_law) $\nabla \cdot \mathbf{E} = 4\pi \rho$ yields $-\nabla^2 \phi - \frac{1}{c} \frac{\partial}{\partial t} (\nabla \cdot \mathbf{A}) = 4\pi \rho$. In the Lorentz [gauge](/page/Gauge), this decouples to the inhomogeneous [wave equation](/page/Wave_equation) $\square \phi = -4\pi \rho$, where the d'Alembertian operator is $\square = \nabla^2 - \frac{1}{c^2} \frac{\partial^2}{\partial t^2}$.[](http://kirkmcd.princeton.edu/examples/maxwell.pdf) Similarly, inserting into Ampère's law $\nabla \times \mathbf{B} = \frac{4\pi}{c} \mathbf{J} + \frac{1}{c} \frac{\partial \mathbf{E}}{\partial t}$ and using vector identities and the Lorentz [gauge](/page/Gauge) gives $\square \mathbf{A} = -\frac{4\pi}{c} \mathbf{J}$.[](http://kirkmcd.princeton.edu/examples/maxwell.pdf) These derivations follow directly from [Maxwell's equations](/page/Maxwell's_equations) by applying the curl and [divergence](/page/Divergence) operators and exploiting the [gauge](/page/Gauge) [condition](/page/Condition) to eliminate cross terms.[](https://physics.byu.edu/faculty/colton/docs/phy441-fall19/lecture-41-gaussian-units.pdf) The wave equations highlight the propagation of electromagnetic disturbances at speed $c$, with sources $\rho$ and $\mathbf{J}$. In vacuum, where $\rho = 0$ and $\mathbf{J} = 0$, the homogeneous equations $\square \phi = 0$ and $\square \mathbf{A} = 0$ describe plane waves where $\mathbf{E}$ and $\mathbf{B}$ are perpendicular, transverse to the propagation direction, and $|\mathbf{E}| = |\mathbf{B}|$.[](https://physics.byu.edu/faculty/colton/docs/phy441-fall19/lecture-41-gaussian-units.pdf) To ensure causality, the solutions are retarded potentials: $\phi(\mathbf{r}, t) = \int \frac{[\rho(\mathbf{r}', t_r)]}{|\mathbf{r} - \mathbf{r}'|} d^3 r'$ and $\mathbf{A}(\mathbf{r}, t) = \frac{1}{c} \int \frac{[\mathbf{J}(\mathbf{r}', t_r)]}{|\mathbf{r} - \mathbf{r}'|} d^3 r'$, evaluated at the retarded time $t_r = t - \frac{|\mathbf{r} - \mathbf{r}'|}{c}$.[](http://kirkmcd.princeton.edu/examples/maxwell.pdf) ## Applications in Circuits and Materials ### Electrical Circuits and [Ohm's Law](/page/Ohm's_law) In the Gaussian system, electrical circuits are analyzed using a combination of electrostatic (esu) and electromagnetic ([emu](/page/Emu)) units, reflecting the hybrid nature of the framework for handling electric and magnetic phenomena separately. [Current](/page/Current) is expressed in statamperes (esu/s) for electrostatic contexts or abamperes ([emu](/page/Emu)/s, where 1 abampere = 10 amperes) for electromagnetic ones, while voltage is measured in statvolts (esu/cm).[](https://caps.gsfc.nasa.gov/simpson/ref/gaussian.html)[](https://uspas.fnal.gov/materials/02Yale/Units.pdf) Resistance adopts the unit of statohm (s/cm), equivalent to approximately $9 \times 10^{11}$ ohms, ensuring dimensional consistency in lumped-element approximations where fields are treated as uniform across components.[](https://physics.nist.gov/cuu/pdf/sp811.pdf) This setup allows circuit analysis to proceed analogously to [SI](/page/Si) but requires careful unit matching to avoid inconsistencies arising from the [speed of light](/page/Speed_of_light) $c$. Ohm's law retains its familiar form, $V = IR$, relating voltage $V$, current $I$, and [resistance](/page/Resistance) $R$ directly in purely electrostatic circuits using esu throughout.[](https://web.mit.edu/sahughes/www/8.022/lec07.pdf) However, in mixed esu-emu applications common to circuits with inductive or magnetic elements, the relation demands attention to the factor of $c$ (approximately $3 \times 10^{10}$ cm/s) for unit compatibility, as emu currents pair with abvolts (statvolts divided by $c$). Power dissipation follows $P = IV$, yielding energy in ergs per second when consistent units are used, though the mixed convention introduces a $1/c$ scaling in cross-domain calculations to align electric and magnetic contributions.