Vacuum permittivity
The physical constant ε0, commonly called the vacuum permittivity, permittivity of free space or electric constant, is an ideal, (baseline) physical constant, which is the value of the absolute dielectric permittivity of classical vacuum. Its value is:
ε0 = 8.854 187 817... x 10−12 [F/m] (farads per meter).
This constant relates the units for electric charge to mechanical quantities such as length and force. For example, the force between two separated electric charges (in the vacuum of classical electromagnetism) is given by Coulomb's law:
1 q1q2
Fc = _______ _______
4πε0 r^2
where q1 and q2 are the charges, and r is the distance between them. Likewise, ε0 appears in Maxwell's equations, which describe the properties of electric and magnetic fields and electromagnetic radiation, and relate them to their sources.
permittivity 誘電率
constant 定数
ideal 理想
absolute 絶対の
dielectric 絶縁体
likewise 同様に
property 財産
radiation 放熱 8
Maxwell's equations
Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies. Maxwell's equations describe how electric and magnetic fields are generated and altered by each other and by charges and currents. They are named after the Scottish physicist and mathematician James Clerk Maxwell, who published an early form of those equations between 1861 and 1862.
The equations have two major variants. The "microscopic" set of Maxwell's equations uses total charge and total current, including the complicated charges and currents in materials at the atomic scale; it has universal applicability but may be unfeasible to calculate. The "macroscopic" set of Maxwell's equations defines two new auxiliary fields that describe large-scale behavior without having to consider these atomic scale details, but it requires the use of parameters characterizing the electromagnetic properties of the relevant materials.
The term "Maxwell's equations" is often used for other forms of Maxwell's equations. For example, space-time formulations are commonly used in high energy and gravitational physics. These formulations, defined on space-time rather than space and time separately, are manifestly compatible with special and general relativity. In quantum mechanics and analytical mechanics, versions of Maxwell's equations based on the electric and magnetic potentials are preferred.
Since the mid-20th century, it has been understood that Maxwell's equations are not exact laws of the universe, but are a classical approximation to the more accurate and fundamental theory of quantum electrodynamics. In most cases, though, quantum deviations from Maxwell's equations are immeasurably small. Exceptions occur when the particle nature of light is important or for very strong electric fields.
partial 部分的な
differential 差異
foundation 創設
electrodynamics 電気力学
optics 光学
underlie 下地
generated 生成された
altered 変更された
currents 電流
variant 変形
microscopic 微視的
complicate 複雑な
applicability 適用性
unfeasible 実施し難い
define 定義する
auxiliary 補助物
behavior 行動
consider 考える
require 要求する
property 財産
relevant 関連した
term 期間
form 形態
formulation 公式化
commonly 一般に
rather かなり
separately 別々に
manifestly 明白に
compatible 適合
general 一般
relativity 相対性
quantum 定量
analytical 分析的
potential 潜在的な
prefer ほうを好む
exact 正確
approximation 近似
accurate 正確
fundamental 基本
though しかし
deviation 偏差
immeasurably 計りがたいほど
exception 例外
occur 起こる
particle 粒子 43