A Torchlight: Cells and Batteries

Class 7 Science · Chapter 3: Electricity: Circuits and their Components · NCERT Curiosity section 3.1–3.2.2

Learn

Before a school trip to a hydroelectric power station, Neel's class lists every use of electricity they can find: cooking (kettle, mixer), lighting (homes, streets), transport (metro, electric scooters, lifts), heating and cooling (fan, geyser, fridge), entertainment and communication (phone, internet). The list keeps growing!

Caution: electricity from wall sockets, poles and generators can kill. Never experiment with them. For experiments we use only cells and batteries, like those in torches, clocks and remotes. Let us start with a torch: slide its switch one way and the lamp glows, the other way and it goes off. Inside are two or more electric cells.

Every cell has two terminals: the small metal cap is positive (+) and the flat metal disc is negative (−). A cell is a portable source of electrical energy. In the circuit symbol, the long line is + and the short line is −.

In a two-cell torch, the lamp glows only when the + of one cell touches the − of the next. Cells joined like this make a battery. More cells give energy for longer and/or more energy. (We also call a single cell, like a phone's, a "battery".)

Explore: uses of electricity.

1. Sort each use of electricity.

Remember

• A cell has a + terminal (cap) and a − terminal (disc); it is a portable energy source.
• A battery is cells joined + to −. Never experiment with mains electricity.

Quick check

How are the cells in a torch arranged so that it works?

1. + of one cell touching − of the next
2. + touching +
3. − touching −
4. in any order

Think about it

If the power supply in your area stopped for two days, which daily activities would be hardest?

Practice

Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.