CBSE • Class 7 Science • NCERT Curiosity

Electricity: Circuits and Their Components

Detailed concept notes with a chapter summary, sticky notes, key definitions, original circuit diagrams, comparisons, activities, mind map and NCERT-based, Exemplar-style and Olympiad-level questions.

Chapter 3

Summary

Sticky Notes

Mind Map

Important Q&A

Summary Definitions Cell & Battery Lamps Circuits Symbols Conductors Differences Mind Map Q&A

Chapter Summary and Sticky Notes

Central idea: Electric current flows only when a source and the components are connected in a complete conducting path. A switch controls this path, while conductors carry current and insulators help keep us safe.

⚡ Energy Source

An electric cell is a portable source of electrical energy. It has a positive terminal and a negative terminal.

🔋 Battery Rule

In a battery, the positive terminal of one cell connects to the negative terminal of the next cell.

🔁 Complete Path

Current needs a closed, unbroken circuit. Any gap makes the circuit open and stops current.

💡 Lamp Rule

An incandescent lamp glows when current heats its thin filament. A broken filament makes the lamp fused.

🔆 LED Polarity

An LED works in one direction: longer lead to positive and shorter lead to negative.

⏻ Switch

ON closes the circuit; OFF creates a gap. A switch controls current but does not produce it.

🧲 Conductors

Metals and graphite allow current to pass. Copper is commonly used inside electric wires.

🛡️ Insulators

Rubber, plastic, glass and dry wood resist current and are used for protective coverings.

Essential safety: Perform classroom circuit activities only with small cells or batteries under adult supervision. Never experiment with wall sockets, mains electricity, portable generators or exposed household wiring.

Important Definitions

ElectricityA useful form of energy that operates lights, machines and many everyday devices.

Electrical energyEnergy supplied by a cell, battery or another electrical source to operate a circuit.

Electric cellA portable source of electrical energy with one positive and one negative terminal.

TerminalA point of an electrical component through which it is connected to a circuit.

BatteryA combination of two or more cells connected together, usually positive-to-negative in sequence.

Electric currentThe flow of electricity through a complete conducting path. Its conventional direction is taken from the positive to the negative terminal outside the cell.

Electrical circuitA complete path made of connected electrical components through which current can flow.

Closed circuitA complete, unbroken circuit in which current flows.

Open circuitAn incomplete circuit containing a gap, so current cannot flow.

Electric switchA device that completes or breaks a circuit to control the flow of current.

Electric lampA component that converts electrical energy into light; an incandescent lamp uses a glowing filament.

FilamentThe thin wire inside an incandescent lamp that becomes hot and glows when current passes through it.

Fused lampAn incandescent lamp whose filament has broken, producing an open path inside the lamp.

LEDLight Emitting Diode; a light-producing component that permits current in one direction and must be connected with correct polarity.

Circuit diagramA simplified representation of a circuit drawn with standard symbols for its components.

ConductorA material through which electric current can flow easily.

InsulatorA material through which electric current does not flow easily; also called a poor conductor.

Conduction testerA simple cell–lamp circuit with two free ends used to test whether a material conducts current.

Electric Cell and Battery

Parts of a Cylindrical Cell

Positive terminal: raised metal cap marked “+”.
Negative terminal: flat metal disc marked “−”.
The two terminals must connect to different terminals of the device to form a circuit.

Making a Battery

Use two or more cells.
Connect the positive terminal of one cell to the negative terminal of the next.
The two free outer terminals become the positive and negative terminals of the battery.

Cell and its terminals

The raised cap is positive; the flat end is negative.

Correct series connection

Positive of one cell touches negative of the next.

Memory line: “Plus meets minus in the middle; the two free ends power the circuit.” Reversing one cell may prevent the device from working correctly.

Incandescent Lamp and LED

💡 Incandescent Lamp

A thin filament is attached to two thicker support wires. One terminal is the metal case and the other is the metal tip at the base. Current heats the filament until it glows.

Polarity: Either lamp terminal may be connected to either cell terminal, provided the circuit is complete.

🔆 LED Lamp

An LED has no filament. Its longer lead is positive and its shorter lead is negative. Current passes through it only in one direction.

