Potential, Equipotentials & Capacitors

📋 Teacher notes

• •Electric potential V at a point is the potential energy per unit positive charge placed there: V = kQ/r for a point charge, defining V = 0 at infinity.
• •Electric potential ENERGY U of a pair of charges is U = kQ₁Q₂/r — this is the energy of the SYSTEM, not of either charge alone.
• •For unlike charges (attractive), U is NEGATIVE — the system loses potential energy as the charges get closer, and that lost PE becomes kinetic energy, exactly like a ball falling under gravity.
• •For like charges (repulsive), U is POSITIVE — pushing them together stores energy, which is released as kinetic energy if they are let go and fly apart.
• •Work done moving a charge q through a potential difference V is W = qV — this is the same principle used to define the volt (1V = 1 joule per coulomb) and underlies how batteries and circuits are analysed.