Chapter 12: Electric Current and Circuits ⚡
A Comprehensive Guide for PSTET Paper-2 (Science)
Chapter Overview
| Section | Topic | PSTET Weightage | Page No. |
|:---:|:---|::---:|:---:|
| 12.1 | Electric Cell and its Terminals | High | 2 |
| 12.2 | A Simple Electric Circuit (Bulb, Wires, Switch) | High | 6 |
| 12.3 | Conductors and Insulators | High | 11 |
| 12.4 | Electric Circuits - Series and Parallel (Basic Concepts) | Medium | 16 |
| 12.5 | Heating and Magnetic Effect of Electric Current | High | 21 |
| 12.6 | Electromagnets | Medium | 26 |
| 12.7 | Safety Measures in Using Electricity | High | 30 |
| Practice Zone | MCQs & Pedagogical Questions | - | 35 |
Learning Objectives 🎯
After studying this chapter, you will be able to:
✅ Explain the structure and functioning of an electric cell, identifying its positive and negative terminals
✅ Construct a simple electric circuit and understand the role of each component (bulb, wires, switch)
✅ Differentiate between conductors and insulators with examples from daily life
✅ Distinguish between series and parallel circuits and their practical applications
✅ Describe the heating and magnetic effects of electric current with examples
✅ Explain the working principle of electromagnets and their applications
✅ Identify essential safety measures for using electricity at home and school
✅ Apply pedagogical strategies to teach electric circuits effectively to upper primary students
Pedagogical Link 🔗
For Teachers: This chapter directly aligns with:
Class 6 Science NCERT Chapter 12: "Electricity and Circuits"
Class 7 Science NCERT Chapter 14: "Electric Current and Its Effects"
Class 10 Science NCERT Chapter 12: "Electricity"
Class 10 Science NCERT Chapter 13: "Magnetic Effects of Electric Current"
Teaching Tips:
Begin with hands-on activities—provide students with cells, wires, bulbs, and let them explore making the bulb glow
Use the water-flow analogy to explain current, voltage, and resistance—very effective for young learners
Create a "Circuit Corner" with different components for experimentation
Use role-play—students act as electrons flowing through wires
Emphasize safety first—always use low-voltage cells (1.5V to 9V) for classroom activities
Section 12.1: Electric Cell and its Terminals 🔋
Introduction
Have you ever wondered how a torch lights up, how a remote control works, or how your clock keeps ticking? The answer lies in a small device called an electric cell. It is the most common source of electricity for portable devices .
12.1.1 What is an Electric Cell?
Definition: An electric cell is a device that converts chemical energy into electrical energy . It provides the force (called voltage) that pushes electric charges through a circuit .
Inside a Cell:
Contains chemical substances (electrolytes) between two electrodes
Chemical reactions within the cell cause a buildup of charges
This creates a difference in electric potential between the two terminals
Common Types of Cells:
| Cell Type | Description | Examples of Use | Rechargeable? |
|---|---|---|---|
| Dry Cell (Zinc-Carbon) | Most common; inexpensive | Torches, clocks, remote controls | No |
| Alkaline Cell | Longer-lasting than zinc-carbon | Toys, portable radios, cameras | No |
| Button Cell | Small, coin-shaped | Watches, calculators, hearing aids | Sometimes |
| Lithium Cell | Lightweight, high energy | Cameras, laptops, mobile phones | Yes (lithium-ion) |
| Nickel-Metal Hydride (NiMH) | Rechargeable | Rechargeable batteries for toys, cameras | Yes |
| Lead-Acid Battery | Heavy, high capacity | Cars, inverters, UPS | Yes |
📝 PSTET Note: A battery is actually a combination of two or more cells connected together . For example, a 9V battery contains six 1.5V cells inside.
12.1.2 Terminals of an Electric Cell
Every electric cell has two metal ends called terminals . These are the points where we connect wires to use the electricity from the cell .
Table 12.1: Cell Terminals and Their Characteristics
Simple Explanation:
The negative terminal has an excess of electrons
The positive terminal has a deficiency of electrons
When we connect a wire between them, electrons flow from negative to positive
Important Fact: By convention, we say that current flows from positive to negative (conventional current). However, actually electrons flow from negative to positive (electron current) . For PSTET, both conventions are accepted—just be consistent.
12.1.3 Voltage of a Cell
Definition: Voltage (also called potential difference) is the electrical pressure that pushes charges through a circuit . It is measured in volts (V) .
Common Voltages:
| Cell/Battery Type | Voltage |
|---|---|
| AA / AAA / C / D cell (single) | 1.5 V |
| Button cell (watch battery) | 1.5 V or 3 V |
| 9V Battery (rectangular) | 9.0 V |
| Car battery | 12 V |
| Mobile phone battery (typical) | 3.7 V |
What Voltage Means:
A 1.5V cell provides 1.5 joules of energy per coulomb of charge
Higher voltage means more "push" to move electrons
Different devices need different voltages—using wrong voltage can damage the device
12.1.4 Cell Life and Capacity
Why Cells Stop Working:
Chemical reactions inside the cell gradually use up the active materials
The voltage drops below the level needed to operate the device
In primary (non-rechargeable) cells, the process is irreversible
In secondary (rechargeable) cells, the chemical reaction can be reversed by passing current through them
Factors Affecting Cell Life:
Type of cell (alkaline lasts longer than zinc-carbon)
Amount of current drawn by the device (more current = shorter life)
Temperature (extreme temperatures reduce performance)
Storage conditions
12.1.5 Pedagogical Implications
| Teaching Strategy | Description | PSTET Focus |
|---|---|---|
| Show and Tell | Bring different types of cells (AA, AAA, button, 9V) for students to examine | Observation skills |
| "What's Inside?" Activity | Cut open an old dry cell (with safety precautions) to show internal structure | Hands-on learning |
| Terminal Identification | Students identify + and - terminals on various cells | Practical skill |
| Battery Sorting | Sort devices by the type/number of cells they use | Classification |
Section 12.2: A Simple Electric Circuit 💡
Introduction
An electric cell by itself doesn't do anything useful. To make it work, we need to connect it to a device (like a bulb) using wires, creating a complete path for electricity to flow. This complete path is called an electric circuit .
