Hinglish

Lamps, Circuits and Switches

Class 7 Science · Chapter 3: Electricity: Circuits and their Components · NCERT Curiosity section 3.2.3–3.2.6

Learn

An old torch lamp is an incandescent lamp: inside the glass is a thin wire, the filament, held by two thicker wires joined to the metal tip and metal case (the lamp's two terminals). Current heats the filament until it glows. If the filament breaks, the lamp has fused. Newer torches use LEDs (light emitting diodes), which have no filament.

Kabir joins a cell to a lamp with wires. The lamp glows only when one terminal of the lamp goes to one terminal of the cell, and the other to the other. This complete path is an electrical circuit. Current flows from the cell's + terminal round to its − terminal. For a filament lamp it does not matter which way round it is joined.

An LED has two legs: the longer one is +, the shorter −. Current passes through an LED in one direction only, so it glows only when its + leg goes to the battery's + terminal. Turn it round and it stays dark. In the symbol, the triangle points the way current can flow.

A home-made switch: a safety pin pivoting on a drawing pin can swing to touch a second drawing pin. When it touches, the gap closes: the circuit is closed (ON) and the lamp glows. When it does not, there is a gap: the circuit is open (OFF). A switch can be placed anywhere in the circuit; switches at home work the same way.

Explore: fix the circuit.

  1. Set the switch and the LED so that the LED glows.

Remember

  • A lamp glows only in a complete circuit; current flows from + to −.
  • An LED works one way only (long leg to +); a switch opens or closes the circuit.

Quick check

An LED stays dark in a complete circuit with a good battery. What should you try first?

  1. turn the LED round
  2. add more wires
  3. remove the battery
  4. paint the LED
Answer

turn the LED round. Current passes through an LED one way only: its long leg must go to +.

Think about it

In a device the cells sit side by side, not end to end. How are they still joined + to −?

One answer

A thick wire or metal strip inside the compartment joins the + of one cell to the − of the next; + and − marks show how to place them.

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.