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.
Electricity needs a path to pass from the positive terminal to the negative terminal of an electric cell. A connection that provides a path outside an electric cell, for the electricity to pass from the positive terminal to the negative terminal of the cell, is called an electrical circuit
Connect 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.1., 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.
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.
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.
Now we know how to make a bulblight up using an electric cell. Would you like to make a torch for yourself?
Take a torch bulb and a piece of wire. Remove the plastic covering at the two ends of the wire as you did before. Wrap one end of a wire around the base of an electric bulb as shown in fig. 2. Fix the other end of the wire to the negative terminal of an electric cell with a rubber band. Now, bring the tip of the base of the bulb that is, its other terminal, in contact with the positive terminal of the cell. Does the bulb glow? Now move the bulb away from the terminal of the electric cell. Does the bulb remain lighted? Is this not similar to what you do when you switch your torch on or off?
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.
You can make a switch using two drawing pins, a safety pin (or a paperclip), two wires and a small sheet of thermo Col or a wooden board. Insert a drawing pin into the ring at one end of the safety pin and fix it on the thermo Col sheet . Make sure that the safety pin can be rotated freely. Now, fix the other drawing pin on the thermo Col sheet in a way that the free end of the safety pin can touch it. The safety pin fixed in this way would be your switch in this activity.
Now, make a circuit by connecting an electric cell and a bulb with this switch. Rotate the safety pin so that its free end touches the other drawing pin. What do you observe? Now, move the safety pin away. Does the bulb continue to glow?
The safety pin covered the gap between the drawing pins when you made it touch two of them. In this position the switch is said to be 'on' . Since the material of the safety pin allows the current to pass through it, the circuit was complete.Hence, the bulb glows.
On the other hand, the bulb did not glow when the safety pin was not in touch with the other drawing pin. The circuit was not complete as there was a gap between the two drawing pins. In this position, the switch is said to be 'off'.
A switch is a simple device that either breaks the circuit or completes it. The switches used in lighting of electric bulbs and other devices in homes work on the same principle although their designs are more complex.
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