Electricity: Circuits and their Components
Explore how electric cells, lamps, LEDs, switches and wires work together to form circuits, and discover why some materials conduct electricity while others do not.
Chapter Snapshot
- An electric cell is a portable source of electrical energy and has positive and negative terminals.
- Two or more appropriately connected cells form a battery.
- Electric current flows through a lamp only when there is a complete electrical circuit.
- A switch controls a circuit by completing or breaking the path for current.
- Electrical components can be represented using standard circuit symbols.
- An LED allows current to pass through it in one direction and must be connected with correct polarity.
- Materials that allow electric current to pass easily are called conductors.
- Materials that do not allow current to pass easily are called insulators.
What You Will Learn
- Identify common uses of electricity in everyday life
- Describe the structure and terminals of an electric cell
- Explain how two or more cells form a battery
- Distinguish between an incandescent lamp and an LED
- Explain how a complete electrical circuit allows current to flow
- Differentiate between open and closed circuits
- Explain the function of an electric switch
- Recognise and use standard electrical circuit symbols
- Classify common materials as electrical conductors or insulators
- Apply basic electrical safety rules while handling electrical devices
Chapter Overview
Electricity is involved in almost every part of modern life. We use it for lighting houses and streets, operating fans and refrigerators, charging mobile phones, running computers, pumping water and operating machines.
However, an electrical device does not work merely because it contains a cell or battery. Its components must be connected in a suitable way so that electric current gets a complete path through which it can flow.
In this chapter, we will begin with a simple electric cell and gradually build our understanding of batteries, lamps, LEDs, switches and complete circuits. We will then learn how scientists and engineers represent circuits using symbols and how materials can be classified as electrical conductors or insulators.
Detailed Explanation
1. Electricity in Everyday Life
Look around your home, classroom or neighbourhood. You will probably find many devices that depend on electricity.
Some common uses include:
- Lighting: lamps, streetlights and torches;
- Heating: electric kettles, heaters and irons;
- Cooling: fans, refrigerators and air conditioners;
- Communication: mobile phones, computers and routers;
- Entertainment: televisions, speakers and gaming devices;
- Transportation: electric trains and electric vehicles;
- Work and industry: pumps, motors, machines and tools.
Different electrical devices may look completely different, but they all require a source of electrical energy and a suitable path through which electric current can flow.
2. Electric Cell
A torch, wall clock, remote control or small toy can often operate even when it is not connected to a wall socket. Such devices may use electric cells.
An electric cell has two terminals:
- positive terminal (+)
- negative terminal (−)
In a commonly used cylindrical dry cell, the raised metal cap is associated with the positive terminal, while the other end forms the negative terminal.
When drawing or describing an electric cell, always identify both terminals. Do not simply write that a cell has two ends.
Why are two terminals necessary?
A circuit requires connections to both terminals of the cell. Current can flow through an external circuit only when a complete path is formed between the two terminals through suitable conducting components.
Show answer
3. Battery — A Combination of Cells
Many devices need more than one electric cell.
When two or more cells are connected appropriately, they form a battery.
In a common battery arrangement, the positive terminal of one cell is connected to the negative terminal of the next cell.
Suppose a torch requires two cells.
The cells are placed so that the positive terminal of one is connected through the holder to the negative terminal of the other. Together, they act as the battery that supplies electrical energy to the torch circuit.
4. Electric Lamp
A small electric lamp is another important circuit component.
Traditional torch lamps are often incandescent lamps.
The filament is connected to two thicker supporting wires. These lead to two separate terminals of the lamp.
For the lamp to glow:
- electric current must enter through one terminal;
- pass through the filament;
- leave through the other terminal.
If the filament breaks, the conducting path inside the lamp is interrupted.
Such a lamp is commonly called a fused lamp.
A broken filament creates a gap in the electrical path.
Even if the cell, wires and switch are working properly, current cannot pass through the broken filament, so the lamp cannot glow.
5. Light Emitting Diode — LED
Many modern torches and indicators use LEDs, or Light Emitting Diodes.
Unlike an incandescent lamp, an LED does not contain a glowing filament.