12.2.1 What is an Electric Circuit?
Definition: An electric circuit is a closed and continuous path through which electric current flows from the positive terminal of the cell, through various components, and back to the negative terminal of the cell .
Key Principle: For current to flow, the circuit must be complete and unbroken . If there is any gap, the current stops—just like water stops flowing if a pipe is cut .
12.2.2 Components of a Simple Circuit
A basic electric circuit requires three essential components :
Table 12.2: Components of a Simple Circuit
| Component | Symbol | Function | Real-life Example |
|---|---|---|---|
| Cell/Battery 🔋 | + | - | Provides electrical energy (voltage) to push current | AA cell, 9V battery |
| Connecting Wires 📌 | ───── | Provide path for current to flow | Copper wires in appliances |
| Bulb/Load 💡 | ⏀ | Converts electrical energy into light (and heat) | Torch bulb, LED |
| Switch 🔌 | ──o/○── | Controls the circuit (open = off, closed = on) | Light switch, TV power button |
12.2.3 Building a Simple Circuit
Step-by-Step Procedure:
| Step | Action | What Happens |
|---|---|---|
| 1 | Take a 1.5V cell, a small bulb, and two insulated copper wires | - |
| 2 | Remove about 1 cm of insulation from both ends of each wire | Bare wire ends will make contact |
| 3 | Connect one wire from the positive (+) terminal of the cell to one terminal of the bulb holder | Path created from cell to bulb |
| 4 | Connect the second wire from the other terminal of the bulb holder to the negative (-) terminal of the cell | Path completed back to cell |
| 5 | Observe the bulb | If connections are correct and tight, the bulb glows |
Visual Representation:
┌─────[Bulb]─────┐
│ │
(+) (-)
└─────[Cell]──────┘What's Happening:
Chemical reactions in the cell create a surplus of electrons at the negative terminal
Electrons flow from negative terminal, through the wire, through the bulb, and back to positive terminal
Inside the bulb, the current makes a thin wire (filament) glow white-hot—producing light
12.2.4 The Switch: Controlling the Circuit
Definition: A switch is a device that can break or complete an electric circuit .
How a Switch Works:
| Switch Position | Circuit State | Current Flow | Bulb State |
|---|---|---|---|
| ON (Closed) | Complete path | Flows continuously | Glows |
| OFF (Open) | Broken path | No flow | Does not glow |
Types of Switches:
| Switch Type | Description | Common Use |
|---|---|---|
| Toggle Switch | Lever that moves up/down | Home light switches |
| Push-button Switch | Press to make/break contact | Doorbells, calculator buttons |
| Slide Switch | Slide back and forth | Torches, small toys |
| Knife Switch | Metal blade that moves | School science labs |
📝 PSTET Note: In circuit diagrams, an open switch is shown with a gap; a closed switch shows the gap bridged by a line .
12.2.5 Why the Bulb Glows
Inside a bulb, there is a thin wire called a filament. When electric current passes through this filament:
Electrons bump into atoms of the filament material
These collisions cause the atoms to vibrate faster
Faster vibration means higher temperature
The filament becomes so hot (about 2,500°C) that it glows white-hot
This glowing produces light
Filament Facts:
Made of tungsten (a metal with very high melting point)
Coiled to increase surface area
Glass bulb filled with inert gas (argon) to prevent filament from burning
12.2.6 Circuit Diagrams vs. Real Circuits
| Real Circuit | Circuit Diagram (Schematic) |
|---|---|
| Shows actual appearance of components | Uses symbols to represent components |
| Wires shown as they actually run | Wires shown as straight lines |
| Difficult to draw quickly | Easy and quick to draw |
| Size and shape matter | Only connections matter |
Example Circuit Diagram:
┌─────────[Switch]─────────┐
│ │
(+) (-)
│ │
└────[Bulb]──[Cell]────────┘12.2.7 Common Problems and Troubleshooting
| Problem | Possible Cause | Solution |
|---|---|---|
| Bulb doesn't glow | Loose connection | Tighten all connections |
| Dead cell | Replace with fresh cell | |
| Bulb fused (burned out) | Replace bulb | |
| Switch is OFF | Turn switch ON | |
| Wires not making proper contact | Scrape wire ends to remove insulation | |
| Bulb glows dimly | Weak cell | Replace cell |
| Too many components in circuit | Reduce load | |
| Wires get hot | Short circuit (direct connection without load) | Check for accidental wire contact |
12.2.8 Pedagogical Implications
| Teaching Strategy | Description | PSTET Focus |
|---|---|---|
| Circuit Construction Kit | Provide cells, wires, bulbs, switches—let students build circuits | Hands-on learning |
| Troubleshooting Challenge | Give students pre-built circuits with hidden faults to diagnose | Problem-solving skills |
| Circuit Diagram Drawing | Students draw diagrams of circuits they build | Symbol recognition |
| Switch Identification Hunt | Find and identify different switches around school | Observation skills |
Section 12.3: Conductors and Insulators 🧪
Introduction
Have you noticed that electric wires are made of metal but covered with plastic? Why not use plastic for the whole wire? The answer lies in understanding which materials allow electricity to pass through them and which do not .