Correct connection: long lead → battery positive; short lead → battery negative.

Why a lamp may not glow: the cell may be exhausted, the filament may be broken, the circuit may contain a gap, the wire ends may still be insulated, a connection may be loose, or an LED may be reversed.

Electrical Circuit and Switch

A lamp glows only when each terminal of the cell is connected to a different terminal of the lamp through an unbroken conducting path. The conventional direction of current in the external circuit is taken from the positive terminal to the negative terminal of the cell.

Closed circuit — switch ON

The path is complete, so current flows and the lamp glows.

Open circuit — switch OFF

The air gap breaks the path, so current stops and the lamp remains off.

Four conditions for a lamp to glow

1. Working source

The cell or battery must supply electrical energy.

2. Working lamp

The filament must be unbroken, or the LED must be functional.

3. Metal contact

Exposed conducting parts must touch firmly at every connection.

4. Closed path

The circuit must form one continuous loop without a gap.

Switch fact: A switch may be placed anywhere in a simple series circuit. Opening it at any point breaks the only path and stops current everywhere in that circuit.

Circuit Diagrams and Standard Symbols

Symbols make electrical circuits quicker to draw and easier to understand. The long line in a cell symbol represents the positive terminal; the short line represents the negative terminal.

Electric cell

Battery

Electric lamp

LED

Switch ON / closed

Switch OFF / open

Connecting wire

Simple circuit

Exam tip: Use a ruler, join wires neatly, avoid pictorial drawings, label the components if asked, and show the switch open or closed exactly as stated in the question.

Conductors, Insulators and a Conduction Tester

Conductors

Materials that let current pass easily.

Examples: copper, aluminium, iron, steel, silver, gold, coins, keys, metal spoons and graphite in pencil lead.

Insulators / Poor Conductors

Materials through which current does not pass easily.

Examples: rubber, plastic, glass, ceramics, wax, dry wood, paper and air.

How to use a conduction tester

Make a simple closed circuit containing a working cell and lamp, but leave two wire ends free.

Touch the free ends briefly to confirm that the lamp glows. The tester is now working.

Place the test object between the free ends without letting the ends touch each other.

If the lamp glows, the object conducts. If it does not, the object is an insulator or a very poor conductor under these test conditions.

Why wires have two materials: the copper or aluminium core conducts current, while the surrounding plastic or rubber prevents accidental contact and reduces the risk of electric shock.

Activities, Investigation and Troubleshooting

Make a Simple Switch

Fix two drawing pins on cardboard.
Attach a safety pin so it can rotate around one pin.
Connect one wire to each pin.
Touching the second pin closes the circuit; moving away opens it.

Test LED Polarity

Use two cells correctly arranged in a holder.
Connect the longer LED lead to battery positive.
Connect the shorter lead to battery negative.
Reverse the LED and compare. It glows only in the correct direction.

Identify Working Cells

First prepare a tester using a known working lamp and wires.
Test each labelled cell separately in the same circuit.
Keep every other component unchanged.
Record which cells make the lamp glow.

Fair-Test Rules

Change only the item being tested.
Use the same lamp, wires and connections.
Ensure bare metal contact.
Repeat doubtful observations before concluding.

Lamp does not glow: diagnostic checklist

Cell may be exhaustedTest it in a known working circuit or replace it with a verified cell.

Filament may be brokenInspect the lamp and test it separately with a working cell.

Insulation blocks contactExpose about 1 cm of metal at the ends of the connecting wires.

Connection may be loosePress and secure each metal-to-metal contact.

Switch may be openClose the switch and ensure its conducting parts touch.

LED may be reversedConnect the longer lead to positive and shorter lead to negative.

Cells may be arranged wronglyCheck that positive of one meets negative of the next.

Wrong lamp connectionsConnect the cell terminals to the two different lamp terminals, not to the same one.

Electrical Safety

Never use mains supply

School investigations must use only low-voltage cells. Wall sockets can cause severe injury or death.

Keep water away

Never operate switches or appliances with wet hands or use electrical devices in wet areas.

Check insulation

Do not touch damaged wires, broken plugs or exposed conducting parts. Inform a responsible adult.

