Electric flux density

Japanese: 電束密度 - でんそくみつど

Electric flux per unit area in a dielectric. Also called electric displacement. In electromagnetic theory, anything related to the electric field is expressed using the electric field strength (electric field E ) and electric flux density D. The electric flux density is proportional to the electric field, and the proportional constant is called the dielectric constant. The dielectric constant represents the electrical properties of the material that fills the space. The electric field is related to the strength of the electric force acting on the charge. The electric flux density is defined in terms of the charge distribution. In a vacuum, the electric flux density is defined as ε ₀ times the electric field E (ε ₀ = 8.85 × 10 ^-12 F/m). With this definition, the total number of electric fluxes passing through a closed surface that encloses a quantity of electricity Q coulombs is Q. This space is filled with a dielectric. When considering this case in the same way as in a vacuum, in addition to the true charge Q (charge that can be added or removed at will), the total charge that constitutes the material must be considered as the charge enclosed by the closed surface. When a material is not polarized, the charges that constitute the material are balanced between positive and negative and are zero. When a material is polarized and positive and negative charges are displaced, especially when this displacement occurs across a closed surface, the balance of positive and negative charges that make up the material within the closed surface is lost. When the number of electric fluxes passing through a closed surface is counted using ε ₀ E , it is calculated by subtracting the polarized charges displaced outward through the closed surface from the true charge Q. Unlike true charges, which can be added or removed, the latter charges cannot be controlled, so it is convenient to limit the number of electric fluxes passing through a closed surface to those related to true charges. To do this, it is necessary to define the electric flux density by correcting ε ₀ E for the displacement of charge due to polarization P. The electric flux density is defined as follows:

D ＝ε ₀ E ＋ P
Using this electric flux density, the number of electric fluxes passing through a closed surface is equal to the amount of electricity in the net charge it encloses.

[Shigeo Yamaguchi]

[Reference] | Charge | Electric field | Dielectric

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

誘電体の中の単位面積当たりの電束。電気変位ともいう。電磁理論では、電界（電場）に関することは、電界の強さ（電界E）と電束密度Dを用いて表す。電束密度は電界に比例し、その比例定数を誘電率という。誘電率は空間を満たしている物質の電気的な性質を表す。電界は電荷に働く電気力の強さに関するものである。電束密度は電荷分布に関して定義される。真空中では、電束密度は、電界Eのε₀倍(ε₀＝8.85×10^-12F/m)と定める。このように定めると、電気量Qクーロンを囲む閉曲面を通る全電束数はQ本となる。この空間を誘電体で満たす。この場合を、真空中の場合と同じように考えるとき、閉曲面で囲まれる電荷としては真電荷Q（勝手に付け加えたり、取り除いたりできる電荷）のほかに物質を構成する全電荷を考慮しなければならない。物質は分極していない状態では、物質を構成する電荷は正負がつり合ってゼロである。物質が分極して正負の電荷が変位し、とくにこの変位が閉曲面をまたいでおこる場合、閉曲面内の物質を構成する電荷の正負のバランスが崩れることになる。閉曲面を通る電束数をε₀Eを用いて数えると、真電荷Qから閉曲面を通って外へ変位した分極電荷を差し引いた値となる。付加したり除いたりできる真電荷と異なり、後者の電荷は制御しえないものであるため、閉曲面を通る電束数を真電荷に関するものに限定すると都合がよい。そのためには、ε₀Eに分極Pによる電荷の変位を補正して電束密度を定義する必要がある。電束密度を次のように定める。