[](https://web2.ph.utexas.edu/~vadim/Classes/2024s-u/Inductance.pdf)[](http://kirkmcd.princeton.edu/examples/loop.pdf) Kirchhoff's laws apply unchanged in form to Gaussian-unit circuits: the current law states that the algebraic sum of currents at a [junction](/page/Junction) is zero ($\sum I = 0$), and the voltage law requires the sum of potential differences around a closed loop to be zero ($\sum V = 0$).[](https://web.mit.edu/8.02t/www/802TEAL3D/visualizations/coursenotes/modules/guide06.pdf) These principles facilitate solving networks of resistors, [inductors](/page/Inductance), and [capacitors](/page/Capacitance), with [inductance](/page/Inductance) $L$ measured in centimeters (where 1 cm = $10^{-9}$ [henry](/page/Henry)) and [capacitance](/page/Capacitance) $C$ also in centimeters (where 1 cm $\approx 1.11$ [pF](/page/PF)).[](https://web2.ph.utexas.edu/~vadim/Classes/2024s-u/Inductance.pdf)[](https://uspas.fnal.gov/materials/02Yale/Units.pdf) A distinctive feature of Gaussian units is that mutual inductance $M$ between two circuits shares the unit of centimeters, mirroring self-inductance and simplifying geometric computations for [coil](/page/Coil) pairs based on their spatial [configuration](/page/Configuration). This dimensional uniformity aids analytical expressions but necessitates explicit inclusion of $c$ when extending to high-frequency effects, such as electromagnetic wave propagation in transmission lines.[](https://web2.ph.utexas.edu/~vadim/Classes/2024s-u/Inductance.pdf) Historically, Gaussian units found extensive application in early radio [engineering](/page/Engineering), where cgs conventions streamlined calculations for vacuum-tube oscillators and [antenna](/page/Antenna) circuits before the widespread adoption of [SI](/page/Si) in the mid-20th century.[](https://nvlpubs.nist.gov/nistpubs/Legacy/MONO/nbsmonograph56.pdf) ### Dielectrics, Magnetics, and Constitutive Relations In Gaussian units, the constitutive relations for linear isotropic media describe the response of dielectrics and magnetics to applied fields. The [electric displacement field](/page/Electric_displacement_field) $\mathbf{D}$ relates to the [electric field](/page/Electric_field) $\mathbf{E}$ and [polarization](/page/Polarization) $\mathbf{P}$ as $\mathbf{D} = \mathbf{E} + 4\pi \mathbf{P}$, and for linear media, $\mathbf{D} = \kappa \mathbf{E}$, where $\kappa$ is the dielectric constant ([permittivity](/page/Permittivity)). Similarly, the [magnetic induction](/page/Magnetic_induction) $\mathbf{B}$ relates to the [magnetic field](/page/Magnetic_field) $\mathbf{H}$ and magnetization $\mathbf{M}$ as $\mathbf{B} = \mathbf{H} + 4\pi \mathbf{M}$, with $\mathbf{B} = \mu \mathbf{H}$ for linear media, where $\mu$ is the magnetic permeability. In [vacuum](/page/Vacuum), $\kappa = \mu = 1$.[](https://pdg.lbl.gov/1999/elecrelarpp.pdf) The electric susceptibility $\chi_e$ and magnetic susceptibility $\chi_m$ quantify the material's response: $\mathbf{P} = \chi_e \mathbf{E}$ and $\mathbf{M} = \chi_m \mathbf{H}$, leading to $\chi_e = (\kappa - 1)/4\pi$ and $\chi_m = (\mu - 1)/4\pi$. For conducting media, the current density $\mathbf{J}$ follows Ohm's law as $\mathbf{J} = \sigma \mathbf{E}$, where $\sigma$ is the conductivity, incorporating free charge motion into the overall response. In dynamic scenarios, dispersion and loss are captured by frequency-dependent complex quantities, such as the complex dielectric constant $\kappa(\omega) = \kappa'(\omega) + i \kappa''(\omega)$, where the real part $\kappa'$ relates to energy storage and the imaginary part $\kappa''$ to dissipation in dielectrics. The Poynting theorem expresses energy flux as the vector $\mathbf{S} = (c/4\pi) \mathbf{E} \times \mathbf{H}$, with $c$ the speed of light, highlighting power flow and absorption in lossy media. The electromagnetic energy density in materials is given by $u = (\mathbf{E} \cdot \mathbf{D} + \mathbf{B} \cdot \mathbf{H})/8\pi$, encompassing both field and material contributions.