12.3.1 What Makes a Material Conduct Electricity?
The Atomic Explanation:
In some materials, the outermost electrons of atoms are loosely bound and can move freely from atom to atom. These are called free electrons .
In conductors, there are many free electrons that can move easily
In insulators, electrons are tightly bound and cannot move freely
Analogy: Think of electrons as marbles in a tube:
Conductors: Marbles can roll freely from one end to the other
Insulators: Marbles are stuck in place with glue
12.3.2 Conductors
Definition: Conductors are materials that allow electric current to pass through them easily .
Characteristics of Conductors:
Have many free electrons
Low resistance to electric current
Usually metals
Table 12.3: Common Conductors
| Material | Conductivity Level | Common Use |
|---|---|---|
| Silver | Best conductor | Specialized electronic contacts |
| Copper | Excellent (second best) | Electric wires, cables |
| Gold | Very good (doesn't corrode) | Electronic connectors |
| Aluminum | Good | Power transmission lines |
| Iron/Steel | Moderate | Structures, some wiring |
| Graphite (Carbon) | Moderate (non-metal) | Pencil "lead" conducts! |
| Dirty water | Poor but conducts | Don't touch switches with wet hands! |
| Concrete | Very poor when dry, better when wet | Grounding |
📝 PSTET Note: Silver is the best conductor, but copper is most commonly used for wires because it's much cheaper .
12.3.3 Insulators
Definition: Insulators are materials that do not allow electric current to pass through them easily .
Characteristics of Insulators:
Have very few free electrons
Very high resistance to electric current
Used to cover conductors for safety
Table 12.4: Common Insulators
| Material | Insulation Quality | Common Use |
|---|---|---|
| Glass | Excellent | Power line insulators, old-fashioned fuses |
| Rubber | Excellent | Wire insulation, gloves for electricians |
| Plastic | Excellent | Wire coating, plug bodies, switches |
| Porcelain/Ceramic | Excellent | High-voltage insulators, sockets |
| Dry wood | Good | Furniture, ladder material (avoid wet wood!) |
| Dry paper | Good | Cable insulation, capacitors |
| Dry cotton | Good | Clothing (protects somewhat) |
| Air | Good | Separates bare wires in air |
| Oil | Good | Transformer insulation |
| Diamond | Excellent | Specialized applications |
📝 PSTET Note: Pure water is an insulator, but impure water (with dissolved salts) becomes a conductor—this is why we must never touch electrical switches with wet hands .
12.3.4 Testing Conductors and Insulators
Simple Classroom Experiment:
| Step | Procedure |
|---|---|
| 1 | Make a simple circuit with a cell, bulb, and wires—but leave a gap |
| 2 | Touch the two free wire ends together—bulb glows (complete circuit) |
| 3 | Now place different materials between the wire ends |
| 4 | Observe whether the bulb glows |
Materials to Test:
| Material | Bulb Glows? | Classification |
|---|---|---|
| Copper strip | Yes | Conductor |
| Iron nail | Yes | Conductor |
| Aluminum foil | Yes | Conductor |
| Pencil lead (graphite) | Yes (dim) | Conductor (poor) |
| Plastic scale | No | Insulator |
| Wooden pencil | No | Insulator |
| Rubber eraser | No | Insulator |
| Glass rod | No | Insulator |
| Salt water | Yes | Conductor |
| Sugar water | No | Insulator |
12.3.5 Semiconductor: The Middle Ground
Some materials are neither good conductors nor good insulators—they are called semiconductors.
| Material | Type | Use |
|---|---|---|
| Silicon | Semiconductor | Computer chips, transistors |
| Germanium | Semiconductor | Diodes, transistors |
Key Point: Semiconductors can be made to conduct under certain conditions—this is what makes modern electronics possible.
12.3.6 Why Do We Need Insulators?
Insulators are just as important as conductors because they:
| Function | Example |
|---|---|
| Prevent electric shock | Plastic coating on wires |
| Prevent short circuits | Separating wires in a cable |
| Keep electricity in the path | Insulation around copper cores |
| Allow safe handling | Rubber handles on tools |
12.3.7 Conductors and Insulators Summary
METALS ───→ Mostly CONDUCTORS
↓
┌───┴───┐
Good Poor
(Silver, (Iron,
Copper) Steel)
NON-METALS ──→ Mostly INSULATORS
↓
┌───┴───┐
Good Special Cases
(Plastic, (Graphite conducts,
Rubber) Salt water conducts)12.3.8 Pedagogical Implications
| Teaching Strategy | Description | PSTET Focus |
|---|---|---|
| Conductor/Insulator Testing | Students test various objects and classify them | Scientific investigation |
| "Why This Material?" Discussion | Discuss why wires have metal core + plastic covering | Critical thinking |
| Sorting Activity | Sort given materials into conductors and insulators | Classification skills |
| Safety Connection | Discuss why we must never touch switches with wet hands | Real-life application |
Section 12.4: Electric Circuits - Series and Parallel 🔗
Introduction
When we connect more than one component in a circuit, we have two basic ways to arrange them: series and parallel . Each arrangement has different characteristics and applications .
