Lorentz–Heaviside units

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Lorentz–Heaviside units (or Heaviside–Lorentz units) constitute a system of units (particularly electromagnetic units) within CGS, named for Hendrik Antoon Lorentz and Oliver Heaviside. They share with CGS-Gaussian units the property that the electric constant ε0 and magnetic constant µ0 do not appear, having been incorporated implicitly into the unit system and electromagnetic equations. Lorentz–Heaviside units may be regarded as normalizing ε0 = 1 and µ0 = 1, while at the same time revising Maxwell's equations to use the speed of light c instead.

Lorentz–Heaviside units, like SI units but unlike Gaussian units, are rationalized, meaning that there are no factors of 4π appearing explicitly in Maxwell's equations.[1][2] The fact that these units are rationalized partly explains their appeal in quantum field theory: the Lagrangian underlying the theory does not have any factors of 4π in these units.[1] Consequently Lorentz–Heaviside units differ by factors of 4π in the definitions of the electric and magnetic fields and of electric charge. They are particularly convenient when performing calculations in spatial dimensions greater than three such as in string theory. They are often used in relativistic calculations.

Length–Mass–Time Framework[edit]

As in the Gaussian units, the Heaviside–Lorentz units use the length–mass–time dimensions. This means that all of the electric and magnetic units are derived units, dependent on the sizes of length and force.

Coulomb's equation, used to derive the unit of charge, is F = QQ/r2 in the Gaussian system, and F = qq/4πr2 in the HLU. The unit of charge then connects to 1 dyn cm2 = 1 esu2 = 4π hlu. The HLU charge is then 4π larger than the Gaussian (see below), and the rest follow.

When the dimensional analysis for the Gaussian units are used, including ε and μ are used to convert units, the result gives the conversion to and from the Heaviside–Lorents units. For example, charge is ε L3MT−2. When one puts ε = 8.854 pF/m, L = 0.01 m, M = 0.001 kg, and T = 1 second, this evaluates as 9.409669×10−11 C. This is the size of the HLU unit of charge.

Because the Heaviside–Lorentz units continue to use separate electric and magnetic units, an additional constant is needed when electric and magnetic quantities appear in the same formula. As in the Gaussian system, this constant appears as the electromagnetic velocity c.

Rationalization[edit]

In system-independent form, the Maxwell equations are

 \begin{align}
\nabla \cdot \mathbf{D} &= \rho / \beta, \\
 \quad \nabla \cdot \mathbf{B} &= 0, \\
 \quad \kappa \nabla \times \mathbf{E} &= -\frac{\partial \mathbf{B}}{\partial t}, \\
 \quad \kappa \nabla \times \mathbf{H} &= \frac{\partial \mathbf{D}}{\partial t} + \mathbf{J} / \beta,
\end{align}

along with D = ε0E and B = μ0H. The constants β and κ vary from system to system. One can show that ε0μ0c2 = κ2.

The Gaussian system puts β = 1/4π, κ = c.
The HLU system puts β = 1, κ = c.
The SI system puts β = 1, κ = 1.

What rationalisation does is to replace the radiance constant (γ = intensity at radius2 / source) with the gaussian divergence constant (β = flux through a surface / enclosed sources). One can easily show that γ = 4πβ, by considering the case of a sphere around a point, and intensity as density of flux. The older models set γ = 1, while the rationalised systems have β = 1. Rationalized equations in physics generally have a factor related to the effective spatial symmetry: 1 for planar symmetry, 2π for cylindrical symmetry and 4π for spherical symmetry.

The constant κ connects the electric and magnetic units through Q = Iκt. When electric and magnetic systems are defined as in the Gaussian or Heaviside–Lorentz systems, κ = c derives from the electromagnetic wave equations. Most systems have κ = 1, where the electric and magnetic systems are connected by Q = It.

Maxwell's equations with sources[edit]

With Lorentz–Heaviside units, Maxwell's equations in free space with sources take the following form:

 \nabla \cdot \mathbf{E} = \rho \,
 \nabla \cdot \mathbf{B} = 0 \,
 \nabla \times \mathbf{E} = -\frac{1}{c} \frac{\partial \mathbf{B}} {\partial t} \,
 \nabla \times \mathbf{B} = \frac{1}{c} \frac{ \partial \mathbf{E}} {\partial t} + \frac{1}{c} \mathbf{J} \,

where c is the speed of light in vacuum. Here E = D is the electric field, H = B is the magnetic field, ρ is charge density, and J is current density.