[](https://ethz.ch/content/dam/ethz/special-interest/itet/photonics-dam/documents/lectures/EandM/ConstitutiveRelations.pdf)[](https://pdg.lbl.gov/1999/elecrelarpp.pdf)[](https://arxiv.org/pdf/0707.3421v4) Boundary conditions at interfaces incorporate these relations distinctly in Gaussian units. The normal component of $\mathbf{D}$ jumps by $4\pi \sigma_f$, where $\sigma_f$ is the free surface charge density, while the tangential $\mathbf{E}$ is continuous. For magnetics, the normal $\mathbf{B}$ is continuous, and the tangential component of $\mathbf{H}$ jumps by $\frac{4\pi}{c} \mathbf{K} \times \hat{\mathbf{n}}$, where $\mathbf{K}$ is the free surface current density, with $4\pi$ factors arising from the unit system's structure. These conditions ensure consistency in solving field problems across material boundaries.[](https://pdg.lbl.gov/1999/elecrelarpp.pdf)[](https://bohr.physics.berkeley.edu/classes/221/1112/notes/emunits.pdf) For anisotropic media, the relations generalize to tensor forms, with $\mathbf{D} = \overleftrightarrow{\kappa} \cdot \mathbf{E}$ and $\mathbf{B} = \overleftrightarrow{\mu} \cdot \mathbf{H}$, where $\overleftrightarrow{\kappa}$ and $\overleftrightarrow{\mu}$ are [permittivity](/page/Permittivity) and permeability tensors reflecting directional dependencies. Susceptibilities become tensors as well, $\overleftrightarrow{\chi_e} = (\overleftrightarrow{\kappa} - \overleftrightarrow{I})/4\pi$ and $\overleftrightarrow{\chi_m} = (\overleftrightarrow{\mu} - \overleftrightarrow{I})/4\pi$, with $\overleftrightarrow{I}$ the identity tensor. In photonics research, such tensor descriptions are essential for designing metamaterials and nanostructures enabling [polarization](/page/Polarization) control and enhanced light-matter interactions.[](https://bohr.physics.berkeley.edu/classes/221/1112/notes/emunits.pdf) ## Constants, Named Units, and Conversions ### Fundamental Physical Constants In Gaussian units, several fundamental physical constants take on forms that simplify electromagnetic equations by setting the [vacuum permittivity](/page/Vacuum_permittivity) ε₀ and [vacuum permeability](/page/Vacuum_permeability) μ₀ to unity (dimensionless). This choice eliminates explicit factors of 4π in [Coulomb's law](/page/Coulomb's_law) and related expressions, making the [elementary charge](/page/Elementary_charge) e a more "natural" scale for interactions. The [speed of light](/page/Speed_of_light) c serves as a linking constant, explicitly appearing in [Maxwell's equations](/page/Maxwell's_equations) to connect electric and magnetic phenomena. The exact value of the speed of light in vacuum is c = 2.99792458 × 10^{10} cm/s, which is pivotal in Gaussian units for defining the relationship between electrostatic and electromagnetic units.[](https://physics.nist.gov/cgi-bin/cuu/Value?c) Post-2019 SI redefinition, the elementary charge is exactly e = 1.602176634 × 10^{-19} C in SI units, corresponding to e = 4.80320425 × 10^{-10} statcoulombs (esu) via the conversion factor 1 statC = 3.335641 × 10^{-10} C.[](https://physics.nist.gov/cgi-bin/cuu/Value?e)[](https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9) Planck's constant h = 6.62607015 × 10^{-27} erg·s provides the quantum scale, with the reduced form ℏ = h / 2π.[](https://physics.nist.gov/cgi-bin/cuu/Value?h) A key dimensionless constant in Gaussian units is the fine-structure constant α = e² / (ℏ c) ≈ 7.2973525693 × 10^{-3} (or 1/α ≈ 137.035999206), which quantifies the strength of electromagnetic interactions and lacks the 4π ε₀ factor present in [SI](/page/Si) formulations.