12.4.1 Series Circuit
Definition: In a series circuit, components are connected end-to-end, one after another, forming a single path for current flow .
Characteristics:
| Feature | Series Circuit |
|---|---|
| Current path | Single path only |
| Current through all components | Same current flows through each component |
| Voltage across components | Total voltage divides among components |
| Effect of one component failing | Entire circuit breaks (all components stop) |
| Number of paths | One |
Diagram:
┌────[Bulb 1]────[Bulb 2]────┐
│ │
(+) (-)
└──────────[Cell]─────────────┘Example:
Old-style Christmas tree lights—when one bulb fused, all lights went out
Simple torch with one bulb (single component is series by default)
12.4.2 Parallel Circuit
Definition: In a parallel circuit, components are connected across the same two points, creating multiple paths for current flow .
Characteristics:
| Feature | Parallel Circuit |
|---|---|
| Current path | Multiple paths (branches) |
| Current through components | Divides among branches (total = sum of branch currents) |
| Voltage across components | Same voltage across each branch |
| Effect of one component failing | Other components continue working |
| Number of paths | Two or more |
Diagram:
┌────┬────[Bulb 1]────┬────┐
│ │ │ │
(+) └────[Bulb 2]────┘ (-)
│ │
└─────────[Cell]──────────┘Example:
Household wiring—lights in different rooms are connected in parallel
If one light fails, others keep working
12.4.3 Comparison: Series vs. Parallel
Table 12.5: Series vs. Parallel Circuits
| Aspect | Series Circuit | Parallel Circuit |
|---|---|---|
| Current | Same everywhere | Divides among branches |
| Voltage | Divides among components | Same across all branches |
| Resistance | Total = R₁ + R₂ + R₃ | Total = 1/(1/R₁ + 1/R₂ + 1/R₃) |
| Number of paths | One | Two or more |
| If one component fails | All stop working | Others keep working |
| Adding more components | Reduces total current (dim bulbs) | Increases total current (more power used) |
| Common use | Torches, simple toys | Household wiring |
12.4.4 Advantages and Disadvantages
Series Circuit Advantages:
Simple to design and understand
Requires less wiring
Same current through all components
Series Circuit Disadvantages:
If one component fails, everything stops
Adding more components reduces brightness/performance
Cannot control components independently
Parallel Circuit Advantages:
Components operate independently
If one fails, others continue
All components receive full voltage
Can add more without affecting others
Parallel Circuit Disadvantages:
More complex wiring
Higher total current drawn from source
More expensive to wire
12.4.5 Household Wiring: A Practical Example
In our homes, electrical appliances are connected in parallel. Why?
| Reason | Explanation |
|---|---|
| Independent operation | Each appliance can be switched on/off without affecting others |
| Same voltage | Every appliance gets the full 220V supply |
| Safety | If one appliance develops a fault, others keep working |
Diagram of Parallel Household Circuit:
┌─────────────────────────────────┐
│ │
(+)────[Light]────[Fan]────[TV]────(-)
│ │
└──────────[Mains Supply]─────────┘12.4.6 Cells in Series and Parallel
Cells in Series:
| Connection | Resulting Voltage | Resulting Current | Application |
|---|---|---|---|
| Two 1.5V cells in series | 3.0 V (adds) | Same as single cell | Higher voltage devices |
Cells in Parallel:
| Connection | Resulting Voltage | Resulting Current | Application |
|---|---|---|---|
| Two 1.5V cells in parallel | 1.5 V (same) | Twice the current capacity | Longer-lasting operation |
📝 PSTET Note: Never connect cells of different voltages in parallel—this can cause overheating and damage.
12.4.7 Pedagogical Implications
| Teaching Strategy | Description | PSTET Focus |
|---|---|---|
| Build Both Circuits | Students build series and parallel circuits with 2-3 bulbs and observe differences | Hands-on comparison |
| Christmas Light Discussion | Discuss why old lights went out when one failed vs. new ones stay on | Real-world connection |
| House Wiring Diagram | Draw a simple house wiring diagram with parallel connections | Practical application |
| Prediction Activity | Predict what happens when a bulb is removed from series vs. parallel | Scientific thinking |
Section 12.5: Heating and Magnetic Effect of Electric Current 🔥🧲
Introduction
When electric current flows through a conductor, it produces various effects. The two most important effects are the heating effect and the magnetic effect .
12.5.1 Heating Effect of Electric Current
Definition: When electric current passes through a conductor, the conductor gets hot. This is called the heating effect of electric current .
Why Does Heating Occur?
As electrons move through a conductor, they collide with the atoms of the material. These collisions transfer energy to the atoms, making them vibrate more vigorously. Increased vibration means increased temperature—the material heats up .
