Circuit Diagrams

A circuit diagram is a functional map, not a picture. Wires are drawn as straight lines with right-angled corners, and the physical layout of the components is irrelevant — only what is connected to what.

Series — components connected one after another in a single loop, so the same current passes through each in turn.

Parallel — components connected across the same two points, so each has the same potential difference across it and the current divides between them.

The ammeter — measures current, so it must be placed in series with the component, in the path the current takes.

Why an ammeter needs low resistance — it is added to the circuit, so it changes the circuit. If it had appreciable resistance it would reduce the very current it was inserted to measure. An ideal ammeter has zero resistance.

The voltmeter — measures potential difference, which is a difference between two points, so it must be connected in parallel across the component.

Why a voltmeter needs high resistance — connecting it in parallel offers the current an alternative route. If its resistance were low it would divert a significant current and alter the p.d. it was measuring. An ideal voltmeter has infinite resistance.

Example: the pattern is worth noticing. Each meter is designed so that adding it disturbs the circuit as little as possible — a measurement should not change what it measures, and here that principle determines the hardware.

Switches, fuses and the rest — a switch breaks the circuit; a fuse is a deliberate weak link that melts above a set current; a variable resistor lets the current be adjusted; an LDR and a thermistor change resistance with light and temperature.

Reading a diagram — trace from one terminal of the supply and follow every possible route back to the other. Anywhere the path splits and rejoins, the components in the branches are in parallel; anywhere there is only one route, they are in series.