electricity and circuits

13 Electricity and Circuits

Can you imagine life without electricity? Electric bulbs, fans, televisions, refrigerators, air conditioners, mixers and juicers-all work on electricity. Electricity is generated in power stations and brought to our houses by wires. What would your life be like without these electrical appliances? What problems do you have when there is a power failure, especially at night? Which appliance helps you the most at that time? Probably the electric torch. The electric torch also works on electricity. But, the electricity it uses is not generated by power stations. Let us further examine how an electric torch works.

What else does the torch have? Let us find out. Unscrew the torch cover from the back. What can you see inside? Can you see two or three cylindrical objects? Take them out. These are electric cells. Now screw the cover back without putting the cells back in. Switch the torch on. Does it work now? Why not? The electric cells are necessary to make the bulb glow.

Examine an electric cett. It has a small metal cap on one side and a metal disc on the other side. These are the two terminals of the cell. One of them is marked with a '+' and the other with a '-'.

Unscrew the cover of the torch again and look inside it. Put the switch on and off and see what happens inside. You will notice that a strip of metal moves when you switch on or off

Unscrew the front of the torch. It houses the electric bulb. Look at the bulb. Can you see a thin wire fixed in the middle of the glass bulb? It is this thin wire that glows when the torch is switched on. It is called the filament of the bulb. Two thicker wires provide support to the filament. One of these is connected to the metal case of the bulb. The other is connected to the metal tip at the bottom. These are the two terminals of the bulb.

Let us now note down our findings about the torch.

A torch has a bulb, electric cells, a switch and of course the outer body.

→ When switched on, a metal strip moves beneath the switch and the bulb glows, giving light and heat.

You have made your torch and it works! It has a bulb, an electric cell, electric wires (instead of metal strips present in a torch) and a switch-the metal clip-drawing pin arrangement that can be used to put on or put off the bulb is your switch.

Electric current

Why do you think that the bulb glows when the wires are connected?

There must be something flowing in the wires. This 'something' must have energy; otherwise it cannot produce heat and light energy in the bulb. We call this 'something' electric current. An electric current has electrical energy. It gets converted into light and heat energy when the current flows through an electric bulb. The electric current flows from the end of the cell marked + (called the positive terminal) through the metal wires, to the end marked (called the negative terminal).

ELECTRIC CIRCUITS

When you connect the two ends of a cell to a bulb using copper wires, the bulb lights up. This is because you have provided a path for electric current to flow from the positive terminal of the battery to the negative terminal through the bulb. Such a path of an electric current is known as a circuit (Fig. 13.4a). When the circuit allows the flow of current, it is said to be complete or closed. Does the bulb glow if you disconnect one of the wires from the cell or break one of the wires? It does not, because the path of the current is now obstructed and it can no longer flow through the wire. The circuit is now said to be broken or open.

When you want to put off a light or a fan, what do you do? The switch that you use for this purpose works by breaking the electric circuit. The switch is an important part of an electric circuit. It is present in all circuits (Fig. 13.4c).

Components, such as bulbs, wires, cells and switches are known as the elements of a circuit. Drawing pictures of these elements while drawing a circuit is very cumbersome We, therefore, use symbols to represent the circuit elements. Figure 13.5 shows symbols of the common circuit elements, and an electric circuit shown in Figure 13.6 is drawn using these symbols.

You probably know that a fused bulb does not glow even when connected in a closed circuit. One of the reasons why a bulb fuses is when its filament breaks. Why do you think such a bulb does not glow?

CONDUCTORS AND INSULATORS

In the last section, you read that when you broke the metal wire connecting the cell to the bulb, the bulb stopped glowing. What does this show? This shows that electric current can flow through metal wires, but not through air. We say that metal is a conductor of electric current. whereas air is a non-conductor or an insulator.

Both conductors and insulators are important in using electricity. Whenever we want to make a circuit to use electric current in some way, for example, to light a bulb, run a fan, work an oven or run a washing machine, we use wires made of a metal, such as copper or aluminium, which are good conductors. However, we cannot use bare wires as this would be dangerous. A bare wire carrying current can give you an electric shock. Therefore, wires are always coated with an insulating material, such as plastic. You would also have noticed that the outer coverings of switches, regulators of fans, plugs, sockets, etc. are made of an insulating material, such as plastic or bakelite (Fig. 13.8). This saves you from getting electric shocks.

The electricity used at home is generated by large generators far away from the city. It is brought to our houses using thick wires (Fig. 13.9). You should never do any experiment with the electricity used at home, as this can be very dangerous.

ELECTRIC CELLS

The electric cell that is normally used in torches and other electrical appliances is known as a dry cell. It produces energy from the chemicals stored inside it. A dry cell, when connected in a closed circuit, causes electric current to flow through the circuit. A dry cell is therefore a source of electrical energy. Most appliances use between 2 and 4 such cells.

You have seen that a dry cell has two ends or terminals. The end of the cell that has a small, circular metal cap is the positive (or +) terminal. The end that has a flat metal plate is the negative (or -) terminal. As you have seen, a cell, connected in a circuit, causes electric current to flow from the positive to the negative terminal through the circuit.

The dry cell can supply current in a circuit for a certain time. After that, its chemicals are exhausted and it cannot be used any more.

Many other types of cells are available.

The car battery used in a car is big and heavy. It normally has 6 cells inside it. It can supply more current than a dry cell. Also, once exhausted, it can be charged with the help of an electric charger and used again and again. The smaller cells used in mobile phones can also be charged and used again

and again. Another type of cell is the button cell.

It is small, almost the size of a button. It is used in small electronic devices such as wrist watches, calculators, hearing aids, etc. Some button cells have to be thrown away after using them once Others can be charged again and again.

A solar cell is a cell that directly converts solar energy into electrical energy. Many solar cells together form a solar panel. You may have seen a calculator that works on solar cells. A large number of solar cells are used to give electrical power to human-made satellites in outer space. Solar cells in a solar panel give power to street lights and water pumps in remote places where there is no electricity.