Mathematical Relationship:
Heat Produced (H) = I² × R × t
Where:
I = Current flowing (in amperes)
R = Resistance of the conductor (in ohms)
t = Time for which current flows (in seconds)
Key Points:
Heat is proportional to the square of current—doubling current makes 4× heat
Higher resistance materials produce more heat
Longer time = more heat
12.5.2 Applications of Heating Effect
| Device | How It Works | Practical Use |
|---|---|---|
| Electric Bulb 💡 | Current heats tungsten filament to white-hot (2500°C) producing light | Lighting homes and streets |
| Electric Iron | Current heats a metal plate; thermostat maintains temperature | Ironing clothes |
| Electric Heater | Current heats a coil (nichrome wire) which glows red-hot | Room heating |
| Immersion Rod | Current heats a coil immersed in water | Heating water |
| Electric Kettle | Current heats element to boil water | Making tea/coffee |
| Electric Toaster | Current heats nichrome wires to toast bread | Breakfast preparation |
| Geyser | Current heats element to warm water | Bathing |
| Electric Oven | Current heats elements for baking | Cooking |
| Hair Dryer | Current heats air with heated coil, fan blows hot air | Drying hair |
| Electric Fuse | Deliberate use—wire melts if current too high | Circuit protection |
Material for Heating Elements:
Nichrome (alloy of nickel and chromium)
High resistance
Does not oxidize easily at high temperatures
High melting point
12.5.3 The Electric Fuse: Safety Device
Definition: A fuse is a safety device that protects circuits from excessive current .
How It Works:
A thin wire of low melting point (usually tin or alloy) is placed in the circuit
If current exceeds safe limit, the wire heats up and melts (blows)
This breaks the circuit, stopping current flow
Prevents damage to appliances and prevents fires
Types of Fuses:
| Type | Description | Use |
|---|---|---|
| Cartridge Fuse | Glass tube with metal caps | Electronic equipment, cars |
| Kit Kat Fuse | Porcelain holder with replaceable wire | Household circuits |
| MCB (Miniature Circuit Breaker) | Automatic switch—trips on overload | Modern homes (resettable) |
📝 PSTET Note: Fuses must be rated for the correct current. Using a higher-rated fuse (e.g., 15A instead of 5A) defeats the safety purpose.
12.5.4 Magnetic Effect of Electric Current
Definition: When electric current flows through a conductor, it creates a magnetic field around it. This is called the magnetic effect of electric current .
Discovery: This effect was discovered by Hans Christian Oersted in 1820. He noticed that a compass needle deflected when placed near a current-carrying wire .
12.5.5 Oersted's Experiment
Conclusion: An electric current produces a magnetic field around it. The strength and direction depend on current magnitude and direction.
12.5.6 Magnetic Field Patterns
For a Straight Conductor:
The magnetic field lines are concentric circles around the wire .
Closer to wire: field lines closer together (stronger field)
Farther from wire: field lines spread out (weaker field)
Imagine holding the wire in your RIGHT hand with THUMB pointing in direction of CURRENT. Your FINGERS curl in the direction of the MAGNETIC FIELD.
For a Circular Loop:
Field lines pass through the center of the loop
All lines pass through the loop in same direction
Creates a field similar to a short bar magnet
For a Solenoid (Coil of Wire):
Many turns of wire close together
Produces strong, uniform magnetic field inside
12.5.7 Strength of Magnetic Field Depends On:
| Factor | Effect on Field Strength |
|---|---|
| Current | More current → stronger field |
| Number of turns | More turns → stronger field |
| Distance from wire | Farther distance → weaker field |
| Core material | Iron core → much stronger field |
12.5.8 Applications of Magnetic Effect
| Device | Application of Magnetic Effect |
|---|---|
| Electric Motor | Converts electrical energy to mechanical energy |
| Electric Bell | Electromagnet strikes the gong |
| Loudspeaker | Electromagnet vibrates to produce sound |
| Telephone | Electromagnets in receiver produce sound |
| MRI Scanner | Strong magnetic fields for medical imaging |
| Maglev Train | Magnetic levitation for frictionless travel |
| Electric Generator | Converts mechanical energy to electrical energy |
| CRT TV/Monitor | Magnetic deflection of electron beams |
12.5.9 Pedagogical Implications
| Teaching Strategy | Description | PSTET Focus |
|---|---|---|
| Heating Effect Demo | Show wire getting warm (low voltage) | Observation |
| Fuse Activity | Demonstrate fuse blowing with excessive current | Safety learning |
| Oersted Experiment | Compass near current-carrying wire | Historical significance |
| Iron Filings Pattern | Sprinkle iron filings around magnetized solenoid | Visual learning |
Section 12.6: Electromagnets 🧲
Introduction
An electromagnet is a magnet that works only when electric current flows through it. It is one of the most useful inventions based on the magnetic effect of electric current .
12.6.1 What is an Electromagnet?
Definition: An electromagnet is a type of magnet in which the magnetic field is produced by an electric current .
Basic Structure:
A coil of insulated wire (copper) wound around a core
Core made of magnetic material (usually soft iron)
Current through wire creates magnetic field
12.6.2 History of Electromagnet
| Year | Scientist | Contribution |
|---|---|---|
| 1820 | Hans Christian Ørsted | Discovered electric currents create magnetic fields |
| 1820 | André-Marie Ampère | Showed iron can be magnetized by inserting it into a current-carrying coil |
| 1824 | William Sturgeon | Invented first electromagnet—horseshoe iron with 18 turns of bare copper wire; could lift 9 pounds with 7-ounce magnet |
| 1830 | Joseph Henry | Improved design with insulated wire, multiple layers of turns; created magnet lifting 2,063 pounds |
12.6.3 How an Electromagnet Works
Step-by-Step Explanation:
Why Soft Iron?
