Class 7 Science NCERT Notes – Chapter 3: Electricity, Circuits, and Components (PDF, MindMap, Q&A, Quizzes)

Chapter 3 (Science): Electricity, Circuits, and Components – CBSE Class 7 NCERT Science Detailed Study Notes.

1. Introduction to Electricity

Electricity is a fundamental part of modern life, used in countless applications across various sectors.

Common Uses of Electricity:

• Entertainment: Television, radio.
• Communication: Mobile phones, Internet.
• Lighting: Homes, offices, streets, markets, factories.
• Cooking: Electric kettle, mixer grinder, toaster, oven, microwave.
• Transportation: Trains, buses, cars, scooters, lifts, escalators.
• Heating and Cooling: Fans, room heaters, immersion rods, geysers, refrigerators, air conditioners.
• Other Applications: Water pumps, cranes, computers.

Sources of Electrical Energy: Electricity is generated through various methods, including:

• Hydroelectric power houses (e.g., Bhakra Nangal Dam), using the force of falling water.
• Windmills, harnessing wind energy.
• Solar panels, capturing the Sun’s energy.
• Burning natural gas or coal.

Safety Precautions: Electricity can be extremely dangerous if not handled carefully. For all experiments and learning activities, it is crucial to only use batteries or cells like those found in torchlights, clocks, or remotes. Never experiment with the power supply from wall sockets, electrical outlets, or portable generators, as this can cause severe injury or death.

2. Core Electrical Components

2.1 The Electric Cell

An electric cell is a portable source of electrical energy. It is the fundamental power unit for many small devices.

• Terminals: Every electric cell has two terminals: a positive (+ve) terminal and a negative (–ve) terminal.
• Identification: The metal cap on the cell is the positive terminal, and the flat metal disc at the base is the negative terminal.

2.2 The Battery

A battery is a combination of two or more electric cells.

• Connection: To create a battery, cells must be connected in a specific order: the positive terminal of one cell must be connected to the negative terminal of the next cell. This series connection allows the battery to function correctly.

2.3 The Electric Lamp

Electric lamps convert electrical energy into light. There are different types, such as incandescent lamps and LEDs.

• Incandescent Lamp:
  • Filament: Contains a thin wire called the filament, which is located inside a glass bulb. When electric current passes through the filament, it gets hot and glows, producing light.
  • Terminals: The filament is supported by two thicker wires. One wire connects to the metal case at the lamp’s base, and the other connects to the metal tip at the center of the base. These form the two terminals of the lamp.
  • Fused Bulb: An incandescent lamp that does not glow, even when connected to a power source, is called ‘fused’. This is usually because the filament has broken, which interrupts the flow of current.
• Light Emitting Diode (LED):
  • Structure: LEDs do not have a filament.
  • Terminals: They have two terminals, identified by the length of their wires: a positive terminal (longer wire) and a negative terminal (shorter wire).
  • Directional Current: Unlike incandescent lamps, electric current can only pass through an LED in one direction. It will only glow if its positive terminal is connected to the positive terminal of the battery and its negative terminal to the negative terminal of the battery.

2.4 The Electric Switch

A switch is a simple device used to either complete (close) or break (open) an electrical circuit.

• Function: It allows users to control the flow of electricity as required.
• ‘ON’ Position: When a switch is in the ‘ON’ position, it closes the circuit, completing the path for the current to flow.
• ‘OFF’ Position: When a switch is in the ‘OFF’ position, it creates a gap in the circuit, which is then considered ‘open’. This gap prevents the current from flowing.
• Placement: A switch can be placed anywhere within an electrical circuit.

3. Electrical Circuits

An electrical circuit is a complete and continuous path for electric current to flow. The lamp or device in a circuit will only operate when the current can pass through the entire circuit without interruption.

• Direction of Current: By convention, the direction of electric current in a circuit is taken to be from the positive (+ve) terminal to the negative (–ve) terminal of the electric cell or battery.
• Open Circuit: A circuit with a break or gap (such as an open switch) that prevents the flow of current.
• Closed Circuit: A complete, unbroken circuit that allows current to flow.

4. Circuit Diagrams

A circuit diagram is a simplified representation of an electrical circuit that uses standardized symbols for its components. Using symbols makes it easier to draw, read, and understand circuits globally. International organizations like the International Electrotechnical Commission (IEC) and the Institute of Electrical and Electronics Engineers (IEEE) help create these standard symbols.