Human body conducts electricity. Never test conductivity using the body, never place wires in the mouth, and never connect the two terminals of a cell directly for more than an instant because the wire and cell can become hot.

Important Differences

Electric cell and battery

Basis	Electric cell	Battery
Meaning	A single portable source of electrical energy	A combination of two or more cells
Terminals	One positive and one negative terminal	Two outer free terminals after cells are connected
Arrangement	No combination required	Positive of one cell connects to negative of the next
Use	Suitable for devices needing one cell	Used where more cells are required

Open and closed circuit

Basis	Open circuit	Closed circuit
Path	Incomplete; contains a gap	Complete and continuous
Switch position	OFF	ON
Current	Does not flow	Flows through the circuit
Lamp	Does not glow	Glows if all components work

Incandescent lamp and LED

Basis	Incandescent lamp	LED
Full form / type	Lamp that produces light using a heated filament	Light Emitting Diode
Filament	Present	Absent
Polarity	Works with either terminal orientation	Must be connected in the correct direction
Terminal clue	Metal case and metal tip	Longer lead positive; shorter lead negative
If damaged	Broken filament creates an open circuit	May fail to emit light; reversing a good LED also prevents glow

Conductors and insulators

Basis	Conductors	Insulators
Current	Allows current to flow easily	Does not allow current to flow easily
Use in a wire	Forms the inner current-carrying core	Forms the protective outer covering
Examples	Copper, aluminium, iron, graphite	Rubber, plastic, glass, ceramic
Safety role	Carries energy where required	Separates people and objects from conducting parts

Actual circuit and circuit diagram

Basis	Actual circuit	Circuit diagram
Form	Real components joined by wires	Drawing made with standard symbols
Appearance	Components have their real shapes and sizes	Symbols are simple and standardised
Purpose	Performs the electrical task	Communicates how components are connected

Direct current and alternating current

Basis	Direct current (DC)	Alternating current (AC)
Typical source at this level	Cells and batteries	Electricity supplied to wall sockets
Direction	Current has one fixed direction	Direction changes repeatedly
Common use	Small portable devices	Household and larger electrical appliances
Safety	Only small cells should be used for school activities	Never use mains AC for experiments

Complete Chapter Mind Map

Electricity: Circuits & ComponentsSource → path → control → output → materials → safety

🔋 Source

Electric cell
Positive and negative terminals
Battery: 2 or more cells
+ of one to − of next

💡 Output Devices

Incandescent lamp
Hot glowing filament
LED has no filament
LED needs correct polarity

🔁 Circuit

Complete conducting path
Current: + to −
Closed = current
Open = no current

⏻ Control

Switch completes/breaks
ON = closed
OFF = air gap
May be placed anywhere

🧲 Materials

Conductors carry current
Metals and graphite
Insulators resist current
Plastic, rubber, glass

✏️ Communication

Standard symbols
Cell, battery, lamp
LED, switch, wire
Circuit diagram

Key Points for Examination

A cell is the source; a switch is not the source of current.

Every electric cell has one positive and one negative terminal.

A battery contains two or more correctly connected cells.

Current needs a complete conducting loop.

Conventional current is taken from positive to negative outside the cell.

An incandescent lamp glows because its filament becomes hot.

A broken filament stops current and makes the lamp fused.

An LED has a positive long lead and negative short lead.

An LED permits current in only one direction.

Switch ON closes; switch OFF opens the circuit.

Standard symbols make circuits easy to draw and understand.

Copper is used as a conductor and plastic as protective insulation.

Graphite is a conducting non-metal worth remembering.

Never perform circuit experiments using a wall socket.

Important Questions and Answers

These questions are independently written or adapted to match the chapter. They cover NCERT exercise concepts, Exemplar-style reasoning and Olympiad-level application without requiring the original textbook figures.

NCERT-Based Questions Core Textbook

Q1.Which statement is incorrect: a switch supplies current, completes or breaks a circuit, controls use of electricity, or creates a gap when OFF?

Answer: “A switch supplies current” is incorrect. The cell or battery supplies electrical energy. A switch only completes or breaks the circuit and therefore controls whether current flows.

Q2.Why does a torch lamp glow in only one position of its switch?