Soft iron is used because it can be magnetized quickly and loses magnetism just as quickly when current stops
Steel would retain some magnetism (permanent magnet), which is not desirable for most electromagnet applications
12.6.4 Factors Affecting Strength of an Electromagnet
| Factor | Effect on Strength | Explanation |
|---|---|---|
| Current | Increase current → stronger magnet | More current = more magnetic field |
| Number of turns | More turns → stronger magnet | Each turn contributes to total field |
| Core material | Iron core → much stronger than air core | Iron concentrates magnetic field |
| Spacing of turns | Closer turns → stronger field | Field adds more effectively |
12.6.5 Electromagnet vs. Permanent Magnet
| Feature | Electromagnet | Permanent Magnet |
|---|---|---|
| Magnetic field | Can be turned on/off | Always present |
| Strength | Can be varied by changing current | Fixed strength |
| Poles | Can be reversed by reversing current | Fixed poles |
| Material | Soft iron core + coil | Hard steel, alnico, ferrite |
| Power requirement | Needs continuous current | Needs no power |
| Lifting capacity | Can be extremely powerful | Limited strength |
| Demagnetization | Not a problem when switched off | Can be demagnetized by heat, hammering |
12.6.6 Applications of Electromagnets
Electromagnets are used in countless devices :
| Category | Applications |
|---|---|
| Home Appliances | Electric bell, loudspeaker, headphones, tape recorder, VCR, refrigerator (door seal), induction cooker |
| Industrial | Lifting heavy scrap iron/steel, magnetic separation, cranes in junkyards |
| Medical | MRI (Magnetic Resonance Imaging) scanners, medical research equipment |
| Transport | Maglev trains, electric vehicle motors |
| Scientific | Particle accelerators, mass spectrometers, laboratory equipment |
| Communication | Relays, telegraphs, telephone receivers |
| Automotive | Starter motors, door locks, sensors |
| Data Storage | Hard disk drives, magnetic tape |
12.6.7 The Electric Bell: A Classic Application
Working of an Electric Bell:
| Part | Function |
|---|---|
| Electromagnet | Attracts iron armature when current flows |
| Armature | Moves when attracted, strikes the gong |
| Contact screw | Breaks circuit when armature moves |
| Spring | Returns armature to original position |
| Gong | Produces sound when struck |
Working Cycle:
Press button → circuit complete → current flows
Electromagnet magnetized → attracts armature
Armature moves → strikes gong → produces sound
Armature movement breaks circuit at contact screw
Current stops → electromagnet demagnetized
Spring returns armature → circuit completes again
Cycle repeats rapidly → continuous ringing
12.6.8 Pedagogical Implications
| Teaching Strategy | Description | PSTET Focus |
|---|---|---|
| Make an Electromagnet | Students wrap wire around iron nail, connect to cell, pick up paper clips | Hands-on construction |
| Strength Investigation | Vary turns, current, and test strength | Scientific method |
| Compare Magnets | Compare electromagnet with bar magnet | Analytical thinking |
| Electric Bell Demo | Show working model of electric bell | Real-world application |
Section 12.7: Safety Measures in Using Electricity ⚠️
Introduction
Electricity is a powerful and useful servant, but it can be dangerous if not handled properly. Every year, accidents occur due to electrical shocks, short circuits, and fires. Understanding and following safety measures is essential .
12.7.1 Why Electricity is Dangerous
| Hazard | What Happens | Potential Consequence |
|---|---|---|
| Electric Shock | Current passes through body | Injury, burns, cardiac arrest, death |
| Short Circuit | Current takes unintended path | Sparking, overheating, fire |
| Overloading | Too much current in circuit | Overheating, fire, damage to appliances |
| Fire | Heat from electricity ignites materials | Property damage, injury, death |
12.7.2 Essential Electrical Safety Rules
Table 12.6: Electrical Safety Do's and Don'ts
12.7.3 Identifying Electrical Hazards
Look for these warning signs :
| Sign of Trouble | What It Means | Action Required |
|---|---|---|
| Frayed insulation on cords (especially at ends) | Wire exposed—shock/fire risk | Replace cord immediately |
| Melted plugs or sockets | Overheating—danger of fire | Disconnect, replace, check circuit |
| Exposed metal parts on appliances | May become live—shock risk | Repair or replace |
| Sparks from plug when connecting | Loose connection—fire risk | Replace plug, check socket |
| Frequent fuse blowing / MCB tripping | Overload or fault | Reduce load, check circuit |
| Warm plugs or sockets | Loose connection—overheating | Tighten connections or replace |
| Burning smell from appliance | Internal fault | Unplug immediately, get repaired |
12.7.4 Earthing/Grounding
Definition: Earthing means connecting the metal body of an electrical appliance to the earth through a thick wire .