Component	Symbol
Electric Cell	A long line (positive) and a shorter, thicker parallel line (negative).
Battery	A series of two or more cell symbols.
Incandescent Lamp	A circle with a cross inside it.
LED	A triangle pointing in the direction of current flow, with a line at its tip and two arrows indicating light emission.
Switch (Open/’OFF’)	Two small circles with a line angled away from one, showing a gap.
Switch (Closed/’ON’)	Two small circles connected by a straight line.
Connecting Wire	A straight line.

5. Conductors and Insulators

Materials are classified based on their ability to allow electric current to pass through them.

5.1 Electrical Conductors

These are materials through which electric current can flow easily.

• Examples: Metals such as silver, copper, and gold are excellent conductors. Other examples include metal spoons, coins, keys, pins, aluminium foil, and pencil lead.
• Application: Copper is widely used for making electrical wires because it is an effective conductor and is less costly and more abundant than silver or gold. The human body is also a conductor of electricity.

5.2 Electrical Insulators

These are materials through which electric current cannot pass. They are also known as poor conductors.

• Examples: Rubber, plastic, cork, glass, wood, ceramics, cardboard, and paper.
• Application: Insulators are crucial for safety. They are used to cover electrical wires, plug tops, and switches to protect people from electric shocks.

6. Types of Electric Current

• Direct Current (DC): This is the type of electricity supplied by batteries and cells. It typically powers smaller, portable devices.
• Alternating Current (AC): This is the type of electricity supplied by power plants to homes and factories through wall sockets. It is used to run larger appliances.

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Quiz and Q&A Section

Part 1: Short-Answer Questions

(Instructions: Answer each question in 2-3 complete sentences.)

1. What is an electric cell and what are its main components?
2. Define a battery and explain the correct way to connect multiple cells to form one.
3. Describe the function of the filament in an incandescent lamp.
4. What does it mean when an incandescent lamp is “fused”?
5. How do the terminals of an LED differ from each other, and why is this important?
6. Explain what an electrical circuit is and why it must be a “complete path.”
7. What is the conventional direction of electric current flow in a circuit?
8. Describe the function of an electric switch in a circuit.
9. What is the difference between the ‘ON’ and ‘OFF’ positions of a switch?
10. What is a circuit diagram and why is it useful?
11. Define an electrical conductor and provide three examples.
12. Define an electrical insulator and provide three examples.
13. Why is copper commonly used for electrical wires instead of silver or gold?
14. Explain why electrical wires are typically covered with plastic or rubber.
15. What is the key difference between Direct Current (DC) and Alternating Current (AC)?
16. Name two sources of AC power and two sources of DC power mentioned in the text.
17. What is the primary safety rule for conducting experiments with electricity?
18. How are the two terminals of an incandescent lamp formed?
19. Explain the rule for connecting an LED into a circuit to make it glow.
20. Why is the human body considered a conductor of electricity?
21. List five different categories of electricity usage in daily life.
22. Name three methods of generating electricity mentioned in the text.
23. What materials are needed to construct the simple switch described in the text?
24. What do international organizations like the IEEE and IEC do regarding electrical components?
25. Describe two safety precautions to take when handling electrical appliances at home.

Part 2: Multiple-Choice Questions

(Instructions: Choose the best answer for each question.)

1. Which of the following is the positive terminal of an electric cell?
   • a) The flat metal disc b) The plastic casing c) The metal cap d) The side of the cell
2. A combination of two or more cells is called a(n):
   • a) Circuit b) Battery c) Insulator d) Filament
3. What part of an incandescent lamp glows to produce light?
   • a) Glass bulb b) Metal case c) Filament d) Insulator
4. In an LED, which wire represents the positive terminal?
   • a) The shorter wire b) The thicker wire c) The longer wire d) The coiled wire
5. The conventional direction of current flow in a circuit is from:
   • a) Negative to positive terminal b) Positive to negative terminal c) The switch to the lamp d) The lamp to the cell
6. A device that breaks or completes a circuit is a(n):
   • a) Cell b) Lamp c) Switch d) Wire
7. Which of the following materials is an electrical conductor?
   • a) Rubber b) Wood c) Plastic d) Copper
8. Which of the following materials is an electrical insulator?
   • a) A metal spoon b) Aluminum foil c) A plastic scale d) A sewing needle
9. A circuit in the ‘OFF’ position is also known as a(n):
   • a) Closed circuit b) Fused circuit c) Open circuit d) Short circuit
10. A representation of an electrical circuit using symbols is called a:
    • a) Circuit blueprint b) Circuit diagram c) Electrical map d) Component list
11. Electricity from a wall socket is typically:
    • a) Alternating Current (AC) b) Direct Current (DC) c) Static Current (SC) d) Battery Current (BC)
12. A “fused” incandescent lamp fails to glow because of a:
    • a) Cracked glass bulb b) Broken filament c) Dead battery d) Loose switch
13. How must cells be arranged in a battery for it to work?
    • a) Positive to positive, negative to negative b) Positive of one to negative of the next c) Negative of one to negative of the next d) In any order
14. What safety measure is mentioned for using electrical appliances?
    • a) Using them only during the day b) Never touching switches with wet hands c) Keeping them away from windows d) Unplugging them every night
15. An LED will glow only if:
    • a) It is connected to AC power b) The current flows in one specific direction c) It is hotter than the surrounding air d) The circuit has two switches
16. What is the primary purpose of insulators in electrical systems?
    • a) To make wires stronger b) To protect people from electric shocks c) To help current flow faster d) To make appliances look better
17. Which of these is NOT a source of electricity generation mentioned in the text?
    • a) Windmills b) Solar panels c) Geothermal vents d) Falling water
18. In the symbol for an electric cell, the longer line represents the:
    • a) Negative terminal b) Switch c) Positive terminal d) Filament
19. The human body is a(n):
    • a) Insulator of electricity b) Conductor of electricity c) Source of electricity d) Switch for electricity
20. For school experiments, what is the only safe source of electricity to use?
    • a) A wall socket b) A portable generator c) Cells or batteries d) A solar panel connected to an inverter

