Electricity: Circuits and Their Components
Chapter Notes — Class 7 Science (Curiosity)
Table of Contents
1. Introduction
2. A Torchlight
3. A Simple Electrical Circuit
4. Electric Lamp
5. Making an Electric Lamp Glow Using an Electric Cell or Battery
6. Electrical Circuit
7. Electric Switch
8. Circuit Diagrams
9. Electrical Conductors and Insulators
10. Points to Remember
11. Difficult Words and Their Meanings
1. Introduction
Nihal and his classmates are excited about their upcoming school trip to the Bhakra Nangal Dam. There, they will see how falling water is used to generate electricity at a hydroelectric power house. They will enjoy a scenic 13-kilometre train ride along the Sutlej River and through the Shivalik Hills.
Before the trip, their teacher asks them to prepare a presentation on the uses of electricity. The students explore the applications of electricity at home, at school, in the neighbourhood and in the city. They learn that electricity can be generated from various sources, such as falling water, wind and solar energy. They also learn that electricity is essential in daily life but must be handled safely.
Portable sources of electricity, such as cells and batteries used in torchlights, wall clocks and remote controls, can be used for safe, simple experiments.
Caution: The warning signs on electric poles and on many appliances remind us that electricity can be dangerous if it is not handled carefully. Never perform experiments using the mains power supply at home or at school. Even portable generators can be unsafe. For safe hands-on work, use only batteries or cells such as those found in torchlights, wall clocks, radios or remote controls.
2. A Torchlight
What is a Torchlight?
A torchlight, also called a torch or flashlight, is a portable device that produces light and is commonly used to see in the dark.
Components of a Torchlight
Lamp: The part that produces light when the torch is turned on.
Switch: A control that turns the lamp on or off. Sliding the switch to one position completes the circuit and makes the lamp glow; sliding it back breaks the circuit and turns the lamp off.
Electric cells: Two or more cells placed inside the torch provide the electrical energy needed to make the lamp glow.
How it works
When the switch is in the ON position, it connects the cells to the lamp and allows electric current to flow through the lamp, causing it to glow. In the OFF position, the connection is broken and the lamp does not glow.
3. A Simple Electrical Circuit
What is an Electrical Circuit?
An electrical circuit is a complete path that allows electric current to flow from a power source, such as a cell, through a device, such as a lamp, and back to the power source so that the device works.
Electric Cell
An electric cell is a small, portable source of electrical energy used in torchlights, clocks and remote controls.
Terminals
Positive terminal: It is marked with a (+) sign and often has a raised metal cap.
Negative terminal: It is marked with a (-) sign and often has a flat metal disc.
The cell provides electrical energy produced by chemical reactions inside it. This energy makes current flow through an external circuit, powering the devices connected in the circuit. In a simple circuit, conventional current is shown as flowing from the positive terminal to the negative terminal through the external circuit.
Battery
A battery is a combination of two or more electric cells connected together so that they supply more energy or last longer than a single cell.
Connection: In a battery, the positive terminal of one cell is connected to the negative terminal of the next cell. Connecting cells in this way increases the total voltage available to the circuit.
Example: In many torchlights, two cells are placed end to end so that the positive terminal of one touches the negative terminal of the other. This arrangement makes the lamp glow when the circuit is completed.
Fascinating Fact: The word "battery" is often used for a single cell, for example, a mobile phone battery. Technically, however, a battery is made up of two or more cells connected together.
4. Electric Lamp
1. Incandescent Lamp
Structure: An incandescent lamp has a glass bulb containing a thin wire called a filament. The filament is supported by two thicker wires inside the bulb.
Terminals: The filament is connected to two terminals: one is the metal case of the lamp's base and the other is the metal tip at the centre of the base.
How it works: When electric current flows through the filament, it becomes hot and glows, producing light.
Usage: These lamps are found in older torchlights and other devices. An incandescent bulb glows regardless of which terminal is connected to the positive or negative terminal of the cell because the filament is a simple resistive element.
Fascinating Fact: If the filament breaks, the lamp is said to be "fused". Current cannot flow through the broken filament, so the lamp stops glowing.
2. LED Lamp
Structure: A Light Emitting Diode, or LED, has no filament. It has two leads: a longer lead that is the positive terminal, called the anode, and a shorter lead that is the negative terminal, called the cathode.
How it works: An LED allows current to flow in only one direction. It glows only when connected in the correct way: the LED's positive terminal, or longer lead, must be connected to the battery's positive terminal, and the LED's negative terminal, or shorter lead, must be connected to the battery's negative terminal.
Usage: Many modern torchlights use LEDs because they are energy efficient and long-lasting.
To make a lamp glow, we need a simple circuit containing a cell or battery, a lamp, a cell holder and connecting wires. Each component has a clear role:
Electric cell: It provides the electrical energy required to make the lamp glow. This energy is produced by chemical reactions inside the cell.
Incandescent lamp: It converts electrical energy into light. It contains a filament that glows when current passes through it.
Cell holder: It holds the cell securely and ensures proper contact between the cell terminals and the circuit wires.
Electric wires: Conductive wires connect the cell terminals to the lamp so that electric current can flow.
Connection: The two terminals of the electric cell are connected to the lamp using electric wires to form a closed path.
Current flow: When the circuit is complete, or closed, electric current flows from one terminal of the cell through the wires and lamp and returns to the other terminal of the cell. The filament in an incandescent lamp heats up and produces light.