The Lorentz force equation is:

\mathbf{F}_q = q \left(\mathbf{E} + \frac{\mathbf{v}_q}{c} \times \mathbf{B}\right) \,

here q is a the charge of a test particle with vector velocity vq and Fq is the combined electric and magnetic force acting on that test particle.

In both the Gaussian and Heaviside–Lorentz systems, the electrical and magnetic units are derived from the mechanical systems. Charge is defined through Coulomb's equation, with ε = 1. In the gaussian system, Coulomb's equation is F = QQ/R2. In the Heaviside Lorentz system, F = qq/4πR2. From this, one sees that QQ = qq/4π, that the Gaussian units are larger by a factor of 4π. Other quantities follow as follows.

 q_\mathrm{LH} \ = \ \sqrt{4\pi} \ q_\mathrm{G}
 \mathbf{E}_\mathrm{LH} \ = \ {\mathbf{E}_\mathrm{G} \over \sqrt{4\pi}}
 \mathbf{B}_\mathrm{LH} \ = \ {\mathbf{B}_\mathrm{G} \over \sqrt{4\pi}} .

List of equations and comparison with other systems of units[edit]

This section has a list of the basic formulae of electromagnetism, given in Lorentz-Heaviside, 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.

Maxwell's equations[edit]

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 Lorentz–Heaviside units Gaussian units SI units
Gauss's law
(macroscopic)
\nabla \cdot \mathbf{D} = \rho_\text{f} \nabla \cdot \mathbf{D} = 4\pi\rho_\text{f} \nabla \cdot \mathbf{D} = \rho_\text{f}
Gauss's law
(microscopic)
\nabla \cdot \mathbf{E} = \rho \nabla \cdot \mathbf{E} = 4\pi\rho \nabla \cdot \mathbf{E} = \rho/\epsilon_0
Gauss's law for magnetism: \nabla \cdot \mathbf{B} = 0 \nabla \cdot \mathbf{B} = 0 \nabla \cdot \mathbf{B} = 0
Maxwell–Faraday equation
(Faraday's law of induction):
\nabla \times \mathbf{E} = -\frac{1}{c}\frac{\partial \mathbf{B}} {\partial t} \nabla \times \mathbf{E} = -\frac{1}{c}\frac{\partial \mathbf{B}} {\partial t} \nabla \times \mathbf{E} = -\frac{\partial \mathbf{B}} {\partial t}
Ampère–Maxwell equation
(macroscopic):
\nabla \times \mathbf{H} = \frac{1}{c}\mathbf{J}_{\text{f}} + \frac{1}{c}\frac{\partial \mathbf{D}} {\partial t} \nabla \times \mathbf{H} = \frac{4\pi}{c}\mathbf{J}_{\text{f}} + \frac{1}{c}\frac{\partial \mathbf{D}} {\partial t} \nabla \times \mathbf{H} = \mathbf{J}_{\text{f}} + \frac{\partial \mathbf{D}} {\partial t}
Ampère–Maxwell equation
(microscopic):
\nabla \times \mathbf{B} = \frac{1}{c}\mathbf{J} + \frac{1}{c}\frac{\partial \mathbf{E}} {\partial t} \nabla \times \mathbf{B} = \frac{4\pi}{c}\mathbf{J} + \frac{1}{c}\frac{\partial \mathbf{E}} {\partial t} \nabla \times \mathbf{B} = \mu_0\mathbf{J} + \frac{1}{c^2}\frac{\partial \mathbf{E}} {\partial t}

Other basic laws[edit]

Name Lorentz–Heaviside units Gaussian units SI units
Lorentz force \mathbf{F} = q\left(\mathbf{E}+\frac{1}{c}\mathbf{v}\times\mathbf{B}\right) \mathbf{F} = q\left(\mathbf{E}+\frac{1}{c}\mathbf{v}\times\mathbf{B}\right) \mathbf{F} = q\left(\mathbf{E}+\mathbf{v}\times\mathbf{B}\right)
Coulomb's law \mathbf{F} = \frac{1}{4\pi}\frac{q_1 q_2}{r^2} \mathbf{\hat r} \mathbf{F} = \frac{q_1 q_2}{r^2} \mathbf{\hat r} \mathbf{F} = \frac{1}{4\pi\epsilon_0}\frac{q_1 q_2}{r^2} \mathbf{\hat r}
Electric field of
stationary point charge
\mathbf{E} = \frac{1}{4\pi}\frac{q}{r^2} \mathbf{\hat r} \mathbf{E} = \frac{q}{r^2} \mathbf{\hat r} \mathbf{E} = \frac{1}{4\pi\epsilon_0}\frac{q}{r^2} \mathbf{\hat r}
Biot–Savart law  \mathbf{B} = \frac{1}{4\pi} \frac{1}{c} \oint\frac{I d\mathbf{l} \times \mathbf{\hat r}}{r^2}  \mathbf{B} = \frac{1}{c} \oint\frac{I d\mathbf{l} \times \mathbf{\hat r}}{r^2}  \mathbf{B} = \frac{\mu_0}{4\pi} \oint\frac{I d\mathbf{l} \times \mathbf{\hat r}}{r^2}