A typical basic LED has two leads:
- a longer lead, generally connected to the positive side;
- a shorter lead, generally connected to the negative side.
The LED glows only when it is connected with the correct polarity.
Incandescent Lamp vs LED
| Feature | Incandescent Lamp | LED |
|---|---|---|
| Light-producing part | Heated filament | Light-emitting semiconductor device |
| Filament present | Yes | No |
| Polarity important in a simple cell circuit | Usually no | Yes |
| Current direction | Lamp can work with either terminal orientation in a simple DC circuit | Current must pass through the LED in the permitted direction |
Show answer
6. Making a Simple Electrical Circuit
Let us connect an electric cell to a lamp using wires.
Suppose one terminal of the cell is connected to one terminal of the lamp. If the second lamp terminal is left unconnected, the lamp will not glow.
Now connect the second terminal of the lamp to the remaining terminal of the cell.
The lamp can glow because a complete path has been formed.
Direction of current
By convention, the direction of electric current in the external circuit is taken from the positive terminal of the cell toward the negative terminal.
At this stage, remember the conventional direction rather than trying to study the motion of particles inside wires, which belongs to higher classes.
Open and closed circuits
A circuit can be either closed or open.
Closed Circuit vs Open Circuit
| Feature | Closed Circuit | Open Circuit |
|---|---|---|
| Path | Complete | Broken or incomplete |
| Current | Can flow | Cannot flow through the complete circuit |
| Lamp in a simple working circuit | Glows | Does not glow |
| Typical switch position | ON | OFF |
Show answer
7. Electric Switch
We do not want to disconnect wires every time we want to turn a lamp off.
A switch gives us a convenient way to control the circuit.
When a switch is in the ON position, it completes the path.
When it is in the OFF position, it introduces a gap and breaks the path.
8. Circuit Diagrams
Drawing pictures of real cells, lamps, switches and wires every time would be inconvenient.
Scientists therefore use standard symbols to represent circuit components.
Common components represented by symbols include:
- electric cell;
- battery;
- connecting wire;
- electric lamp;
- LED;
- switch in ON position;
- switch in OFF position.
Symbol of an electric cell
The symbol for a cell contains two parallel lines of unequal length.
The longer line represents the positive terminal.
The shorter line represents the negative terminal.
Why symbols are useful
Circuit symbols:
- save time;
- make diagrams neat;
- reduce unnecessary detail;
- make circuits easier to compare;
- provide a common scientific language.
In a circuit diagram, draw the standard symbol, not a realistic sketch of a battery, bulb or switch unless the question specifically asks for a labelled object diagram.
9. Electrical Conductors and Insulators
Why are connecting wires usually made of metal?
Why are these wires commonly covered with plastic or rubber?
The answer depends on how different materials behave when placed in an electrical circuit.
Many metals are good electrical conductors.
Examples include:
- copper;
- aluminium;
- silver;
- gold;
- iron.
Copper is widely used in electrical wires because it conducts electricity well and is practical for widespread use.
Common insulating materials include:
- plastic;
- rubber;
- dry wood;
- glass;
- ceramics.
Conductors vs Insulators
| Feature | Conductors | Insulators |
|---|---|---|
| Current through material | Passes relatively easily | Does not pass easily |
| Common examples | Copper, aluminium, iron | Plastic, rubber, glass |
| Use in electrical wires | Metal core carries current | Outer covering helps isolate the conductor |
| Importance | Provide conducting paths | Help prevent unwanted contact with conducting parts |
Why does a wire contain both conductor and insulator?
A typical electrical wire has:
- a conducting metal core that carries current;
- an insulating covering that reduces accidental contact with the conductor.
Both materials are therefore important.
10. Testing Materials for Electrical Conductivity
We can make a simple conduction tester using a cell, lamp and wires.
The test material is placed in the gap between two free wire ends.
If the material completes the conducting path sufficiently for the lamp to glow, it behaves as a conductor in that setup.
Show answer
11. Electrical Safety
Electricity is extremely useful, but it must be handled responsibly.
The human body can conduct electricity. A sufficiently large electric current passing through the body can cause serious injury.