[](https://physics.nist.gov/cgi-bin/cuu/Value?alph) This form highlights the natural scale of charge in Gaussian units, where α directly measures the ratio of electromagnetic to quantum scales without additional [vacuum](/page/Vacuum) constants. The [impedance of free space](/page/Impedance_of_free_space) Z_0 emerges as Z_0 = 4π / c in Gaussian units, numerically equivalent to approximately 377 Ω when expressed in [SI](/page/Si)-compatible terms, reflecting the balanced electric-magnetic field strengths in [vacuum](/page/Vacuum) waves. | Constant | Symbol | Value in Gaussian Units | Role | |----------|--------|--------------------------|------| | Speed of light | c | 2.99792458 × 10^{10} cm/s (exact) | Links electrostatic and magnetostatic units; appears explicitly in Maxwell's equations.[](https://physics.nist.gov/cgi-bin/cuu/Value?c) | | Elementary charge | e | 4.80320425 × 10^{-10} statC (exact) | Defines the unit of charge (statcoulomb); scales Coulomb's law without 4π factor.[](https://physics.nist.gov/cgi-bin/cuu/Value?e)[](https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9) | | Planck's constant | h | 6.62607015 × 10^{-27} erg·s (exact) | Sets energy-frequency relation in quantum electrodynamics.[](https://physics.nist.gov/cgi-bin/cuu/Value?h) | | Fine-structure constant | α | ≈ 7.2973525693 × 10^{-3} (dimensionless) | Measures electromagnetic coupling strength; α = e² / (ℏ c).[](https://physics.nist.gov/cgi-bin/cuu/Value?alph) | | Vacuum permittivity | ε₀ | 1 (dimensionless) | Simplifies electrostatic equations in vacuum. | | Vacuum permeability | μ₀ | 1 (dimensionless) | Simplifies magnetostatic equations in vacuum. | | Impedance of free space | Z_0 | 4π / c ≈ 377 Ω (effective) | Ratio of electric to magnetic field amplitudes in plane waves. ### Named Electromagnetic Units In the Gaussian system of units, which is based on the centimeter-gram-second (cgs) framework, several named units derive from historical definitions rooted in 19th-century experiments on [electricity](/page/Electricity) and [magnetism](/page/Magnetism). These units emerged from efforts to quantify electromagnetic phenomena without introducing arbitrary factors like 4π, blending elements of the electrostatic (esu) and electromagnetic (emu) subsystems. Many originated in the works of pioneers such as [Hans Christian Ørsted](/page/Hans_Christian_Ørsted), [Carl Friedrich Gauss](/page/Carl_Friedrich_Gauss), James Clerk Maxwell, and William Gilbert, who advanced early theories of [magnetism](/page/Magnetism) and [electricity](/page/Electricity).[](https://micro.magnet.fsu.edu/electromag/pioneers/index.html)[](https://www.magcraft.com/history-of-magnetism-and-electricity) For electric quantities in the esu subsystem, the [statcoulomb](/page/Statcoulomb) (symbol: statC or esu) serves as the fundamental unit of charge, defined as the charge that exerts a force of 1 [dyne](/page/Dyne) on an identical charge separated by 1 cm in vacuum, per [Coulomb's law](/page/Coulomb's_law).[](https://phys.libretexts.org/Bookshelves/Electricity_and_Magnetism/Electricity_and_Magnetism_(Tatum)/16%3A_CGS_Electricity_and_Magnetism/16.02%3A_The_CGS_Electrostatic_System) The [statvolt](/page/Statvolt) (symbol: statV) is the unit of [electric potential](/page/Electric_potential), representing the potential difference across which 1 [statcoulomb](/page/Statcoulomb) of charge experiences a work of 1 erg.[](https://bohr.physics.berkeley.edu/classes/221/1112/notes/emunits.pdf) Derived from this, the statampere (symbol: statA) denotes current as the flow of 1 [statcoulomb](/page/Statcoulomb) per second, while the statohm (symbol: statΩ) measures resistance as the ratio of 1 [statvolt](/page/Statvolt) to 1 statampere.[](https://www.gnu.org/software/units/manual/html_node/CGS-Units-Systems.html) These esu units prioritize electrostatic interactions and were formalized in the late [19th century](/page/19th_century) to align with cgs mechanical units. Magnetic quantities draw from the emu subsystem, where the oersted (symbol: [Oe](/page/Oe)) defines magnetic field strength [H](/page/H+) as the field exerting a force of 1 [dyne](/page/Dyne) on a [unit](/page/Unit) magnetic pole (1 emu of pole strength).