Prediction of lamp glow: If any part of the circuit is disconnected, the circuit is open. Current cannot flow and the lamp will not glow. If all the connections are proper and the cell has sufficient energy, the lamp will glow.
A circuit is a closed loop that allows current to flow from the positive terminal of a cell, through a device such as a lamp, and back to the negative terminal.
Current flow, conventional: Electric current is often shown as flowing from the positive terminal to the negative terminal of the cell. This is called the conventional direction of current.
Incandescent lamp: It glows when the circuit is complete, regardless of which terminal of the lamp is connected to which cell terminal.
LED: It glows only when connected with the correct polarity: its positive lead must be connected to the battery's positive terminal and its negative lead to the battery's negative terminal, because it conducts in only one direction.
A switch is a device that controls the flow of current by completing, or closing, a circuit and breaking, or opening, a circuit.
How it Works
ON position: When the switch is closed, it completes the circuit and allows current to flow, so the lamp glows.
OFF position: When the switch is opened, it breaks the circuit. Current stops flowing and the lamp does not glow.
Placement: A switch can be placed at any convenient point in the circuit. As long as it opens or closes the path, it will control the flow of current.
Example: In a torch, sliding the switch to "on" completes the circuit and the lamp glows. Sliding it to "off" breaks the circuit and the lamp goes out.
Real-life switches: Home switches for lights operate in the same basic way, but they are designed for safety and convenience when working with higher voltages from the mains.
A circuit diagram is a drawing that uses standard symbols to represent the components in an electrical circuit. It makes the connections and the working of the circuit easy to understand.
Symbols for Components
Purpose: Standard symbols simplify the task of drawing and understanding circuits. A complex circuit can be shown clearly and compactly by using these symbols.
Dive Deeper - Standardization: International bodies such as the International Electrotechnical Commission (IEC), the American National Standards Institute (ANSI) and the Institute of Electrical and Electronics Engineers (IEEE) establish standard symbols so that diagrams can be understood worldwide.
9. Electrical Conductors and Insulators
Materials are classified according to whether they allow electric current to pass through them easily.
Conductors
Materials that allow electric current to flow easily are called conductors or good conductors of electricity.
Examples: Metals such as copper, silver, gold and aluminium; everyday objects such as keys, coins and sewing needles when they are made of metal.
Use in circuits: Conductors, especially copper, are used for wiring because they allow current to flow efficiently with a small loss of energy.
Dive Deeper - Best Conductors: Silver, copper and gold are among the best conductors. Copper is used most commonly because it is less expensive and more widely available than silver or gold.
Insulators
Materials that do not allow electric current to pass through them easily are called insulators or poor conductors of electricity.
Examples: Plastic, rubber, glass, wood, cork, paper, wax, ceramics and objects such as plastic rulers, erasers and candles.
Use in circuits: Insulators are used to cover wires and to make plug tops and switches so that people are protected from electric shocks.
Conductors: They are used for wires, switches, plug pins and sockets to carry current where it is needed.
Insulators: They are used as coverings and supports to prevent accidental contact with live conductors and to ensure safety.
Caution: The human body conducts electricity. Electric current passing through the body can cause serious injury or death. Never touch switches or plugs with wet hands, avoid using electrical devices in wet places and do not handle damaged electrical equipment.
Dive Deeper - Types of Electricity:
Direct current (DC): It is produced by cells and batteries, and current flows in one direction. It is used in torchlights, mobile phones and other portable devices.
Alternating current (AC): It is supplied to homes and buildings from power stations through the mains. The direction of current reverses periodically. AC is used for larger appliances at home and in industry.
An electric cell provides portable electrical energy and has a positive terminal, usually a metal cap, and a negative terminal, usually a metal disc.
A battery is formed by connecting two or more cells so that the positive terminal of one cell touches the negative terminal of the next. This increases the total voltage or the useful life of the arrangement.
The word "battery" is sometimes used for a single cell, for example, in mobile phones, although technically a battery is a group of cells.
An incandescent lamp contains a filament that glows when heated by electric current. It has two terminals, the metal case and the centre tip, and it glows regardless of which terminal is connected to the cell's positive or negative terminal.
A fused incandescent lamp does not glow because its filament has broken and current cannot flow.
An LED, or Light Emitting Diode, has no filament. It glows only when connected with the correct polarity: its positive lead to the battery's positive terminal and its negative lead to the battery's negative terminal, because it conducts in only one direction.
A switch completes a circuit in the ON position or breaks it in the OFF position, thereby controlling current flow. It can be placed at any convenient point in the circuit.
An electrical circuit is a closed path through which current flows from the positive to the negative terminal of a cell to make devices such as lamps glow.
Circuit diagrams use standard symbols, such as long and short lines for cell terminals and standard shapes for lamps, switches and LEDs, so circuits can be understood worldwide. Standard symbols are set by organisations such as IEC, ANSI and IEEE.
Conductors, such as copper and silver, allow current to flow easily and are used for wiring. Copper is most commonly used because of its good conductivity, lower cost and availability.
Insulators, such as plastic and rubber, do not allow current to flow easily and are used to cover conductive parts for safety.
The human body conducts electricity, so unsafe handling of electrical devices, such as using them with wet hands, can be dangerous.
Battery-operated devices use direct current (DC), while wall sockets supply alternating current (AC) for household appliances.
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