Electric circuits and circuit diagrams

Six ways to join a cell and a lamp

Take a cell, a torch lamp in a holder and some wires with about 1 cm of plastic removed from each end. Joining them in six different ways, first predicting and then testing, shows a clear pattern: the lamp glows in only two of the arrangements. In those, one terminal of the lamp is joined to the positive terminal of the cell and the other terminal to the negative. Where a wire is left loose, or both lamp wires go to the same terminal of the cell, the lamp stays dark.

A complete path

When the cell, the wires and the lamp make one unbroken loop, they form an electrical circuit. Electric current flows around it, and its direction is taken to be from the positive terminal of the cell, through the wires and the lamp, to the negative terminal. In an incandescent lamp the current passes through the filament and makes it glow, and it does not matter which lamp terminal goes to which cell terminal. If the filament breaks, the lamp has fused: the break is a gap in the path, so no current flows.

An LED is different, because current can pass through it in one direction only. It glows when its longer wire goes to the battery’s + terminal and its shorter wire to the − terminal; swap them and it stays dark.

Switches open and close a circuit

A switch completes or breaks a circuit. You can make one with two drawing pins, a safety pin and a small piece of cardboard: the safety pin turns about one drawing pin, and its other end can be swung to touch the second pin. When it touches, the gap closes, current flows from the positive to the negative terminal, and the lamp glows; this is the ON position and the circuit is closed. When it does not touch, the gap stops the current; this is the OFF position and the circuit is open. A switch works wherever it sits in the loop, and the switches on your walls do the same job inside a plastic cover.

Because a single loop has only one path, a gap anywhere stops the current everywhere. Two lamps and two switches in one loop light up only when both switches are ON, and if one lamp fuses, the other goes out too.

Circuit diagrams and symbols

Instead of drawing real parts, we use symbols. A cell is a long line and a short line: the long line is positive and the short line negative. A battery is two or more cell symbols together. A lamp is a circle with a cross. An LED is a triangle with a bar, whose triangle points the way current can flow, plus two arrows for the light it gives out. A switch is drawn closed for ON and with a tilted, open line for OFF. A drawing of a circuit with symbols is its circuit diagram, and standard symbols, set by bodies such as the IEC, ANSI and IEEE, help people from different countries understand one another.

Why it matters

Every electrical device, from a torch to a television, depends on complete circuits and switches. What a circuit is made of is explained in cells, batteries and lamps, and which materials let current through in conductors and insulators. Answers to every activity and question are in the NCERT solutions for Chapter 3; see all of the Chapter 3 notes.