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12. ELECTRICITY AND CIRCUITS
We use electricity for many purposes to make our tasks easier. For example, we use electricity to operate pumps that lift water from wells or from ground level to the roof top tank. What are other purposes for which you use electricity? List some of them in your notebook.
Does your list include the use of electricity for lighting? Electricity makes it possible to light our homes, roads, offices, markets and factories even after sunset. This helps us to continue working at night. A power station provides us with electricity. However, the supply of electricity may fail or it may not be available at some places. In such situations, a torch is sometimes used for providing light. A torch has a bulb that lights up when it is switched on. Where does the torch get electricity from?
12.1 ELECTRIC CELL
Electricity to the bulb in a torch is provided by the electric cell. Electric cells are also used in alarm clocks, wristwatches, transistor radios, cameras and many other devices. Have you ever carefully looked at an electric cell? You might have noticed that it has a small metal cap on one side and a metal disc on the other side (Fig. 1). Did you notice a positive (+) sign and a negative (–) sign marked on the electric cell? The
Fig.1: An Electric Cell
metal cap is the positive terminal of the electric cell. The metal disc is the negative terminal. All electric cells have two terminals; a positive terminal and a negative terminal.
An electric cell produces electricity from the chemicals stored inside it. When the chemicals in the electric cell are used up, the electric cell stops
Fig.2 (a) Torch bulb and (b) its inside view
producing electricity. The electric cell then has to be replaced with a new one.
A torch bulb has an outer case of glass that is fixed on a metallic base [Fig. 2 (a)]. What is inside the glass case of the bulb?
12.2 A BULB CONNECTED TO AN ELECTRIC CELL
Let us try to make an electric bulb light up using an electric cell. How do we do that?
12.3 AN ELECTRIC CIRCUIT
In Activity 2 you connected one terminal of the electric cell to the other terminal through wires passing to and from the electric bulb. Note that in the arrangements shown in Fig. 5 (a) and (f), the two terminals of the electric cell were connected to two terminals of the bulb. Such an arrangement is an example of an electric circuit. The electric circuit provides a complete path for electricity to pass (current to flow) between the two terminals of the electric cell. The bulb glows only when current flows through the circuit.
In an electric circuit, the direction of current is taken to be from the positive to the negative terminal of the electric cell as shown in Fig.6. When the terminals of the bulb are connected with that of the electric cell by wires, the current passes through the filament of the bulb. This makes the bulb glow.
Fig.6 Direction of current in an electric circuit
Sometimes an electric bulb does not glow even if it is connected to the cell. This may happen if the bulb has fused. Look at a fused bulb carefully. Is the filament inside it intact?
An electric bulb may fuse due to many reasons. One reason for a bulb to fuse is a break in its filament. A break in the filament of an electric bulb means a break in the path of the current between the terminals of the electric cell. Therefore, a fused bulb does not light up as no current passes through its filament.
Can you now explain why the bulb did not glow when you tried to do so with the arrangements shown in Fig. 5 (b), (c), (d) and (e)?
Now we know how to make a bulb light up using an electric cell. Would you like to make a torch for yourself?
12.4 ELECTRIC SWITCH
We had an arrangement for switching on or off our home made torch by moving the base of the bulb away from the tip of the cell. This was a simple switch, but, not very easy to use. We can make another simple and easier switch to use in our circuit.
12.5 ELECTRIC CONDUCTORS AND INSULATORS
In all our activities we have used metal wires to make a circuit. Suppose we use a cotton thread instead of a metal wire to make a circuit. Do you think that the bulb will light up in such a circuit? What materials can be used in electric circuits so that the current can pass through them? Let us find out.
What do you find? The bulb does not glow when the free ends of the wires are in contact with some of the materials you have tested. This means that these materials do not allow the electric current to pass through them. On the other hand, some materials allow electric current to pass through them, which is indicated by the glowing bulb. Materials which allow electric current to pass through them are conductors of electricity.
Insulators do not allow electric current to pass through them. With the help of Table 1, name the materials that are conductors of electricity and also those which are insulators.
Insulator ,, ,
What do you conclude? Which materials are conductors and which are insulators? Recall the objects that we grouped as those having luster, in Chapter 4. Are they the conductors? It now seems easy to understand why copper, aluminum and other metals are used for making wires.
Let us recall Activity 4 in which we made an electric circuit with a switch (Fig.9). When the switch was in the open position, were the two drawing pins not connected with each other through the thermo Col sheet? But, thermo Col, you may have found is an insulator. What about the air between the gap? Since the bulb does not glow when there is only air in the gap between the drawing pins in your switch, it means that air is also an insulator.
Conductors and insulators are equally important for us. Switches, electrical plugs and sockets are made of conductors. On the other hand, rubber and plastics are used for covering electrical wires, plug tops, switches and other parts of electrical appliances, which people might touch.
- Electric cell is a source of electricity.
- An electric cell has two terminals; one is called positive (+ ve) while the other is negative (– ve).
- An electric bulb has a filament that is connected to its terminals.
- An electric bulb glows when electric current passes through it.
- In a closed electric circuit, the electric current passes from one terminal of the electric cell to the other terminal.
- Switch is a simple device that is used to either break the electric circuit or to complete it.
- Materials that allow electric current to pass through them are called conductors.
- Materials that do not allow electric current to pass through them are called insulators.
- Electric cell
- Electric circuit
- Imagine there were no electric supply for a month. How would that affect your day to day activities and others in your family? Present your imagination in the form of a story or a play. If possible stage the play written by you or your friends in school.
- For your friends, you may set up a game "How steady is your hand?". You will need a cell, an electric bulb, a metal key, two iron nails ( about 5 cm in length), about one and a half metre long thick metal wire (with its plastic insulation scraped off ) and few pieces of connecting wires. Fix two nails nearly one metre apart on a wooden board so that these can be used as a hook. Fix the wire between the nails after inserting it through the loop of the key. Connect one end of this wire to a bulb and a cell. Connect the other terminal of the cell to the key with a wire. Ask your friend to move the loop along the straight wire without touching it. Glowing of the bulb would indicate that the loop of the key has touched the wire.
- Read and find out about Alessandro Volta who invented the electric cell. You may also find out about Thomas Alva Edison who invented the electric bulb.