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Essay Questions & Answers

1. Describe in detail the components of an incandescent lamp and explain how it produces light. Also, explain what causes it to “fuse.” An incandescent lamp consists of several key parts. Inside a glass bulb, there is a thin wire called the filament. This filament is attached to two thicker support wires. These thicker wires connect the filament to the lamp’s two terminals: one wire is connected to the metal case at the base of the lamp, and the other is connected to the metal tip at the very center of the base. To produce light, an electric current must pass through this complete circuit. When current flows from the cell, through the terminals, and into the filament, the filament gets very hot and glows brightly, emitting light. A lamp “fuses” when this filament breaks. A broken filament creates a gap in the circuit, stopping the flow of current and preventing the lamp from glowing.

2. Compare and contrast an incandescent lamp with a Light Emitting Diode (LED) based on their structure, terminals, and how they function in a circuit. Incandescent lamps and LEDs differ significantly. Structurally, an incandescent lamp contains a filament inside a glass bulb, which is the component that heats up to produce light. An LED, however, has no filament. Regarding terminals, an incandescent lamp has two terminals (the metal case and the metal tip) and it doesn’t matter which connects to the positive or negative side of the battery. An LED has a distinct positive terminal (the longer wire) and a negative terminal (the shorter wire). This distinction is critical to its function, as an LED allows current to pass in only one direction. For an LED to glow, its positive terminal must be connected to the positive terminal of the battery, and its negative terminal to the negative terminal of the battery; reversing the connection will prevent it from lighting up.

3. Explain the concept of an electrical circuit, distinguishing between an open and a closed circuit. How does a switch control this state? An electrical circuit is a complete, continuous path that allows electric current to flow from a source, like a battery, through components, like a lamp, and back to the source. The current conventionally flows from the positive terminal to the negative terminal. A circuit must be unbroken for electricity to flow. A closed circuit is one where this path is complete, and current can flow freely, allowing a device like a lamp to glow. An open circuit has a break or gap in the path, which prevents the flow of current, and the device will not operate. An electric switch is a device designed to control this state. When the switch is in the ‘ON’ position, it closes the gap, creating a closed circuit. When it is moved to the ‘OFF’ position, it creates a gap, resulting in an open circuit.

4. What are electrical conductors and insulators? Provide examples of each and explain their importance and application in everyday electrical systems. Electrical conductors are materials that allow electric current to flow through them easily. Examples include metals like copper, silver, and aluminum, as well as pencil lead and the human body. Insulators are materials that do not allow electric current to pass through them, such as plastic, rubber, wood, glass, and ceramics. Both are critically important in electrical systems. Conductors, like copper wires, are used to create the pathways for electricity to travel from the source to the appliance. Insulators are used for safety; they cover wires, switches, and plugs to prevent the electrical current from coming into contact with people, which would cause a dangerous electric shock.