[](https://phys.libretexts.org/Bookshelves/Electricity_and_Magnetism/Electricity_and_Magnetism_(Tatum)/16%3A_CGS_Electricity_and_Magnetism/16.03%3A_The_CGS_Electromagnetic_System) The [gauss](/page/G) (symbol: [G](/page/G)) is the corresponding [unit](/page/Unit) for magnetic flux density [B](/page/List_of_punk_rap_artists), numerically equal to the oersted in [vacuum](/page/Vacuum) due to the dimensionless permeability of free space in Gaussian units; it was named for Gauss's contributions to geomagnetism in the [1830s](/page/1830s).[](https://physics.byu.edu/faculty/colton/docs/phy441-fall19/lecture-41-gaussian-units.pdf) Magnetic flux uses the [maxwell](/page/Maxwell) (symbol: [Mx](/page/.mx)), defined as the flux through a 1 cm² area under a field of 1 gauss, with 1 Mx equivalent to 10^{-8} weber in [SI](/page/Si).[](https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-5-units-outside-si) The [gilbert](/page/Gilbert) (symbol: [Gb](/page/GB)), named after Gilbert's 1600 treatise *[De Magnete](/page/De_Magnete)*, quantifies [magnetomotive force](/page/Magnetomotive_force) as the force driving 1 emu of pole strength across a [unit](/page/Unit) path.[](https://www.qsl.net/g4cnn/units/units.htm) The emu subsystem also includes the abampere (symbol: abA or Bi, after [Jean-Baptiste Biot](/page/Jean-Baptiste_Biot)), the unit of current defined such that two infinite straight wires 1 cm apart, each carrying 1 abampere, experience a force of 1 [dyne](/page/Dyne) per cm of length; it equals 10 amperes.[](https://phys.libretexts.org/Bookshelves/Electricity_and_Magnetism/Electricity_and_Magnetism_(Tatum)/16%3A_CGS_Electricity_and_Magnetism/16.03%3A_The_CGS_Electromagnetic_System) Underlying these are mechanical base units like the [dyne](/page/Dyne) (symbol: dyn), the force accelerating 1 g by 1 cm/s², and the erg (symbol: erg), the energy from 1 [dyne](/page/Dyne) acting over 1 cm—both integral to cgs since the [19th century](/page/19th_century).[](https://physics.byu.edu/faculty/colton/docs/phy441-fall19/lecture-41-gaussian-units.pdf) Although some units like the [oersted](/page/Oersted) and [gilbert](/page/Gilbert) are now obsolete in modern practice, they persist in historical literature and specialized fields such as geomagnetism.[](https://nvlpubs.nist.gov/nistpubs/jres/83/jresv83n1p9_A1b.pdf) | Category | Unit Name | Symbol | Definition | |----------|-----------|--------|------------| | Electric (esu) | [Statcoulomb](/page/Statcoulomb) | statC | Charge repelling equal charge at 1 cm with 1 [dyne](/page/Dyne) force | | Electric (esu) | [Statvolt](/page/Statvolt) | statV | Potential for 1 erg work on 1 [statcoulomb](/page/Statcoulomb) | | Electric (esu) | Statampere | statA | 1 [statcoulomb](/page/Statcoulomb) per second | | Electric (esu) | Statohm | statΩ | statV per statA | | Magnetic (emu) | [Oersted](/page/Oersted) | Oe | H field exerting 1 [dyne](/page/Dyne) on unit pole | | Magnetic (emu) | Gauss | G | B field; equals Oe in [vacuum](/page/Vacuum) | | Magnetic (emu) | [Maxwell](/page/Maxwell) | Mx | Flux of 1 G through 1 cm² | | Magnetic (emu) | [Gilbert](/page/Gilbert) | Gb | [Magnetomotive force](/page/Magnetomotive_force) for unit pole | | Current (emu) | Abampere | abA | Current giving 1 [dyne](/page/Dyne)/cm force on parallel wire 1 cm away | | Mechanical | [Dyne](/page/Dyne) | dyn | Force of 1 g·cm/s² | | Mechanical | Erg | erg | Work of 1 dyn over 1 cm | ### Translation Rules Between Systems Translating formulas and numerical values between SI and Gaussian units requires accounting for the different definitions of electromagnetic quantities, particularly the absence of ε₀ and μ₀ as explicit constants in Gaussian units and the explicit appearance of the speed of light c to unify electric and magnetic sectors. In electrostatics, the SI form of Coulomb's law, F = q_1 q_2 / (4π ε₀ r^2), becomes F = q_1 q_2 / r^2 in Gaussian units by setting 4π ε₀ = 1, effectively incorporating the vacuum permittivity into the definition of charge. For magnetostatics, the SI Biot-Savart law includes a factor of μ₀ / (4π), which is replaced by 1/c in Gaussian units, where c is the speed of light; similarly, the magnetic term in the Lorentz force law F = q (E + v × B) in SI becomes F = q (E + (v/c) × B) in Gaussian units to maintain dimensional consistency. These substitutions ensure that Maxwell's equations retain their symmetric form in Gaussian units without dimensionful constants like ε₀ or μ₀.