Flux surface surrounding a charge "q" is "q"/ε. In free space the electric displacement is given by the constitutive relation D = ε E, so for any bounding surface the D-field flux equals the charge "Q" within it. Here the expression "flux of" indicates a mathematical operation and, as can be seen, the result is not necessarily a "flow", since nothing actually flows along electric field lines. The magnetic flux density (magnetic field) having the unit Wb/m (Tesla) is denoted by B, and magnetic flux is defined analogously: with the same notation above. The quantity arises in Faraday's law of induction,
Flux tube density of the tube increase with the same proportionality. If a flux tube with a configuration of a magnetic field of B and a plasma density of ρ confined to the tube is compressed by a scalar value defined as λ, the new magnetic field and density are given by:formula_10formula_11If λ < 1, known as traverse compression, B and ρ increase and are scaled the same while transverse expansion decreases B and ρ by the same value and proportion where B/ρ = constant value. Extending the length of the flux tube by λ* gives a new length of L =