NCERT Class 7 Science Curiosity Chapter 03 Electricity Circuits and their Components PDF Download

Chapter 3: Electricity: Circuits and their Components

Nihal and his classmates were excited for their school trip to the Bhakra Nangal Dam. There they would visit the hydroelectric power house where the force of falling water was used to generate electricity. They also looked forward to the free 13 kilometre-train ride from Nangal in Punjab to Bhakra in Himachal Pradesh, along the beautiful Sutlej river and through the Shivalik hills.

Prior to the trip, Nihal and his classmates were given a group assignment to prepare a presentation on the uses of electricity. They began by looking around their houses, then their school, followed by their neighbourhood, their city, and finally they searched the internet. To their astonishment, their list kept growing. They decided to organise the uses under different headings.

Can you help Nihal by adding some more uses to his lists? Also, suggest some other ways of grouping the uses of electricity.

We use electricity all the time, so let us learn something more about electricity. You have learnt earlier that electricity is generated in multiple ways - by windmills, by using wind energy, by solar panels capturing the Sun's energy, by falling water and by using natural gas or coal (in the chapter 'Nature's Treasures' in the Grade 6 Science textbook Curiosity). The electric supply from these sources reaches our homes and factories via wires. For example, at home we plug in various devices to the electrical sockets in the wall. However, to learn about electricity, we will focus on a portable source of electricity that most of us may have used. Let us start with its use in a common device like a torchlight.

Caution

The danger signs on electric poles and other appliances warn people that electricity can be dangerous if not carefully handled. Never ever perform experiments with power supply at your home or school. Even electricity from portable generators can be dangerous. Use only batteries or cells, like those in torchlights, wall clocks, radios, or remotes, for experiments with electricity.

3.1 A Torchlight

You might have used a torchlight, also called a torch or a flashlight.

Activity 3.1: Let us explore

• Take a torchlight similar to the one shown in Fig. 3.1.
• Observe it carefully. Do you notice a lamp? And a switch?
• Slide its switch and observe. Does the torch lamp glow?
• Now slide the switch back to its original position and observe the torch lamp.

You might have noticed that in one position of the switch, the torch lamp glows and in the other position the lamp does not glow.

• Now, open the torchlight. What do you find inside?

Inside the torchlight, you may find two or more electric cells.

[Figure 3.1: A torchlight, See in your textbook]

Teacher's Note

The switch controls whether the circuit is complete or broken. When the switch is in one position, it connects all the parts into a complete path for electricity to flow, making the lamp glow. When you flip it to the other position, it breaks the path and the lamp goes dark. Remember this when you design circuits later.

3.2 A Simple Electrical Circuit

To understand how a torch works, let us first find out about its components.

3.2.1 Electric cell

Activity 3.2: Let us observe

• Take an electric cell, turn it around and look at it carefully (Fig. 3.2). Do you notice a positive (+) sign and a negative (-) sign marked on the electric cell? Do you also notice that it has a small protruding metal cap on one side and a flat metal disc on the other side?

All electric cells have two terminals; one is called positive (+ ve) while the other is negative (- ve). The metal cap is the positive terminal of the electric cell and the metal disc is the negative terminal. The electric cell is a portable source of electrical energy.

[Figure 3.2: An electric cell, See in your textbook]

3.2.2 Battery

Activity 3.3: Let us experiment

• Take a torch which uses two cells. Open its cell compartment and take out the cells.
• Put the cells back in a different order. Also, try reversing the direction of one cell. Then, slide the switch and check whether the lamp glows in each case.
• Check the order in which the cells were placed in the torch when the lamp glows.

The lamp glows when the cells are placed in the order as shown in Fig. 3.3. Notice how the terminals of the two cells are connected. The positive terminal of one cell is connected to the negative terminal of the next cell. Such a combination of two or more cells is called a battery.

[Figure 3.3: A battery made up of (a) two cells (b) four cells, See in your textbook]

Teacher's Note

The order of cells matters - they must be connected positive to negative, not positive to positive or negative to negative. If you reverse even one cell, the circuit may not work. Think of it like a one-way road: electricity needs the cells to point in the same direction to flow properly through the circuit.

Key Points

• Electricity is used for many purposes including lighting, cooking, entertainment, communication, heating, cooling, and transportation.
• An electric cell has two terminals: a positive terminal (metal cap) and a negative terminal (flat metal disc) that produce electrical energy.
• A battery is made by connecting two or more cells in series, with the positive terminal of one cell connected to the negative terminal of the next cell.
• A switch in a circuit controls whether electricity can flow by opening or closing the path, making a connected device turn on or off.
• For safety, experiments with electricity should only use batteries or cells from devices like torchlights, never with home or school power supply.

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