Potential of a point charge’s field. Potential difference
If the field strength is the force characteristic of the field, the potential is its energy characteristic: φ = W/q₀, where W is the potential energy of a test charge q₀ in the field. At distance r from a point charge q in vacuum φ = kq/r (φ = 0 at infinity); potential is a scalar, with the same sign as q, and the potentials of several charges add algebraically. Its unit is 1 V = 1 J/C. When a charge moves from point 1 to point 2 the field does work A = q(φ₁ – φ₂) = qU, where U = φ₁ – φ₂ is the potential difference (voltage). In a uniform field E = U/d (unit 1 V/m = 1 N/C). A surface of equal potential is an equipotential surface; for a point charge these are spheres around it. Field lines are always perpendicular to equipotential surfaces and point toward decreasing potential; moving a charge along an equipotential surface requires zero work.
Only in class: on A4 paper mark a positive charge and draw around it its equipotential lines (concentric circles) and field lines (radial). No electrical equipment is used.