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The density of a quantity ρ and its current j can be combined into a 4-vector called a 4-current: = (,,,) where c is the speed of light. The 4- divergence of this current is: ∂ μ J μ = c ∂ ρ ∂ c t + ∇ ⋅ j {\displaystyle \partial _{\mu }J^{\mu }=c{\frac {\partial \rho }{\partial ct}}+\nabla \cdot \mathbf {j} } where ∂ μ is the 4 ...
The Lorentz–Lorenz equation is similar to the Clausius–Mossotti relation, except that it relates the refractive index (rather than the dielectric constant) of a substance to its polarizability. The Lorentz–Lorenz equation is named after the Danish mathematician and scientist Ludvig Lorenz , who published it in 1869, and the Dutch ...
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Density is an intensive property in that increasing the amount of a substance does not increase its density; rather it increases its mass. Other conceptually comparable quantities or ratios include specific density, relative density (specific gravity), and specific weight.
In electromagnetism, current density is the amount of charge per unit time that flows through a unit area of a chosen cross section. [1] The current density vector is defined as a vector whose magnitude is the electric current per cross-sectional area at a given point in space, its direction being that of the motion of the positive charges at this point.
A density meter (densimeter) is a device which measures the density of an object or material. Density is usually abbreviated as either ρ {\displaystyle \rho } or D {\displaystyle D} . Typically, density either has the units of k g / m 3 {\displaystyle kg/m^{3}} or l b / f t 3 {\displaystyle lb/ft^{3}} .
The set of points in the plane at which the density is neither 0 nor 1 is non-empty (the square boundary), but it is negligible. The Lebesgue density theorem is a particular case of the Lebesgue differentiation theorem. Thus, this theorem is also true for every finite Borel measure on R n instead of Lebesgue measure, see Discussion.
A United States Navy Aviation boatswain's mate tests the specific gravity of JP-5 fuel. Relative density, also called specific gravity, [1] [2] is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material.