5. Describe how to build the simple switch shown in the text. Explain the principle behind its operation in both the ‘ON’ and ‘OFF’ positions. To build the simple switch, you need two drawing pins, a safety pin (or paper clip), two wires, and a small piece of cardboard. First, insert one drawing pin through the ring at the end of the safety pin and fix it to the cardboard, ensuring the safety pin can rotate freely. Then, fix the second drawing pin to the cardboard in a position where the free end of the safety pin can touch it. Finally, connect one wire to each drawing pin. The principle of its operation is based on completing or breaking a circuit. In the ‘OFF’ position, the safety pin does not touch the second drawing pin, creating a gap that makes the circuit open, so no current flows. To turn it ‘ON’, you rotate the safety pin until its free end touches the second drawing pin. This closes the gap, completes the circuit (making it a closed circuit), and allows current to flow from the cell to the lamp.

6. Explain what a battery is and the critical rule for connecting cells to form one. What happens if the cells are connected incorrectly? A battery is defined as a combination of two or more electric cells used to power a device. There is a critical rule for connecting these cells: the positive terminal of one cell must be connected to the negative terminal of the next cell in the series. This creates a continuous path for the electrical energy to flow with increased voltage. If the cells are connected incorrectly, for instance by connecting a positive terminal to another positive terminal or by reversing the direction of one cell in the series, the circuit will not be properly formed. As a result, the electric current will not flow, and the device, such as a torch lamp, will not glow.

7. What is a circuit diagram? Draw a simple circuit diagram for a torch with one cell, a switch, and an incandescent lamp, and label each symbol. A circuit diagram is a representation of an electrical circuit that uses standard, simplified symbols for its various electrical components. This makes circuits easier to draw and understand universally. A simple circuit diagram for a torch would include:

• The symbol for one electric cell (a long line for positive and a shorter, thicker line for negative).
• The symbol for an incandescent lamp (a circle with a cross inside it).
• The symbol for a switch (two small circles connected by a line when ‘ON’ or with a broken line when ‘OFF’).
• Lines representing the connecting wires. (A drawing is not possible, but the description is as follows: The symbol for the cell would be connected by a line (wire) to the symbol for the switch. The switch symbol would then be connected by another line to the lamp symbol. Finally, the lamp symbol would be connected by a line back to the other terminal of the cell symbol, completing the loop.)

8. Why is it extremely dangerous to experiment with electricity from a wall socket, and what is the recommended alternative for learning activities? Explain the difference in the types of current involved. It is extremely dangerous to experiment with electricity from a wall socket because it is Alternating Current (AC) from a power plant, which is of a much higher voltage and power than that from a battery. This level of electricity can cause severe injury, electric shock, or even death if handled improperly. Our body is a conductor, and this powerful current can easily pass through it. The recommended alternative for learning activities and experiments is to use only batteries or electric cells. These provide Direct Current (DC), which is a much lower and safer voltage suitable for powering small devices like torchlights and for educational purposes without the risk of serious harm.

9. The text mentions several ways electricity is generated. Describe three of these methods. The text outlines multiple methods for generating electricity. One method is hydroelectric power, which is used at facilities like the Bhakra Nangal Dam. This process uses the force of falling water to turn turbines and generate electricity. A second method involves using windmills, which capture the kinetic energy from wind to turn blades connected to a generator. A third method is through solar panels, which contain photovoltaic cells that capture the sun’s energy and convert it directly into electrical energy.

10. What safety advice does the text provide regarding the use of everyday electrical appliances? Explain why this advice is important. The text provides several critical pieces of safety advice for handling everyday electrical appliances. It warns to never touch switches or plugs with wet hands and to avoid using electrical devices in wet areas. It also stresses the importance of not handling equipment that has damaged insulation or broken plugs. This advice is important because water reduces the body’s resistance to electricity, and our bodies are natural conductors. Touching an electrical device with wet hands or in a wet environment creates a very easy path for current to flow through the body, leading to a severe electric shock. Damaged insulation or plugs expose the live conductors, and touching them can also result in a direct, dangerous shock.

