Electricity: Circuits and their Components

Every electric cell has two ends, called terminals, and they are not identical. One terminal is marked positive (+) and is usually a small raised metal button; the other is negative (−) and is a flat metal disc. This small, sealed unit is a compact, portable store of electrical energy — it is what powers torches, wall clocks, remote controls, and toy cars.

A single cell is often not enough to run a device, so two or more cells are connected end to end — the positive button of one touching the negative disc of the next. This chain of cells is called a battery. Line four cells up this way inside a torch, and you have made one small battery out of four cells. Interestingly, in everyday speech, people often call even a single cell a "battery" too — the single cell that runs a TV remote or a wristwatch is commonly called a battery even though, strictly speaking, it is just one cell.

Lamps are the part of the circuit that turns electrical energy into light, and torches use one of two very different kinds. An older torch is likely to use an incandescent lamp: inside its glass bulb is an extremely thin coiled wire called a filament, held up by two thicker support wires that connect to the lamp's two terminals. When current flows through this thin filament, it heats up so much that it glows and gives off light. If that hair-thin filament ever snaps — which we say means the lamp has "fused" — the gap stops current from flowing, and the lamp goes dark for good.

Most torches sold today instead use an LED (Light Emitting Diode). An LED has no filament at all. Instead, it has two thin wire leads of different lengths — look closely and you will notice one lead is longer than the other. The longer lead is the LED's positive terminal, and the shorter one is its negative terminal. This difference in lead length is not decorative; it matters a great deal, as you will see in the next section.

Connect one terminal of a cell to one terminal of a lamp with a wire, and connect the lamp's other terminal back to the cell's other terminal with a second wire — and the lamp lights up. This closed loop, which gives electric current an unbroken path to travel all the way around, is called an electric circuit. Current only flows, and the lamp only glows, when this loop has no gaps anywhere in it.

By convention, we describe the direction of current in a circuit as flowing from the cell's positive terminal, around the loop, and back into its negative terminal. This is just a convention scientists agreed on long ago to describe circuits consistently — it does not mean anything physically moves in only that direction inside the cell itself.

Does it matter which way round you connect an incandescent lamp? Not really — current can flow through its filament in either direction, heating it up and making it glow regardless of which of its two terminals faces the cell's positive end. An LED behaves completely differently: current can only pass through it in one direction. Connect an LED so that its longer (positive) lead faces the cell's positive terminal, and its shorter (negative) lead faces the cell's negative terminal, and it glows. Swap the two connections around, and no current flows through it at all — the LED simply stays dark, no matter how fresh the cell is.

A circuit can also fail to light up for less obvious reasons: a strip of plastic insulation accidentally left on a wire's tip, a loose connection at a holder, or a cell inserted the wrong way round in its holder (so that its negative terminal, instead of its positive one, faces the spring contact) can each silently break the loop. Whenever a lamp refuses to glow, checking each of these joints, one at a time, is usually how the fault gets found.

A switch is nothing more than a controllable gap in a circuit. You can build a simple one yourself: pin a safety pin loosely to a small board so it can swing freely, and fix a second pin nearby so the safety pin's free end can be rotated to touch it. When the safety pin touches the second pin, the gap closes and current can flow — we call this the ON position. Rotate the safety pin away, and the gap reopens; the circuit is now OFF, and no current flows. The switches on home walls work on exactly this idea, just built more neatly and safely.

Drawing a torch's actual wires, cells, and bulbs every time you want to explain a circuit gets tedious and messy, so instead we draw a circuit diagram — a drawing that uses an agreed set of symbols for each component. A plain wire is drawn as a straight line. An electric cell is drawn as one long line next to one short line, side by side — the long line is always the positive terminal, and the short line is the negative terminal. Several of these long-short pairs in a row represent a battery. A lamp is drawn as a small circle, usually with a filament mark inside it. An LED is drawn as a triangle pointing in the direction current can flow through it, with two small arrows beside it to show that it gives off light. A switch is drawn as a small break in the line, shown closed (ON) or open (OFF).

Why bother with symbols instead of realistic drawings? Because engineers across the world — whether building a torch in Chennai or a satellite in California — agree on roughly the same set of symbols for electrical components. That shared language lets anyone look at a diagram and immediately understand how a circuit is wired, without needing to see a photograph of the actual parts.

When a cell, a switch, and one or more lamps are all connected one after another, forming a single loop with no branches, we say they are connected in series. Current has only one path available, and it must pass through every single component in that loop, one after the other, to complete the circuit.

This has an important consequence: in a series loop, a break anywhere stops current everywhere in that loop. It does not matter whether the break is a lamp with a snapped filament, a wire that has come loose from its holder, or simply a switch that has been turned OFF — the effect on the rest of the loop is identical. If two lamps, say L1 and L2, are wired one after another in the same series loop along with a single switch, then opening that one switch turns both lamps off together, and so does a break at either lamp. There is no way to control L1 and L2 separately if they share the same loop and the same switch.

Now compare this with two lamps that do not share a loop at all — each lamp has its own separate cell (or its own separate connection back to a shared cell) and its own separate switch, with the two loops never crossing. In that arrangement, opening the switch for L1 only breaks L1's own loop; current in L2's completely separate loop is unaffected, and L2 keeps glowing exactly as before. Whether turning off one lamp affects another, then, depends entirely on whether they share the same unbroken loop or sit in loops of their own.

Why are electric wires made of metal, and why are they almost always coated in plastic or rubber? The answer lies in how different materials respond to electric current. You can build a simple tester to check this yourself: connect a cell and a lamp with wires, but leave the two free ends of one wire unconnected. Touch those two free ends together, and the lamp should glow — your tester is ready. Now touch the two free ends to either side of some object instead (a coin, a pencil, a rubber eraser, a steel key) and watch whether the lamp glows.

You will find the lamp glows for some objects and stays dark for others. Materials that let current pass through them easily — and make the lamp glow — are called conductors. Materials that block current and leave the lamp dark are called insulators (also called poor conductors). Nearly every metal you test — iron keys, steel spoons, aluminium foil, copper coins — will turn out to be a conductor. Silver, copper, and gold are the three best conductors of all, but wires are almost always made of copper rather than silver or gold, simply because copper is far cheaper and much more easily available in large quantities.

Plastic, rubber, glass, dry wood, and ceramic, on the other hand, are all insulators. That is exactly why the copper wire inside an electric cable is wrapped in a layer of rubber or plastic, and why the handles of pliers and screwdrivers used by electricians are covered in rubber or plastic grips: the conducting copper core does the job of carrying current, while the insulating cover keeps that current from reaching — and shocking — the person holding it.

Our own bodies are fairly good conductors of electricity too, which is precisely why electricity can be dangerous. Never touch a switch, plug, or appliance with wet hands, never handle equipment that has a cracked plug or torn insulation, and never attempt circuit experiments using the electricity supply from a wall socket — experiment only with cells or batteries. The current available from a wall socket is far more powerful than anything a cell can supply, and a shock from it can cause serious injury or worse.

One more difference worth knowing: the electricity stored in a cell or battery flows steadily in one fixed direction and is called direct current (DC) — it is what powers small, portable devices like torches and toys. The electricity delivered through the wires to a wall socket constantly reverses direction many times every second and is called alternating current (AC) — it is what can run heavier appliances like fans, refrigerators, and washing machines.