Electricity: Circuits and Their Components – Study module with Revision Notes

1. What is Electricity and an Electric Circuit?

Electricity is a form of energy that can be used to produce light, heat, sound and mechanical work. In simple words, when tiny charged particles (electrons) move through a material like a metal wire, we say an electric current flows.

Electric Circuit – Definition

An electric circuit is a closed and continuous path along which electric current can flow. For current to flow, the circuit must:

• Contain a source of electricity (like a cell or battery),
• Contain conducting wires to provide a path,
• Contain an electric component such as a bulb, and
• Be closed (no break in the path).

Open Circuit vs Closed Circuit

• Closed Circuit: A circuit in which the path from one terminal of the cell, through the components, back to the other terminal is complete. Current flows and the bulb glows.
• Open Circuit: A circuit in which the path is broken (for example, switch is open or a wire is disconnected). Current does not flow and the bulb does not glow.

2. Electric Cell and Battery

Electric Cell

The most common source of electricity in simple circuits is an electric cell. A cell has two terminals:

• Positive terminal (+) – usually the raised metal cap.
• Negative terminal (–) – usually the flat metal base.

Inside the cell, a chemical reaction takes place which produces electrical energy. This energy pushes the charges through the circuit.

Conventional direction of current: In diagrams, current is taken to flow from the positive terminal of the cell to the negative terminal through the external circuit.

Battery

When two or more cells are connected together in the proper way, the combination is called a battery.

• Cells are usually connected so that the positive terminal of one cell is connected to the negative terminal of the next.
• The free negative terminal of the first cell and the free positive terminal of the last cell act as the two terminals of the battery.
• A battery provides more energy than a single cell and can run devices for a longer time or make bulbs glow brighter (depending on circuit).

3. Electric Bulb and Its Filament

An electric bulb is a common device in which electric energy is changed into light and heat energy. The main parts of a small electric bulb are:

• Glass bulb – protects the inner parts.
• Filament – a very thin wire (usually made of tungsten) that glows when current passes through it.
• Support wires and metal cap – help in fixing the bulb in a holder and connecting it to the circuit.

When current flows through the filament, it becomes hot and starts glowing. If the circuit is open or the filament is broken, the bulb does not glow. A bulb with a broken filament is said to be fused.

4. Conductors and Insulators

Not all materials allow electric current to pass through them. Based on this, materials are classified as:

Conductors

• Materials that allow electric current to pass through them are called conductors.
• Examples: most metals like copper, aluminium, iron; also human body, earth, salted water etc.
• Conductors are used to make wires and connections in circuits.

Insulators

• Materials that do not allow electric current to pass through them are called insulators.
• Examples: rubber, plastic, dry wood, glass, pure distilled water, air (in normal conditions).
• Insulators are used to cover wires and handles of tools so that we can use them safely.

5. Switches (Keys) and Circuit Diagrams

Switch (Key)

A switch or key is a device used to open or close an electric circuit.

• When the switch is ON (closed), the circuit is complete and current flows.
• When the switch is OFF (open), the circuit is broken and current does not flow.

Circuit Diagrams

Drawing real pictures of cells, bulbs, and switches is time-consuming. So we use standard symbols to represent them. A circuit drawn using these symbols is called a circuit diagram.

Common symbols (as per your level):

• Cell – two parallel lines (long line = positive, short line = negative).
• Battery – combination of cells drawn together.
• Connecting wire – straight line.
• Bulb – circle with a small curved filament or cross inside.
• Switch (open) – gap in wire with a tilted line.
• Switch (closed) – continuous line without gap or line joining two points.

6. Series and Parallel Connections (Simple Idea)

In circuits, cells and bulbs can be connected in different ways. In Class 7, we mainly need a simple idea of series and parallel connections.

Series Connection

• Components are connected end-to-end, one after another, in a single path.
• If one component (like a bulb) breaks or is removed, the entire circuit breaks and current stops.
• In series connection of cells, the battery gives more energy (bulb can glow brighter).

Parallel Connection

• Components are connected in different branches between the same two points.
• If one bulb fails, current can still pass through other branches and other bulbs keep glowing.
• Commonly used in house wiring so that each light and fan works independently.

7. Heating Effect of Current and Fuse (Concept Overview)

When electric current flows through a conductor, it can produce heat. This is called the heating effect of electric current. Devices like electric iron, electric heater and electric kettle work on this principle.

Electric Fuse

A fuse is a very important safety device used in electric circuits.

• It is a thin wire which is easily fusible (melts quickly when current is too high).
• It is connected in series in the circuit.
• If current becomes too high (due to short circuit or overloading), the fuse wire melts and breaks the circuit, stopping the flow of current and preventing damage.

8. Safe Use of Electricity – Do’s and Don’ts

Electricity is very useful but can be dangerous if not handled carefully. In Class 7, you should know some basic safety rules.

Do’s

• Always switch off the main switch before repairing or changing any electrical appliance.
• Use dry hands while touching switches or plugs.
• Use proper insulated wires and avoid broken wire coverings.
• Inform elders or an electrician if you notice any spark, burning smell or loose wires.

Don’ts

• Never touch switches, plugs or wires with wet hands.
• Never insert fingers or metal objects into a plug point or socket.
• Do not overload a socket with too many plugs at a time.
• Do not try to repair major faults on your own – call a trained electrician.

9. Summary – Quick Revision Points

• Electricity is a form of energy produced by the flow of electric charges.
• An electric circuit is a closed path through which current flows from the positive terminal of a cell to the negative terminal.
• In an open circuit, the path is broken and the current does not flow; the bulb does not glow.
• An electric cell has two terminals – positive (+) and negative (–). A combination of cells is called a battery.
• An electric bulb has a filament; when current passes through it, it glows. A bulb with a broken filament is called a fused bulb.
• Conductors allow current to pass (metals), while insulators do not (rubber, plastic).
• A switch is used to open or close a circuit. Closed switch → current flows; open switch → current stops.
• Circuit diagrams use standard symbols for cells, batteries, bulbs, wires and switches.
• In a series connection, components are connected one after another; if one fails, the whole circuit is broken.
• In a parallel connection, components are in separate branches; if one fails, others can still work.
• The heating effect of current is used in devices like heaters and irons.
• A fuse is a thin wire that melts when current is too high, protecting the circuit from damage.
• We must follow safety rules while using electricity to avoid shocks and accidents.