Therefore:
- never touch switches or plugs with wet hands;
- never insert objects into electrical sockets;
- never touch exposed electrical wires;
- do not use appliances with damaged insulation;
- keep electrical devices away from water unless specifically designed for such use;
- inform an adult if you notice damaged wires, loose sockets or sparks;
- never perform school experiments using household mains electricity.
12. Understanding a Torch as a Complete System
A torch is a useful example because several ideas from this chapter work together inside one device.
A simple torch contains:
- one or more cells;
- a lamp or LED;
- conducting connections;
- a switch;
- a casing that holds the components in position.
When the switch is moved to ON:
- the switch completes the circuit;
- the battery provides electrical energy;
- current flows through the conducting path;
- the lamp or correctly connected LED produces light.
When the switch is moved to OFF:
- the circuit is broken;
- current no longer flows through the complete circuit;
- the lamp or LED stops producing light.
Concept Map
Troubleshooting a Circuit
Sometimes a circuit appears complete, but the lamp still does not glow.
A scientist does not immediately assume that one particular component is faulty. Instead, possible causes are checked systematically.
Why Might a Lamp Fail to Glow?
Possible reasons include:
-
Cell is exhausted The cell may no longer be able to operate the lamp.
-
Lamp filament is broken A fused incandescent lamp has an incomplete path inside it.
-
Loose connection One or more wires may not be touching the metal terminals properly.
-
Insulation not removed If plastic insulation remains between the metal wire and terminal, electrical contact may not occur.
-
Switch is OFF The circuit may contain an intentional gap.
-
Incorrect LED orientation An LED connected in reverse may not glow.
-
Cells placed incorrectly Multiple cells may not be arranged correctly in the holder.
A Good Troubleshooting Method
Check one possible cause at a time:
- inspect the circuit path;
- check the switch;
- check all metal-to-metal connections;
- test the lamp with a known working cell;
- test the cell with a known working lamp;
- reverse the LED if polarity may be incorrect.
Changing only one factor at a time helps identify the actual fault.
Foundation Extension
Exam Focus
Important Exam Areas
- definition and terminals of an electric cell;
- difference between a cell and a battery;
- structure and working of an incandescent lamp;
- identification of LED polarity;
- meaning of electrical circuit;
- open circuit and closed circuit;
- function of an electric switch;
- standard electrical circuit symbols;
- definition of circuit diagram;
- conductors and insulators with examples;
- reason for using metal inside wires and insulating material outside;
- troubleshooting a circuit in which a lamp does not glow;
- electrical safety precautions.
Key Terms
Key Terms
Quick Revision
Quick Revision
- An electric cell is a portable source of electrical energy.
- Every electric cell has a positive terminal and a negative terminal.
- Two or more appropriately connected cells form a battery.
- An incandescent lamp contains a thin filament that glows when current passes through it.
- A broken filament makes an incandescent lamp fused.
- An LED has polarity and must be connected in the correct direction.
- An electrical circuit provides a complete path for electric current.
- A closed circuit allows current to flow.
- An open circuit contains a gap that prevents current from flowing through the complete path.
- A switch completes or breaks an electrical circuit.
- Conventional current direction is taken from the positive terminal toward the negative terminal in the external circuit.
- A circuit diagram represents an electrical circuit using standard symbols.
- The longer line in a cell symbol represents the positive terminal.
- Conductors allow electric current to pass relatively easily.
- Metals such as copper and aluminium are common electrical conductors.
- Plastic, rubber, glass and ceramics are common electrical insulators.
- Electrical wires use a conducting core and an insulating covering.
- Household mains electricity must never be used for Class 7 circuit experiments.
Practice Questions
A. Multiple Choice Questions
Which component acts as the source of electrical energy in a simple torch circuit?
A. Switch B. Electric cell C. Connecting wire D. Lamp holder
View Solution
Correct Answer: B. Electric cell
An electric cell provides electrical energy to the circuit. A switch controls the path, while wires connect the components.
A lamp glows in a simple cell-lamp circuit when:
A. only one lamp terminal is connected to the cell B. both cell terminals are connected directly to each other C. a complete path passes through the lamp D. the switch creates a gap in the circuit
View Solution
Correct Answer: C. a complete path passes through the lamp
Current must pass through the lamp as part of a complete circuit.