Dielectric and magnetic materials[edit]

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.

Lorentz–Heaviside units Gaussian units SI units
\mathbf{D} = \mathbf{E}+\mathbf{P} \mathbf{D} = \mathbf{E}+4\pi\mathbf{P} \mathbf{D} = \epsilon_0 \mathbf{E}+\mathbf{P}
\mathbf{P} = \chi_\text{e}\mathbf{E} \mathbf{P} = \chi_\text{e}\mathbf{E} \mathbf{P} = \chi_\text{e}\epsilon_0\mathbf{E}
\mathbf{D} = \epsilon\mathbf{E} \mathbf{D} = \epsilon\mathbf{E} \mathbf{D} = \epsilon\mathbf{E}
\epsilon = 1+\chi_\text{e} \epsilon = 1+4\pi\chi_\text{e} \epsilon/\epsilon_0 = 1+\chi_\text{e}

where

The quantities \epsilon in both Lorentz-Heaviside and Gaussian units and \epsilon/\epsilon_0 in SI are dimensionless, and they have the same numeric value. By contrast, the electric susceptibility \chi_e is unitless in all the systems, but has different numeric values for the same material:

\chi_\text{e}^\text{SI} = \chi_\text{e}^\text{LH} = 4\pi \chi_\text{e}^\text{G}

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.

Lorentz–Heaviside units Gaussian units SI units
\mathbf{B} = \mathbf{H}+\mathbf{M} \mathbf{B} = \mathbf{H}+4\pi\mathbf{M} \mathbf{B} = \mu_0 (\mathbf{H}+\mathbf{M})
\mathbf{M} = \chi_\text{m}\mathbf{H} \mathbf{M} = \chi_\text{m}\mathbf{H} \mathbf{M} = \chi_\text{m} \mathbf{H}
\mathbf{B} = \mu\mathbf{H} \mathbf{B} = \mu\mathbf{H} \mathbf{B} = \mu\mathbf{H}
\mu = 1+\chi_\text{m} \mu = 1+4\pi\chi_\text{m} \mu/\mu_0 = 1+\chi_\text{m}

where

The quantities \mu in both Lorentz-Heaviside and Gaussian units and \mu/\mu_0 in SI are dimensionless, and they have the same numeric value. By contrast, the magnetic susceptibility \chi_\text{m} is unitless in all the systems, but has different numeric values for the same material:

\chi_\text{m}^\text{SI} = \chi_\text{m}^\text{LH} = 4\pi \chi_\text{m}^\text{G}

Vector and scalar potentials[edit]

The electric and magnetic fields can be written in terms of a vector potential A and a scalar potential φ:

Name Lorentz–Heaviside units Gaussian units SI units
Electric field
(static)
\mathbf{E} = -\nabla\phi \mathbf{E} = -\nabla\phi \mathbf{E} = -\nabla\phi
Electric field
(general)
\mathbf{E} = -\nabla\phi-\frac{1}{c}\frac{\partial \mathbf{A}}{\partial t} \mathbf{E} = -\nabla\phi-\frac{1}{c}\frac{\partial \mathbf{A}}{\partial t} \mathbf{E} = -\nabla\phi-\frac{\partial \mathbf{A}}{\partial t}
Magnetic B field \mathbf{B} = \nabla \times \mathbf{A} \mathbf{B} = \nabla \times \mathbf{A} \mathbf{B} = \nabla \times \mathbf{A}

General rules to translate a formula[edit]

To convert any formula from Lorentz–Heaviside units to Gaussian or to SI units, replace the quantity in the Lorentz-Heaviside column by the quantity in the Gaussian column or in the SI column (vice-versa to convert the other way). This will reproduce any of the specific formulas given in the list above, such as Maxwell's equations. It may also be necessary to use the relation \mu_0\epsilon_0 = 1/c^2 to simplify.

