A level Physics · Edexcel 9PH0 · Topic 3A

Current, charge and potential difference

Key points

Electric current is the rate of flow of charge: $I = \frac{\Delta Q}{\Delta t}$ . Charge is measured in coulombs (C); an electron carries $-1.6 \times 10^{-19}$ C.
In metals the charge carriers are free electrons; in electrolytes they are ions. Conventional current flows from + to −.
Drift velocity equation: $I = nqvA$ , where $n$ is the number of charge carriers per unit volume. A good conductor has a large $n$ , so a small drift speed is enough.
Potential difference is the energy transferred per unit charge: $V = \frac{W}{Q}$ , so 1 V = 1 J C $^{-1}$ .
Conservation of charge: total current into a junction equals total current out (Kirchhoff's first law).
Conservation of energy: around any closed loop, the sum of e.m.f.s equals the sum of p.d.s (Kirchhoff's second law).
Electrical power $P = VI$ and energy transferred $W = VIt$ .

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