The Path Must Be Complete: Circuits and their Components

A school trip to a hydroelectric power station is a natural moment to ask just how much of daily life secretly runs on electricity, cooking, lighting, transport, heating and cooling, entertainment, communication, and a long list of odd extras that keep growing the moment you start looking around your own house. But mains electricity, the kind flowing through wall sockets and powering an entire dam's worth of machinery, is genuinely dangerous to experiment with directly, and should never be touched at home or school. To actually learn how electricity works hands-on, this chapter uses something far safer and more portable instead: a plain torchlight. Open one up and slide its switch, and a lamp glows; slide it back, and the lamp goes dark. Open the torch itself, and inside you will typically find two everyday components doing all the work: one or more electric cells, and a lamp. Understanding exactly how those two simple parts cooperate to produce light, and nothing more exotic than that, is the whole story of this chapter.

Pick up any ordinary electric cell and look closely at its two ends: one has a small protruding metal cap, the positive terminal, and the other a flat metal disc, the negative terminal. An electric cell is a portable source of electrical energy, but a single cell often is not enough for a device like a torch, which typically needs two or more. Open a two-cell torch and look carefully at how those cells are actually arranged: they are not placed in just any order. The positive terminal of one cell is connected to the negative terminal of the next, forming a chain. A combination of two or more cells connected this way is called a battery, and using more than one cell provides more energy, or energy for a longer time, than a single cell could on its own. Get the order wrong, insert one cell backwards, and the torch simply will not light, which is exactly why torches and toys are always built with a clearly marked, correct-only slot for each cell. Interestingly, the word 'battery' has drifted a little in everyday speech: people commonly call even a single cell, like the one inside a mobile phone, its 'battery', even though strictly speaking a battery is a combination of two or more cells.

Take out the lamp from an older torch and examine it closely: a thin wire runs through the middle of a small glass bulb, held in place by two thicker support wires. That thin wire is called the filament, and it is the part that actually glows, becoming hot enough to produce light when current flows through it. This kind of lamp is called an incandescent lamp, and it has one convenient property: it does not matter which of its two terminals connects to the positive or negative terminal of the cell, the filament glows either way, as long as the circuit is complete. Many torches today use a different kind of lamp instead, a Light Emitting Diode, or LED, which has no filament at all, and behaves quite differently. An LED has two wires of visibly different lengths: the longer wire is its positive terminal, the shorter wire its negative terminal. Unlike an incandescent lamp, an LED only allows current to pass in one single direction, from its positive terminal to its negative terminal; connect it backwards, longer wire to the cell's negative terminal instead, and it simply will not glow at all, no matter how correctly everything else is wired. This one-way behaviour is the single biggest practical difference between the two kinds of lamp, and it matters every time you build a circuit with an LED rather than an incandescent bulb.

Given a cell, an incandescent lamp, and some wires, there are actually several different ways you might try connecting them, and only some of those ways will make the lamp glow. Try connecting just one wire between the cell and the lamp, and nothing happens. Try connecting both wires but to the same terminal of the cell, and nothing happens either. The lamp only glows in the one arrangement where one terminal of the lamp is connected to one terminal of the cell, and the lamp's other terminal is connected to the cell's other terminal, forming a single, unbroken, complete loop. This complete loop, providing an unbroken path for current to travel all the way from the cell, through the lamp, and back to the cell, is called an electrical circuit, and the lamp only glows once that loop is fully closed. The direction electric current is taken to flow in a circuit is a fixed convention: from the positive terminal of the cell, around through the outer circuit, to the negative terminal. Sometimes a lamp fails to glow even when it is connected correctly, and the circuit looks complete: the lamp itself may have 'fused', meaning its filament has broken somewhere inside the glass bulb, which stops current from completing the loop at all, no matter how correctly everything outside the bulb is wired.

A working circuit is not always meant to stay on forever; sometimes the whole point is to turn it on and off at will, which is exactly the job of a switch. A simple switch can be built from little more than two drawing pins and a safety pin: one drawing pin fixed so the safety pin can rotate freely around it, a second drawing pin fixed close enough for the safety pin's free end to touch. Wire this arrangement into a circuit with a cell and a lamp, and its behaviour depends entirely on one thing: whether the safety pin is touching the second drawing pin or not. When it is touching, the gap is closed, the loop is complete, current flows, and the lamp glows, a state called ON, or a closed circuit. When it is not touching, the gap stays open, breaking the loop, current cannot flow, and the lamp stays dark, a state called OFF, or an open circuit. A switch can be placed absolutely anywhere along a circuit's path, not just next to the lamp, and still do exactly the same job: closing or opening that one deliberate gap. The switches on the walls of homes and schools work on this exact same principle, closing or breaking a circuit, even though their outward design looks nothing like two drawing pins and a safety pin.

Realistic pictures of cells, lamps and switches are slow to draw and can get cluttered fast, especially in a circuit with several components. Electrical circuits are instead usually drawn using a small set of agreed symbols, standing in for each component: a long line and a short line for a cell's terminals, an X-marked circle for a lamp, a triangle-with-a-bar for an LED, a break in a line for a switch, and so on. A circuit drawn this way, using symbols instead of realistic pictures, is called a circuit diagram. In the symbol for a cell, the long line represents the positive terminal and the short line the negative terminal; in the symbol for an LED, the triangle points in the one direction current is allowed to flow through it. These are not symbols any single textbook invented on its own: international organisations such as the International Electrotechnical Commission and the Institute of Electrical and Electronics Engineers maintain standard symbols for electrical components specifically so that circuit diagrams drawn in one country can be read and understood correctly by anyone, anywhere, working from the same shared visual language.

Wires used in circuits are, without exception, made of metal, and it is worth confirming exactly why, rather than just accepting it. Build a simple tester from a cell, a lamp, and two free wire ends, then touch those two free ends briefly to different objects, a metal spoon, a piece of cork, a rubber eraser, a glass bangle, a metal key: the lamp glows for some of these objects and stays dark for others. Materials through which electric current can flow easily, letting the tester's lamp glow, are called conductors, and metals are conductors, which is exactly why wires are made of metal, most commonly copper, since it conducts electricity well and is comparatively cheap and abundant, even though silver and gold actually conduct slightly better. Materials through which current cannot pass, leaving the tester's lamp dark, are called insulators, and this category includes plastic, rubber, and ceramic, all of which show up constantly around household wiring, as the covering on wires themselves, and on switches, plug tops and sockets. This is not a cosmetic design choice; the human body is itself a conductor of electricity, and current passing through it can cause severe injury or worse, which is exactly why insulators are deliberately used to cover any part of a circuit a person might touch. Never touch a switch or plug with wet hands, and never use electrical devices in wet areas or with damaged insulation. One more distinction is worth knowing before this thread of chapters goes further: electricity from a cell or battery is called Direct Current, DC for short, while the electricity from power plants that reaches a wall socket is called Alternating Current, or AC, a difference that becomes important once these chapters move beyond cells and torches into mains electricity itself.