Which material is most likely to be an electrical conductor?
A. Rubber B. Plastic C. Copper D. Glass
View Solution
Correct Answer: C. Copper
Copper is a metal and a good electrical conductor. It is widely used for electrical wiring.
A working LED connected to a working battery does not glow. The circuit has no visible gap. What should be checked first?
A. Colour of the LED B. Orientation of the LED C. Length of the wires D. Shape of the battery holder
View Solution
Correct Answer: B. Orientation of the LED
An LED allows current through it in one direction. Reversing its connections may solve the problem.
When a switch is in the OFF position in a simple circuit:
A. the cell stops existing B. the lamp becomes a conductor C. a gap is present in the circuit D. current becomes stronger
View Solution
Correct Answer: C. a gap is present in the circuit
The OFF switch breaks the path and produces an open circuit.
Why is plastic commonly used around a metal electrical wire?
A. Plastic produces electric current. B. Plastic is an electrical insulator. C. Plastic increases the size of the cell. D. Plastic makes metal a better conductor.
View Solution
Correct Answer: B. Plastic is an electrical insulator.
The conducting metal core carries current, while the insulating plastic covering helps prevent unwanted electrical contact.
B. Very Short Answer Questions
-
Name the two terminals of an electric cell.
-
What is a battery?
-
What is the thin glowing wire inside an incandescent lamp called?
-
What is meant by a closed circuit?
-
What is the function of a switch?
-
Give two examples each of electrical conductors and insulators.
Answers
-
Positive terminal and negative terminal.
-
A battery is a combination of two or more electric cells connected together.
-
The thin wire is called the filament.
-
A closed circuit is a circuit containing a complete conducting path through which current can flow.
-
A switch completes or breaks an electrical circuit.
-
Conductors: copper and aluminium. Insulators: plastic and rubber.
C. Short Answer Questions
Why does an incandescent lamp fail to glow when its filament is broken?
View Solution
A broken filament creates a gap inside the lamp. The electrical path is therefore incomplete, so current cannot pass through the filament. As a result, the lamp does not glow.
Differentiate between an open circuit and a closed circuit.
View Solution
A closed circuit contains a complete conducting path and allows electric current to flow.
An open circuit contains a break or gap, so current cannot flow through the complete circuit.
In a simple working lamp circuit, the lamp glows when the circuit is closed and does not glow when the circuit is open.
Why must the insulation at the end of a connecting wire be removed before making an electrical connection?
View Solution
The metal inside the wire is the conductor. The plastic or rubber covering is an insulator.
If the insulating covering remains between the metal wire and the terminal, proper electrical contact may not be formed. Removing a small amount of insulation exposes the conducting metal.
Why are standard symbols used in circuit diagrams?
View Solution
Standard symbols:
- make circuit diagrams simpler;
- save drawing time;
- clearly show how components are connected;
- make circuits easier to understand;
- provide a common way of representing components.
D. Long Answer Questions
Explain how a simple circuit containing a cell, lamp and switch works.
View Solution
A simple circuit contains an electric cell, connecting wires, a lamp and a switch.
The electric cell provides electrical energy.
When the switch is in the ON position, it completes the path. The circuit becomes closed and current can pass through the lamp. The lamp then glows.
When the switch is moved to the OFF position, a gap is created. The circuit becomes open and current cannot pass through the complete path. Therefore, the lamp does not glow.
Thus, the switch controls the lamp by completing or breaking the electrical circuit.
Describe an activity to determine whether a given material is an electrical conductor or insulator.
View Solution
Materials
A small electric cell, lamp, connecting wires and the object to be tested.
Procedure
- Connect the cell and lamp using wires but leave a gap between two free wire ends.
- Briefly touch the free ends together to ensure the tester is working.
- Separate them again.
- Place the test material between the two free ends.
- Ensure both wires touch the material properly.
- Observe the lamp.
Observation and conclusion
If the lamp glows, the material allows current to pass and can be classified as a conductor in the test.
If the lamp does not glow, the material is likely an insulator or poor conductor under those conditions.
