Here, q 1 and q 2 are the charges, r is the distance between their centres, and the value of the constant fraction 1 / ( 4 π π ε ε 0 ) is approximately 9 × 10 N⋅m ⋅C. Likewise, ε 0 appears in Maxwell's equations, which describe the properties of electric and magnetic fields and electromagnetic radiation, and relate them to their sources. In electrical engineering, ε 0 itself is used as a unit to quantify the permittivity of various dielectric materials.