Answer: In the ON position, the switch closes the circuit and provides an unbroken path from one battery terminal, through the lamp, back to the other terminal. In the OFF position, a gap opens, current stops and the lamp does not glow.

Q3.What is the correct arrangement of cells in a battery?

Answer: Connect the positive terminal of one cell to the negative terminal of the next. The free negative terminal at one end and free positive terminal at the other end act as the battery terminals.

Q4.Two lamps are connected in one series loop. If the filament of one lamp breaks, will the other glow? Explain.

Answer: No. A broken filament creates a gap in the only path. The circuit becomes open, so current cannot flow through either lamp.

Q5.A student connects a working cell and lamp but leaves the plastic covering on the ends of both wires. Will the lamp glow?

Answer: No. Plastic is an insulator and prevents the metal conductor inside the wire from touching the terminals. Without metal-to-metal contact, the circuit remains incomplete.

Q6.Describe a circuit diagram for a simple torch.

Answer: Draw the battery symbol, a switch symbol and a lamp symbol connected in one closed loop by straight wire lines. For a glowing torch, show the switch in its ON or closed position. The long line of the cell symbol represents positive and the short line negative.

Q7.List possible reasons why a lamp fails to glow even after the switch is closed.

Answer:

The cell is exhausted.
The lamp filament is broken.
A wire is broken or loosely connected.
Insulation has not been removed from a wire end.
The switch contacts do not touch properly.
Both wires are connected to the same terminal of the lamp or cell.
The cells in the battery are arranged incorrectly.
If the device is an LED, its polarity may be reversed.

Test one component at a time in a known working circuit.

Q8.How can the positive and negative terminals of an unmarked low-voltage battery be identified?

Answer: Use a known working LED of suitable rating. Connect it in one direction and then reverse it. It glows only when its longer positive lead is connected to the battery’s positive terminal and its shorter negative lead to the battery’s negative terminal. Conduct this only with small classroom batteries under supervision.

Q9.Design an activity to identify working cells from a group of labelled cells.

Answer:

Use a verified lamp, holder and connecting wires to make a simple tester.
Place each labelled cell into the same circuit one at a time.
Keep the lamp, wires and contact positions unchanged.
Close the circuit briefly and record whether the lamp glows.
Cells that produce a clear glow are working; retest doubtful cells to confirm.

Q10.Why does an LED glow in one orientation but not after its connections are reversed?

Answer: An LED allows current through it in only one direction. It glows when its positive, longer lead is connected to battery positive and its negative, shorter lead to battery negative. Reversing these connections blocks current through the LED.

Q11.With which type of material between the two free ends of a tester will the lamp not glow?

Answer: It will not glow with an insulator such as rubber, plastic, glass, wax or dry wood, because such a material does not provide an easy path for electric current.

Q12.Why are the conducting and insulating parts of a wire both necessary?

Answer: The metal core carries current to the required component. The plastic or rubber covering separates the live conductor from people and nearby objects, helping prevent accidental contact, unwanted current paths and electric shock.

Exemplar-Style Questions Reasoning & Application

Q13.Both wires from a cell are attached to the metal case of a torch bulb. Predict and explain.

Answer: The bulb will not glow. The two bulb terminals are the metal case and the metal tip. Both must be included separately in the circuit. Attaching both wires to the same terminal does not send current through the filament.

Q14.A bulb glows with the switch placed near the cell. Will it work if the same switch is moved near the bulb without changing other connections?

Answer: Yes. In a simple series loop, a switch can be placed anywhere. Closing it completes the entire path, and opening it breaks the only path.

Q15.Why can an incandescent bulb glow even after the cell connections are reversed, while an LED may not?

Answer: Current can heat the incandescent filament in either direction, so terminal orientation does not matter. An LED is direction-sensitive and conducts only when connected with correct polarity.

Q16.A metal key covered completely with paint is placed in a conduction tester. The lamp remains off. Is metal an insulator?

Answer: No. The tester ends are touching the insulating paint rather than bare metal. Remove a tiny amount of paint at both contact points and repeat. If the ends touch exposed metal, the lamp should glow.

Q17.Why must a conduction tester first be checked by touching its free wire ends together?