Name Lorentz–Heaviside units Gaussian units SI units
Electric field, Electric potential  \left(\mathbf{E}, \varphi\right)  \frac{1}{\sqrt{4\pi}} \left(\mathbf{E}, \varphi\right)  \sqrt{\varepsilon_0} \left(\mathbf{E}, \varphi\right)
Electric displacement field  \mathbf{D}  \frac{1}{\sqrt{4\pi}}\mathbf{D}  \frac{1}{\sqrt{\varepsilon_0}}\mathbf{D}
Charge, Charge density, Current,
Current density, Polarization density,
Electric dipole moment
 \left(q, \rho, I, \mathbf{J},\mathbf{P}, \mathbf{p}\right)  \sqrt{4\pi} \left(q, \rho, I, \mathbf{J},\mathbf{P}, \mathbf{p}\right)  \frac{1}{\sqrt{\varepsilon_0}}\left(q, \rho, I, \mathbf{J},\mathbf{P},\mathbf{p}\right)
Magnetic B field, Magnetic flux,
Magnetic vector potential
 \left(\mathbf{B}, \Phi_\text{m},\mathbf{A}\right)  \frac{1}{\sqrt{4\pi}} \left(\mathbf{B}, \Phi_\text{m},\mathbf{A}\right)  \frac{1}{\sqrt{\mu_0}}\left(\mathbf{B}, \Phi_\text{m},\mathbf{A}\right)
Magnetic H field  \mathbf{H}  \frac{1}{\sqrt{4\pi}} \mathbf{H}  \sqrt{\mu_0}\mathbf{H}
Magnetic moment, Magnetization  \left(\mathbf{m}, \mathbf{M}\right)  \sqrt{4\pi} \left(\mathbf{m}, \mathbf{M}\right)  \sqrt{\mu_0}\left(\mathbf{m}, \mathbf{M}\right)
Relative permittivity,
Relative permeability
 \left(\varepsilon, \mu\right)  \left(\varepsilon, \mu\right)  \left(\varepsilon_\text{r}, \mu_\text{r}\right) \equiv \left(\frac{\varepsilon}{\varepsilon_0}, \frac{\mu}{\mu_0}\right)
Electric susceptibility,
Magnetic susceptibility
 \left(\chi_\text{e}, \chi_\text{m}\right)  4\pi \left(\chi_\text{e}, \chi_\text{m}\right)  \left(\chi_\text{e}, \chi_\text{m}\right)
Conductivity, Conductance, Capacitance  \left(\sigma, S, C\right)  4\pi \left(\sigma, S, C\right)  \frac{1}{\varepsilon_0}\left(\sigma,S,C\right)
Resistivity, Resistance, Inductance  \left(\rho,R,L\right)  \frac{1}{4\pi} \left(\rho,R,L\right)  \varepsilon_0\left(\rho,R,L\right)

Replacing CGS with natural units[edit]

When one takes standard SI textbook equations, and sets ε = μ = c = 1 to get natural units, the resulting equations follow the Heaviside–Lorentz formulation and sizes. The conversion requires no changes to the factor 4π, unlike for the Gaussian equations. Coulomb's inverse-square law equation in SI is F = q1q2/4πεr2. Set ε = 1 to get the HLU form: F = q1q2/4πr2. The Gaussian form does not have the 4π in the denominator.

By setting c = 1 with HLU, Maxwell's equations and the Lorentz equation become the same as the SI example with ε = μ = c = 1.

 \nabla \cdot \mathbf{E} = \rho \,
 \nabla \cdot \mathbf{B} = 0 \,
 \nabla \times \mathbf{E} = -\frac{ \partial \mathbf{B}} {\partial t} \,
 \nabla \times \mathbf{B} =  \frac{ \partial \mathbf{E}} {\partial t} + \mathbf{J} \,
\mathbf{F}_q = q (\mathbf{E} + \mathbf{v}_q \times \mathbf{B}) \,

Because these equations can be easily related to SI work, HLU-style (i.e. rationalized) systems are becoming more fashionable.

References[edit]

  1. ^ a b Littlejohn, Robert (Fall 2011). "Gaussian, SI and Other Systems of Units in Electromagnetic Theory" (pdf). Physics 221A, University of California, Berkeley lecture notes. Retrieved 2008-05-06. 
  2. ^ Kowalski, Ludwik, 1986, "A Short History of the SI Units in Electricity," The Physics Teacher 24(2): 97–99. Alternate web link (subscription required)

External links[edit]