[](https://uspas.fnal.gov/materials/02Yale/Units.pdf)[](https://bohr.physics.berkeley.edu/classes/221/1112/notes/emunits.pdf) Numerical conversions between the systems also involve scaling factors derived from the base unit differences (e.g., meter to centimeter for length) and the electromagnetic definitions. The numerical value of charge in Gaussian esu (statcoulombs) is q_G = q_SI × (c / 10), where c ≈ 2.99792458 × 10^{10} cm/s is the [speed of light](/page/Speed_of_light), yielding q_G ≈ q_SI × 2.99792458 × 10^9 for q_SI in coulombs; this factor arises from equating the force in both systems while adjusting for [dyne](/page/Dyne) (10^{-5} [N](/page/N+)) and centimeter units. For the [electric field](/page/Electric_field), the numerical value in Gaussian units (statvolts per centimeter) is E_G = E_SI / (c / 10^6), where c in m/s gives E_G ≈ E_SI / 2.99792458 × 10^4 for E_SI in volts per meter, since 1 statV/cm = (c / 10^6) V/m with c ≈ 3 × 10^8 m/s. The [magnetic induction](/page/Magnetic_induction) B in gauss is B_G = B_SI × 10^4 for B_SI in teslas, as 1 G = 10^{-4} T. [Current](/page/Current) in electromagnetic units (emu) converts as I_G = I_SI / 10 for I_SI in amperes, reflecting that 1 emu of [current](/page/Current) = 10 A. Length scales as L_G = L_SI × 100 (cm to m), while time and mass units remain identical (seconds and grams). These factors use the 2022 CODATA values for precision, with c exact at 299792458 m/s and ε₀ = 8.8541878188(14) × 10^{-12} F/m, ensuring accurate scaling since μ₀ = 4π × 10^{-7} H/m exactly./16%3A_CGS_Electricity_and_Magnetism/16.05%3A_Conversion_Factors)[](https://www.nist.gov/document/magneticunitspdf)[](https://physics.nist.gov/cuu/pdf/wallet_2022.pdf) A unique aspect of Gaussian units is the dimensional analysis for charge, derived directly from the force law without separate base units for current or charge. The dimension of charge [Q]_G satisfies [F] = [Q]^2 [L]^{-2}, with [F] = M L T^{-2} and [L] = L, yielding [Q]_G = M^{1/2} L^{3/2} T^{-1}; this contrasts with SI, where [Q]_{SI} = I T (independent via the ampere). This dimensional structure facilitates formula translations by matching powers of mass, length, and time, eliminating ε₀ (which has dimensions in SI) from equations. For example, in SI, ε₀ has [ε₀] = M^{-1} L^{-3} T^4 I^2, but it disappears in Gaussian expressions due to the embedded charge dimension.[](https://bohr.physics.berkeley.edu/classes/221/1112/notes/emunits.pdf)[](https://uspas.fnal.gov/materials/02Yale/Units.pdf) The following table summarizes common conversion factors for numerical values (Gaussian numerical value = SI numerical value × factor), using 2022 CODATA precision where applicable: | Quantity | SI Unit | Gaussian Unit | Factor | |----------------|-------------|---------------|-------------------------| | Charge q | C | esu | 2.99792458 × 10^9 | | Electric field E | V/m | statV/cm | 1 / 2.99792458 × 10^4 | | Magnetic field B | T | G | 10^4 | | Current I | A | [emu](/page/Emu) | 1 / 10 | | Length L | m | cm | 100 | | Force F | N | dyne | 10^5 | | Energy | J | erg | 10^7 | These conversions apply directly to measured values, with ε₀ and μ₀ absorbed into the unit definitions, allowing seamless numerical translation once formulas are adjusted./16%3A_CGS_Electricity_and_Magnetism/16.05%3A_Conversion_Factors)[](https://www.nist.gov/document/magneticunitspdf)[](https://physics.nist.gov/cuu/pdf/wallet_2022.pdf)

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