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Answer Keys

Part 1: Short-Answer Questions – Answer Key

1. An electric cell is a portable source of electrical energy. Its main components are two terminals: a positive (+ve) terminal, which is the metal cap, and a negative (–ve) terminal, which is the flat metal disc.
2. A battery is a combination of two or more electric cells. To form a battery, the positive terminal of one cell must be connected to the negative terminal of the next cell.
3. The filament is a thin wire inside the glass bulb of an incandescent lamp. When electric current passes through it, the filament gets very hot and glows, which produces light.
4. A “fused” incandescent lamp means its filament is broken. A broken filament creates a gap in the circuit, which stops the flow of current and prevents the lamp from glowing.
5. An LED has a positive terminal (attached to a longer wire) and a negative terminal (the shorter wire). This is important because current can only flow through an LED in one direction, from positive to negative.
6. An electrical circuit is the complete path along which an electric current can flow. It must be a complete, or “closed,” path without any gaps for the current to travel from the source, through the components, and back again.
7. The conventional direction of electric current flow in a circuit is from the positive (+ve) terminal of the electric cell to the negative (–ve) terminal.
8. An electric switch is a simple device that either completes a circuit or breaks it. It allows a user to control the flow of electricity, turning a device on or off.
9. The ‘ON’ position means the switch closes the circuit, allowing current to flow. The ‘OFF’ position means the switch creates a gap, opening the circuit and stopping the current.
10. A circuit diagram is a representation of an electrical circuit using standard symbols for its components. It is useful because it provides a simple, clear, and universally understood way to show how a circuit is connected.
11. An electrical conductor is a material that allows electric current to flow through it easily. Three examples are copper, aluminum foil, and a metal spoon.
12. An electrical insulator is a material that does not allow electric current to pass through it. Three examples are rubber, plastic, and wood.
13. Although silver and gold are better conductors, copper is used for electrical wires mainly due to its comparatively lower cost and abundant supply.
14. Electrical wires are covered with insulators like plastic or rubber for safety. These materials prevent the electric current from passing to someone who might touch the wire, thereby protecting them from electric shocks.
15. Direct Current (DC) is the type of electricity from batteries that powers small devices. Alternating Current (AC) is the type from power plants and wall sockets that runs larger appliances.
16. Sources of AC power include power plants using coal or falling water. Sources of DC power are electric cells and batteries.
17. The primary safety rule is to never perform experiments with the power supply from a home or school outlet. One must only use batteries or cells for experiments.
18. The two terminals of an incandescent lamp are the metal case at its base and the metal tip at the center of the base. These are connected to the filament by thicker support wires.
19. To make an LED glow, its positive terminal (the longer wire) must be connected to the positive terminal of the battery, and its negative terminal (the shorter wire) must be connected to the negative terminal of the battery.
20. The human body is considered a conductor because electric current can pass through it. This is why handling electricity carelessly can cause severe injury or death.
21. Five categories are: Entertainment (TV), Communication (phone), Lighting (homes), Cooking (oven), and Transportation (train).
22. Three methods of generating electricity are using the force of falling water (hydroelectric), using windmills to capture wind energy, and using solar panels to capture the sun’s energy.
23. To build the simple switch, you need two drawing pins, a safety pin or paper clip, two wires, and a small piece of cardboard.
24. International organizations like the IEEE and IEC create standard symbols for electrical and electronic parts. This standardization helps people from different countries and industries understand circuit diagrams easily.
25. Two safety precautions are to never touch switches or plugs with wet hands and to not use electrical equipment with damaged insulation or broken plugs.

Part 2: Multiple-Choice Questions – Answer Key

1. c) The metal cap
2. b) Battery
3. c) Filament
4. c) The longer wire
5. b) Positive to negative terminal
6. c) Switch
7. d) Copper
8. c) A plastic scale
9. c) Open circuit
10. b) Circuit diagram
11. a) Alternating Current (AC)
12. b) Broken filament
13. b) Positive of one to negative of the next
14. b) Never touching switches with wet hands
15. b) The current flows in one specific direction
16. b) To protect people from electric shocks
17. c) Geothermal vents
18. c) Positive terminal
19. b) Conductor of electricity
20. c) Cells or batteries

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Glossary of Key Terms

Term	Definition
Alternating Current (AC)	The type of electricity from power plants that comes to the wall socket and can run larger appliances.
Battery	A combination of two or more electric cells.
Circuit Diagram	A representation of an electrical circuit using symbols.
Closed Circuit	A complete, unbroken electrical circuit that allows current to flow.
Conductor	A material through which electric current can flow easily.
Direct Current (DC)	The type of electricity from batteries that usually powers small devices.
Electric Cell	A portable source of electrical energy with a positive and a negative terminal.
Electric Circuit	A complete path for electric current to flow.
Filament	The thin wire inside an incandescent lamp that gets hot and glows to produce light when electric current passes through it.
Insulator	A material through which electric current cannot pass.
Light Emitting Diode (LED)	An electric lamp without a filament that allows current to pass in only one direction. It has a positive (longer wire) and negative (shorter wire) terminal.
Negative Terminal (–ve)	One of the two terminals of an electric cell or battery, typically the flat metal disc on a cell.
Open Circuit	An electrical circuit with a break or gap that prevents the flow of current.
Positive Terminal (+ve)	One of the two terminals of an electric cell or battery, typically the metal cap on a cell.
Switch	A simple device that either completes (closes) or breaks (opens) a circuit.
Terminals	The connection points of an electrical component, such as a cell or lamp, where wires are attached.