The activity must be performed only with a small low-voltage cell, never with household mains electricity.
Application and HOTS Questions
Riya constructs a circuit using a working cell, working lamp and two wires. Both wires are covered with plastic from end to end. She presses the plastic-covered ends against the lamp and cell terminals, but the lamp does not glow. Explain why.
View Solution
Plastic is an electrical insulator.
Because the conducting metal inside the wires is not making contact with the metal terminals, the circuit does not obtain a proper conducting path.
Riya should remove a small amount of insulation from the wire ends so that the metal conductor can make proper contact.
A circuit contains a cell, switch and lamp. The lamp does not glow when the switch is ON. List at least four possible causes.
View Solution
Possible causes include:
- the cell may be exhausted;
- the lamp filament may be broken;
- a wire may be loose;
- insulation may prevent metal-to-metal contact;
- the switch may not actually be completing the circuit;
- a connection may be attached to the wrong part of the lamp;
- the conducting path may contain an unnoticed gap.
The best approach is to test one possible cause at a time.
A student tests a metal spoon using a circuit tester, but the lamp does not glow. Should the student immediately conclude that the spoon is an insulator? Explain.
View Solution
No.
The student should first check whether the tester itself is working.
Possible problems include:
- exhausted cell;
- fused lamp;
- loose wires;
- poor contact with the spoon;
- insulation covering the wire ends.
Only after confirming that the tester works properly should the student use the observation to classify the material.
Why is it useful for an electrical wire to contain both conducting and insulating materials instead of being made entirely from one type of material?
View Solution
The two materials perform different functions.
The conducting metal core provides a path through which electric current can flow.
The insulating outer covering helps prevent unwanted contact between the conducting metal and people or other conductors.
Therefore, combining a conductor with an insulator makes the wire both functional and safer to use.
Assertion–Reason
Assertion: A lamp in a simple circuit does not glow when the switch is OFF.
Reason: An OFF switch creates a gap in the conducting path.
Choose the correct option:
A. Both Assertion and Reason are true, and Reason correctly explains Assertion. B. Both are true, but Reason does not explain Assertion. C. Assertion is true, but Reason is false. D. Assertion is false, but Reason is true.
View Solution
Correct Answer: A
An OFF switch breaks the electrical circuit. The gap prevents current from flowing through the complete path, so the lamp does not glow.
Assertion: Plastic is commonly used as the outer covering of electrical wires.
Reason: Plastic is a good electrical conductor.
Choose the correct option:
A. Both statements are true. B. Assertion is true, but Reason is false. C. Assertion is false, but Reason is true. D. Both statements are false.
View Solution
Correct Answer: B
Plastic is used as an outer covering because it is an electrical insulator, not a good conductor.
Apply Your Learning
Observe battery-operated objects around your home or classroom, such as:
- torch;
- wall clock;
- remote control;
- battery-operated toy.
Do not open any mains-powered appliance.
For each suitable battery-operated object, answer:
- How many cells does it use?
- How are the positive and negative markings shown?
- Does it contain a visible switch?
- What happens when the switch is OFF?
- Does the device use a lamp, LED, motor, buzzer or another output component?
- Which visible parts appear to be conductors?
- Which visible parts appear to be insulators?
Common Mistakes to Avoid
Final Chapter Summary
Chapter Summary
- Electricity is used for lighting, heating, cooling, communication, transportation and many other purposes.
- An electric cell is a portable source of electrical energy and has positive and negative terminals.
- Two or more connected cells form a battery.
- An incandescent lamp produces light when current heats its filament.
- An LED does not contain a filament and must be connected with correct polarity.
- An electrical circuit is a complete path through which electric current can flow.
- A closed circuit provides a complete path, while an open circuit contains a break.
- A switch controls a circuit by completing or breaking the conducting path.
- Circuit diagrams use standard symbols to represent electrical components clearly.
- Conductors such as metals allow current to pass relatively easily.
- Insulators such as plastic and rubber do not allow current to pass easily and are useful for electrical protection.
- Circuit faults should be investigated systematically by checking the source, component, switch, wires and connections.
- Electrical activities at this level must use only small cells or safe low-voltage batteries, never household mains electricity.