Current, Voltage and Resistance Model

Voltage: number of 1.5 V cells 2: 3.0 V

Length of the wire l 1.00 m

Thickness: cross-sectional area A 0.10 mm²

p.d. across the component V 0.0 V

cells	V / V	I / A	R = V / I / Ω

Mean resistance = (15 + 15 + 15 + 15) ÷ 4 = 15 Ω

free electrons (on the wires, each dot is 0.1 C) conventional current energy carried by the charge metal ions I–V line: resistor filament lamp diode

Not to scale. The circuit diagram uses the standard symbols; its sizes mean nothing. In the close-up, the length of the wire is to scale with the ruler, but the wire is drawn about 270 times too thick for its length. The metal ions are drawn about 100 million times larger than real. The free electrons are shown as enlarged dots; their size does not show the real size of an electron. Far fewer ions and electrons are shown than in a real wire. The electrons are drawn drifting several hundred times faster than real free electrons, which drift at less than a millimetre per second. The connecting wires, the ammeter and the cells are treated as wasting no energy.

How to write it in the exam

An electric current is a flow of charge. In a metal, the charge is carried by free electrons, which move through the metal.
Current is the charge passing a point per unit time: I = Q / t, with Q in coulombs (C), t in seconds (s) and I in amperes (A).
Conventional current is from positive to negative. The free electrons flow from negative to positive.
An ammeter is connected in series. A voltmeter is connected in parallel, across the component. Choose the smallest range that is larger than the reading you expect.
The e.m.f. of a source is the electrical work it does in moving unit charge round a complete circuit: E = W / Q.
The p.d. across a component is the work done by unit charge passing through it: V = W / Q. E.m.f. and p.d. are both measured in volts; 1 V = 1 J/C.
Resistance R = V / I, in ohms (Ω). To measure it, read V and I for several settings, calculate R for each pair and take the mean.
For a metal wire at constant temperature, resistance is directly proportional to length and inversely proportional to cross-sectional area.
I–V graphs: a resistor of constant resistance gives a straight line through the origin. A filament lamp gives a curve that gets less steep, because the filament gets hotter and its resistance increases. A diode lets current through in one direction only: it has a very high resistance in the other direction.