Potential difference, the voltmeter and emf
Charge moves from a point of higher potential to a point of lower potential. Think of two containers of water joined by a pipe with a valve: water flows until the levels are equal, then stops. The potential difference between two points is the work W needed to move one coulomb of charge between them: V = W ÷ Q. W is in joules (J), Q in coulombs (C) and V in volts (V), so 1 V = 1 J/C. A voltmeter measures potential difference and is connected in parallel across the two points. The total work that a source does in moving one coulomb round the whole circuit is its electromotive force, or emf, symbol ε, in volts. A source also pushes the current through itself, so the voltage across its terminals is a little less than ε. In this unit we ignore this loss and take V = ε. Say “voltage across the lamp”, never “voltage flows through the lamp”, and do not mix up V (a difference between two points) with ε (a property of the source).
Water-levels model, with a grown-up. Put two clean cups at different heights and join them with a clean tube that the adult fills with water by siphoning; no electricity. Describe how the water moves and when it stops. Then explain: water level = potential, height difference = potential difference, flow stops when the difference is zero. Say what this model cannot show (electrons are not water).