Answer: This confirms that the cell, lamp and wires are functioning and that the connections are complete. Otherwise, a faulty tester could make a conductor appear to be an insulator.

Q18.A bulb’s filament looks unbroken, yet the bulb does not glow. State a scientific troubleshooting sequence.

Answer: First test the cell with a known working bulb. Then test the bulb in a known working circuit. Next check exposed wire ends, firm metal contacts and the switch. Finally check whether each wire reaches a different bulb terminal. Change only one component at a time.

Q19.Why is a circuit diagram preferable to a realistic drawing of a circuit?

Answer: Standard symbols are quick to draw, unambiguous and understood widely. They show electrical connections clearly without being distracted by the actual size, shape or appearance of components.

Q20.Pencil lead completes a tester circuit even though it is not a metal. What conclusion follows?

Answer: Pencil lead contains graphite, a form of carbon that conducts electricity. Therefore, not all conductors are metals; graphite is an important conducting non-metal.

Olympiad Challenge Higher-Order Thinking

Q21.Two identical cells are placed in a holder with their positive terminals facing each other. Why may the lamp fail to glow?

Answer: The cells are not arranged positive-to-negative in a proper series sequence. Their electrical effects oppose rather than support each other, so the battery may not provide the required energy to the lamp. Reorient one cell according to the + and − marks.

Q22.Two switches are connected in series with one lamp. Which switch positions make the lamp glow?

Answer: The lamp glows only when both switches are ON. If either switch is OFF, that switch introduces a gap into the only path and the circuit is open.

Q23.Two bulbs lie on separate parallel paths. One filament breaks. Must the other bulb go off?

Answer: No, provided the second path remains complete and connected to the source. A broken filament opens only its own branch. This differs from two bulbs in one series path, where one break opens the entire loop.

Q24.A tester bulb does not glow with a material. Can we always declare the material a perfect insulator?

Answer: Not always. The cell may be weak, the contacts may be poor, or the material may conduct such a tiny current that the bulb cannot visibly glow. At Class 7 level it is recorded as an insulator or poor conductor under the given test conditions, but the conclusion should mention the limits of the tester.

Q25.Why is it unsafe to connect the two terminals of a cell directly with a thick metal wire?

Answer: A very easy path allows a large current to flow. The wire and cell can become hot, the cell can discharge rapidly and may be damaged. This connection should not be made as an experiment.

Q26.An LED does not glow. Reversing it makes it glow. What does this prove about the cell, LED and first connection?

Answer: It proves the cell and LED are working. The first connection had the LED in reverse polarity, which blocked current. The second connection placed its longer positive lead towards battery positive and shorter lead towards battery negative.

Q27.Three objects are tested: a steel spoon, a plastic ruler and a pencil lead. Predict the results.

Answer: The steel spoon conducts because it is metal. The plastic ruler is an insulator, so the lamp remains off. Pencil lead contains graphite and should conduct, although the glow may be weaker depending on the tester.

Q28.A complete-looking circuit contains a working cell, switch and LED, but the LED remains off. Give the shortest two-step diagnosis.

Answer: First reverse the LED connections because polarity is the most likely direction-specific fault. If it still remains off, replace the LED with a known working incandescent lamp or LED to separate a component fault from a connection fault.

Q29.A student says, “An open switch uses up the cell slowly because it is still connected by wires.” Evaluate the statement.

Answer: The statement is false for the simple circuit studied here. An open switch creates a gap, so there is no complete path and current does not flow through the lamp circuit. Therefore, the lamp circuit is not using the cell.

Q30.Case study: A torch works when shaken but goes off again. What is the most likely type of fault and how should it be checked?

Answer: The intermittent behaviour suggests a loose or dirty contact rather than a permanently broken filament. Open the torch with adult supervision, check cell orientation, press the cells firmly, inspect the spring and metal contacts, and clean or tighten contacts safely. Do not connect it to mains electricity.

One-Minute Revision Capsule

SOURCE
Cell or correctly arranged battery supplies electrical energy.

→

PATH + CONTROL
Conducting wires and an ON switch create a closed circuit.

→

OUTPUT
Current passes through the lamp or correctly oriented LED and produces light.

Final recall: Source supplies → wires carry → switch controls → lamp/LED responds → insulators protect.