Introduction to Electricity and the Electric Cell

We use electricity for many purposes in our daily lives to make our tasks easier, such as operating pumps to lift water and for lighting our homes, roads, and workplaces after sunset. The electricity we use is typically provided by a power station.

However, when the main supply of electricity is unavailable, a torch is often used for light. A torch has a bulb that lights up when switched on. The electricity for the torch is provided by an electric cell.

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The Electric Cell

An electric cell is the source of electricity in many portable devices like alarm clocks, wristwatches, cameras, and torches. It produces electricity from the chemicals stored inside it.

Structure of an Electric Cell

A typical electric cell has two ends, called terminals.

• It has a small metal cap on one side, which is the positive terminal ($+$).
• It has a flat metal disc on the other side, which is the negative terminal ($-$).

All electric cells have these two terminals. When all the chemicals inside a cell are used up, it stops producing electricity and needs to be replaced.

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The Electric Bulb

A torch bulb is a device that converts electrical energy into light. Like an electric cell, it also has two terminals.

Structure of a Bulb

• Filament: Inside the glass case of the bulb, there is a thin wire called the filament. When electricity passes through it, the filament heats up and gives off light. It is supported by two thicker wires.
• Terminals: The bulb has two terminals that are fixed in a way that they do not touch each other.
  1. The metal case at the base of the bulb.
  2. The metal tip at the centre of the base.

One of the thick support wires is connected to the metal case, and the other is connected to the metal tip, forming the two terminals of the bulb.

A Bulb Connected to an Electric Cell

To make an electric bulb light up, we need to connect it to an electric cell. This requires creating a complete path for the electricity to flow from the cell, through the bulb, and back to the cell.

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Connecting a Bulb and a Cell

An activity involving connecting a bulb to a cell in six different ways helps us understand how a complete path is necessary.

Analysis of Different Arrangements

The bulb glows only in the arrangements where there is an unbroken path for electricity to flow from one terminal of the cell, through the bulb, and back to the other terminal of the cell.

• Arrangements where the bulb glows [Fig. 12.5 (a) and (f)]: In these cases, one wire connects one terminal of the cell to one terminal of the bulb, and a second wire connects the other terminal of the cell to the other terminal of the bulb. This creates a complete, closed loop.
• Arrangements where the bulb does not glow [Fig. 12.5 (b), (c), (d), and (e)]: In these cases, the path is broken. Either the wires are connected to only one terminal of the cell or only one terminal of the bulb, or the path does not form a complete loop.

This shows that for a bulb to glow, there must be a continuous path from the positive terminal of the cell to the negative terminal, passing through the filament of the bulb.

An Electric Circuit

A complete path for electricity to pass between the two terminals of an electric cell is called an electric circuit. The bulb glows only when electric current flows through the circuit. Electric current is the flow of electricity.

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Direction of Current Flow

By convention, in an electric circuit, the direction of current is taken to be from the positive ($+$) terminal to the negative ($-$) terminal of the electric cell. The current passes through the wires and the filament of the bulb, causing it to glow.

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Open and Closed Circuits

• Closed Circuit: When there is an unbroken path from one terminal of the cell, through the bulb, and back to the other terminal, the circuit is complete or "closed". Current flows, and the bulb lights up.
• Open Circuit: If there is a break anywhere in the path, the circuit is incomplete or "open". No current can flow, and the bulb will not light up.

Fused Bulb

Sometimes a bulb does not glow even in a closed circuit. This can happen if the bulb is fused. A fused bulb has a break in its filament. This break in the filament means there is a break in the path of the current, creating an open circuit. Therefore, a fused bulb will not light up.

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Making a Home-Made Torch

A simple torch can be made with just an electric cell, a torch bulb, and a piece of wire. By wrapping one end of the wire around the base of the bulb and the other end to the negative terminal of the cell, we can create a simple switch. The bulb will glow only when its other terminal (the metal tip) is brought into contact with the positive terminal of the cell, completing the circuit.

The Electric Switch

A switch is a simple device that is used to either break an electric circuit or to complete it. Switches used in our homes for lighting and other appliances work on this same basic principle, though their designs are more complex.

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Making a Simple Switch

A simple switch can be made using two drawing pins, a safety pin (or paper clip), two wires, and a small sheet of thermo Col or a wooden board.

1. Fix one drawing pin through the ring of the safety pin into the board.
2. Fix the second drawing pin on the board in a way that the free end of the safety pin can touch it.
3. Connect this setup into a circuit with a cell and a bulb.

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'ON' and 'OFF' Positions

• 'ON' Position: When the safety pin is rotated to touch both drawing pins, it covers the gap and completes the circuit. Since the safety pin is made of metal, it allows current to pass through it. In this position, the switch is said to be 'ON', and the bulb glows.
• 'OFF' Position: When the safety pin is moved away from the second drawing pin, there is a gap in the circuit. The circuit is broken, and no current flows. In this position, the switch is said to be 'OFF', and the bulb does not glow.

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Inside a Torch

A torch has a sliding switch that completes or breaks the circuit. When the switch is pushed to the 'ON' position, it completes the path for the current to flow from the electric cells, through the bulb, and back, causing the bulb to light up.

Electric Conductors and Insulators

Different materials have different properties when it comes to allowing electric current to pass through them. Based on this, they can be classified as either conductors or insulators.

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Conductors

Materials which allow electric current to pass through them are called conductors of electricity. When a conductor is used to bridge the gap in an open circuit, the circuit becomes complete, and the bulb will glow.

Most metals are good conductors of electricity. This is why materials like copper and aluminium are used for making electric wires.

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Insulators

Materials which do not allow electric current to pass through them are called insulators. If an insulator is placed in the gap of an open circuit, the circuit remains broken, and the bulb will not glow.

Examples of insulators include rubber, plastic, wood, glass, cork, and paper. Air is also an insulator.

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Importance and Applications

Both conductors and insulators are equally important for us.

• Conductors are used to make the parts of electrical devices that need to carry current, such as wires, plugs, and sockets.
• Insulators are used for safety. Materials like rubber and plastic are used to cover electrical wires, plug tops, switches, and other parts of appliances that people might touch. This prevents electric shock.

Caution: The human body is a conductor of electricity. Therefore, great care must be taken when handling any electrical appliance.

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Table of Conductors and Insulators

Object used in place of the switch	Material it is made of	Bulb glows? (Yes/No)	Type (Conductor/Insulator)
Key	Metal	Yes	Conductor
Eraser	Rubber	No	Insulator
Scale	Plastic	No	Insulator
Matchstick	Wood	No	Insulator
Glass bangle	Glass	No	Insulator
Iron nail	Metal	Yes	Conductor

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