Electricity: Circuits and their Components – Class 7 Science Chapter 3 Notes & NCERT PDF

Introduction

Nihal and his classmates are going on a school trip to the Bhakra Nangal Dam, where falling water is used to generate electricity in a hydroelectric power house. They are also excited about the free 13 km train ride from Nangal (Punjab) to Bhakra (Himachal Pradesh) along the Sutlej river through the Shivalik hills. Before the trip they list uses of electricity under headings — cooking (kettle, mixer, toaster), lighting, entertainment (TV, radio), transportation (train, lift, escalator), communication (mobile, Internet), heating and cooling (fan, geyser, refrigerator, AC) and others (water pump, crane, computer).

Electricity is generated by windmills, solar panels, falling water, and natural gas or coal, and reaches us through wires.

Caution: Never experiment with the power supply at home or school — even portable generators can be dangerous. Use only cells or batteries (like those in torches, clocks, remotes).

Key Concepts

3.1 A torchlight

A torch has a lamp, a switch and two or more electric cells. In one position of the switch the lamp glows; in the other it doesn't.

3.2 A simple electrical circuit

Electric cell — a portable source of electrical energy with two terminals:

Battery — two or more cells joined so that the positive terminal of one touches the negative terminal of the next. More cells give energy for a longer time and/or more energy. (We also call a single cell, like the one in a mobile phone, a "battery".)

Incandescent lamp — has a thin wire called the filament inside a glass bulb, held by two thicker wires. One connects to the metal case and the other to the metal tip of the base — these are the lamp's two terminals and must not touch. The filament gets hot and glows. A lamp with a broken filament is said to be "fused" and won't glow.

LED (Light Emitting Diode) — has no filament. Its longer wire is positive and shorter wire is negative. Current passes through an LED in one direction only, so it glows only when its longer wire goes to the battery's positive terminal.

Electrical circuit (Activity 3.6) — the lamp glows only when one lamp terminal is joined to one cell terminal and the other to the other, making a complete path. The direction of current is taken from the positive to the negative terminal of the cell. For an incandescent lamp it doesn't matter which terminal goes where.

Electric switch (Activities 3.8–3.9) — made with two drawing pins, a safety pin and cardboard.

A switch can be placed anywhere in a circuit; it only completes or breaks the circuit — it is not a source of current.

3.3 Circuit diagrams

Components have standard symbols (cell, battery, lamp, LED, switch ON/OFF, wire). In the cell symbol, the long line is positive and the short line negative. In the LED symbol, the triangle points in the direction current can flow and the two arrows show it gives out light. A drawing of a circuit using symbols is a circuit diagram. Organisations like IEC, ANSI and IEEE set standard symbols so everyone understands them.

3.4 Conductors and insulators (Activity 3.11)

A conduction tester (cell + lamp with two free wire ends) is touched to different objects. The lamp glows for metal objects (key, coin, spoon, pin, aluminium foil) but not for wood, plastic, glass, paper, wax or rubber.

Silver, copper and gold are the best conductors; copper is mainly used for wires because it is cheaper and plentiful.

Insulators cover wires, plug tops and switches to protect us from shocks. Our body is a conductor — never touch switches with wet hands or use damaged wires.

DC and AC: Batteries give Direct Current (DC) for small devices; the wall socket supplies Alternating Current (AC) from power plants for larger appliances.

Activities in the Chapter

Key Terms

Common Misconceptions

Keep the Curiosity Alive (exercise hints)