Purpose of Earthing:
If a live wire touches the metal body, current flows to earth instead of through a person touching it
Provides a safe path for fault current
Trips the circuit breaker/fuse, cutting off power
Three-Pin Plug Connections:
| Pin | Position | Wire Color (International) | Wire Color (India) | Function |
|---|---|---|---|---|
| Live (Phase) | Right | Brown | Red | Carries current to appliance |
| Neutral | Left | Blue | Black | Completes circuit |
| Earth | Top (longer) | Green/Yellow | Green | Safety connection |
12.7.5 Fuses and Circuit Breakers
Fuse:
Thin wire that melts if current exceeds safe limit
One-time use—must be replaced after "blowing"
Must be correctly rated for the circuit
MCB (Miniature Circuit Breaker):
Automatic switch that trips (turns off) on overload
Can be reset by switching back on
More convenient than fuses
Earthing-Leakage Circuit Breaker (ELCB) / Residual Current Circuit Breaker (RCCB):
Detects when current is leaking to earth (through a person)
Trips in milliseconds—can save lives
Highly recommended for home safety
12.7.6 First Aid for Electric Shock
If someone gets an electric shock, follow these steps:
| Step | Action | Important Notes |
|---|---|---|
| 1 | Switch off power immediately | If switch is accessible, turn off |
| 2 | Do NOT touch the person directly | You could also get shocked |
| 3 | Move person away using non-conductor | Use wooden stick, rubber mat, dry cloth—NOT metal or wet material |
| 4 | Check breathing and pulse | If not breathing, start CPR if trained |
| 5 | Call for medical help | Rush to hospital; electric shock can cause internal injuries |
| 6 | Treat burns if present | Cool with water, cover with sterile cloth |
12.7.7 Safety at Home and School
| Location | Specific Safety Measures |
|---|---|
| Kitchen | Keep appliances away from water; dry hands before touching switches |
| Bathroom | No sockets near bath/shower; use safety sockets |
| Children's Room | Use safety covers on unused sockets; teach children not to poke objects into sockets |
| Outdoors | Use weatherproof sockets; keep cables out of water |
| Laboratory | Low-voltage circuits only; teacher supervision essential |
12.7.8 What to Do in Case of Electrical Fire
NEVER use water on an electrical fire—water conducts electricity and can cause shock.
| Do ✅ | Don't ❌ |
|---|---|
| Switch off main power if possible | Don't throw water |
| Use fire extinguisher (Class C for electrical) | Don't use metal objects |
| Use fire blanket to smother small fires | Don't panic |
| Call fire department | Don't re-enter burning area |
12.7.9 Pedagogical Implications
| Teaching Strategy | Description | PSTET Focus |
|---|---|---|
| Safety Poster Project | Students create posters illustrating safety rules | Creative learning |
| Hazard Hunt | Identify potential electrical hazards in classroom/home | Observation skills |
| Role Play: First Aid | Practice responding to electric shock scenario | Life skills |
| Plug Wiring Demo | Show correct three-pin plug wiring (teacher demo only) | Practical knowledge |
Chapter Summary: Key Points for Revision 📝
Quick Revision Table
| Topic | Key Points | Common PSTET Questions |
|---|---|---|
| Electric Cell | Converts chemical to electrical energy; has + and - terminals; voltage in volts | What are cell terminals? |
| Simple Circuit | Complete path: cell → wires → bulb → back to cell; needs switch for control | Draw a simple circuit diagram |
| Conductors | Allow current; have free electrons; metals (copper, silver, aluminum) | Name three conductors |
| Insulators | Don't allow current; plastic, rubber, glass, wood | Name three insulators |
| Series Circuit | Single path; current same everywhere; one break stops all | Compare series and parallel |
| Parallel Circuit | Multiple paths; voltage same across branches; independent operation | Why are homes wired in parallel? |
| Heating Effect | I²Rt; used in bulbs, irons, heaters, fuses | Applications of heating effect |
| Magnetic Effect | Current produces magnetic field; Oersted's experiment | What is magnetic effect? |
| Electromagnet | Coil + iron core; field controlled by current | How to make an electromagnet |
| Safety Measures | No wet hands; proper earthing; correct fuse rating; three-pin plug | Electrical safety rules |
Practice Zone: PSTET-Style Questions 🎯
Content-Based MCQs
Q1. Which of the following is a conductor of electricity?
a) Rubber
b) Plastic
c) Copper
d) Wood
Q2. In a series circuit with three bulbs, if one bulb fuses, what happens to the other bulbs?
a) They glow brighter
b) They glow dimmer
c) They stop glowing
d) They remain same
Q3. The heating effect of electric current is used in:
a) Electric bell
b) Electric iron
c) Electromagnet
d) Motor
Q4. Which material is commonly used for making the filament of an electric bulb?
a) Copper
b) Aluminium
c) Tungsten
d) Iron
Q5. The device that automatically breaks the circuit in case of excessive current is called:
a) Switch
b) Cell
c) Fuse
d) Bulb
Q6. An electromagnet loses its magnetism when:
a) Current is increased
b) Current is stopped
c) More turns are added
d) Iron core is used
Q7. In a three-pin plug, which pin is for earthing?
a) Top pin (longer)
b) Bottom left
c) Bottom right
d) All pins
Q8. Which scientist discovered the magnetic effect of electric current?
