Charge and electric field strength

In a closed system the algebraic sum of the charges of all bodies does not change: q₁ + q₂ + … + qₙ = const (the idea was put forward by B. Franklin in the mid-18th century). In electrification, the charge one body loses is gained by the other. Charges create an electric field around them; its force characteristic is the field strength E = F/q₀ (a vector, unit 1 N/C = 1 V/m), directed like the force on a positive test charge. At distance r from a point charge q in vacuum, E = k|q|/r², where k = 9·10⁹ N·m²/C². E points away from a positive charge and toward a negative one; field lines start on positive charges and end on negative ones. The field of several charges follows the superposition principle: E = E₁ + E₂ + … (a vector sum). On one line we add or subtract the E values; if they are perpendicular, E = √(E₁² + E₂²).