a) Newton
b) Oersted
c) Faraday
d) Edison
Q9. Pure water is:
a) Good conductor
b) Poor conductor (insulator)
c) Semiconductor
d) Superconductor
Q10. In a parallel circuit, if one bulb is removed:
a) All bulbs stop working
b) Other bulbs continue working
c) Current increases in other bulbs
d) Voltage decreases
Pedagogical MCQs
Q11. A teacher wants to demonstrate that water conducts electricity. The safest method would be:
a) Use tap water with low-voltage cell and LED
b) Use direct mains supply with bulb
c) Show a video only
d) Tell students without demonstration
Q12. While teaching series and parallel circuits, the best approach is:
a) Only draw diagrams on board
b) Students build both circuits and observe differences
c) Give notes to memorize
d) Show pictures in textbook
Q13. A student asks, "Why don't birds sitting on a high-voltage wire get shocked?" The teacher should explain:
a) "Birds are special"
b) Both of bird's feet are on same wire (no potential difference), and no path to ground
c) "I don't know"
d) "Don't touch electricity"
Q14. To teach electrical safety effectively, the teacher should:
a) Only list rules
b) Demonstrate safe practices and discuss real accidents (age-appropriate)
c) Show scary pictures
d) Read from textbook
Q15. The most effective way to teach the concept of electromagnets is:
a) Show a diagram
b) Students make simple electromagnets using nail, wire, and cell
c) Lecture only
d) Video only
Answer Key with Explanations
| Q.No. | Answer | Explanation |
|---|---|---|
| 1 | c) Copper | Copper is a metal with free electrons—good conductor |
| 2 | c) They stop glowing | Series circuit has single path; break anywhere stops current everywhere |
| 3 | b) Electric iron | Iron uses heating effect; bell, electromagnet, motor use magnetic effect |
| 4 | c) Tungsten | Tungsten has high melting point and does not oxidize easily |
| 5 | c) Fuse | Fuse melts on excessive current, breaking circuit |
| 6 | b) Current is stopped | Electromagnet works only when current flows |
| 7 | a) Top pin (longer) | Top pin is earth; longer to ensure connection first |
| 8 | b) Oersted | Oersted discovered magnetic effect in 1820 |
| 9 | b) Poor conductor (insulator) | Pure water is insulator; impurities make it conductive |
| 10 | b) Other bulbs continue | Parallel circuits have independent paths |
| 11 | a) Low-voltage demonstration | Safety first—never use mains for experiments |
| 12 | b) Hands-on construction | Experiential learning is most effective |
| 13 | b) Correct scientific explanation | No potential difference, no path to ground |
| 14 | b) Demonstrate and discuss | Practical demonstration with discussion is effective |
| 15 | b) Hands-on making | Making electromagnet is engaging and memorable |
Pedagogical Reflection for Teachers 🤔
Think-Pair-Share Activity:
Think: How would you explain to students why they must never touch electrical switches with wet hands?
Pair: Discuss with a colleague how you would set up a "Circuit Building Station" with various components for exploration.
Share: Design a 15-minute activity to teach the difference between conductors and insulators using everyday objects.
NCERT Textbook Linkages 📚
| Class | Chapter | Topic |
|---|---|---|
| Class 6 | Chapter 12 | Electricity and Circuits |
| Class 7 | Chapter 14 | Electric Current and Its Effects |
| Class 10 | Chapter 12 | Electricity |
| Class 10 | Chapter 13 | Magnetic Effects of Electric Current |
Chapter End Notes
Key Terminology Glossary
| Term | Definition |
|---|---|
| Electric Cell | Device converting chemical to electrical energy |
| Circuit | Complete path for electric current |
| Conductor | Material allowing current to pass |
| Insulator | Material not allowing current to pass |
| Series Circuit | Components connected end-to-end; single path |
| Parallel Circuit | Components connected across same points; multiple paths |
| Heating Effect | Production of heat when current flows |
| Magnetic Effect | Production of magnetic field when current flows |
| Electromagnet | Magnet working only when current flows |
| Fuse | Safety device melting on excess current |
| Earthing | Connecting appliance body to earth for safety |
Quick Tips for PSTET Aspirants ⚡
✅ Memorize with Mnemonics:
Good Conductors: "Some Cats Are Iron" = Silver, Copper, Aluminum, Iron
Good Insulators: "Plastic Rubber Glass Wood" = Plastic, Rubber, Glass, Wood
Series vs Parallel: "Single path = Series; Parallel = Paths"
Three-Pin Colors: "Red - Live, Black - Neutral, Green - Earth" = RL, BN, GE
✅ Common Exam Traps:
Electron flow vs. Conventional current: Electrons flow negative to positive; conventional current positive to negative
Pure water is insulator; impure water is conductor
Fuse must be in live wire, not neutral
Earth pin is longer to ensure connection first
Filament made of tungsten, not copper
Nichrome used in heating elements (high resistance)
✅ Important Facts:
Silver is best conductor; copper most commonly used
Tungsten melting point: 3422°C
Oersted discovered magnetic effect (1820)
Sturgeon invented first electromagnet (1824)
MCB is reusable; fuse is one-time use
House wiring is parallel; old Christmas lights were series
Answers to "Check Your Understanding"
[To be filled by student]
📝 Note for Self-Study: After completing this chapter, ensure you can:
Explain how a cell works and identify its terminals
Draw and label a simple circuit with cell, bulb, wires, switch
List 5 conductors and 5 insulators with examples
Compare series and parallel circuits with advantages/disadvantages
Explain heating effect with 5 applications
Explain magnetic effect and Oersted's experiment
Describe how to make an electromagnet and factors affecting its strength
List 10 electrical safety rules
Explain three-pin plug wiring
Describe first aid for electric shock
