Magnetic Effects of Electric Current: Complete CBSE Class 10 Notes

Introduction to Magnetic Effects of Electric Current

The relationship between electricity and magnetism is one of the most fundamental discoveries in physics. When electric current flows through a conductor, it produces a magnetic field around it a phenomenon known as the magnetic effect of electric current or electromagnetism. This chapter explores how current-carrying conductors behave in magnetic fields, the principles behind electric motors and generators, and practical applications in everyday devices.

Historical Context

The discovery of electromagnetism is credited to several pioneering scientists:

• Romagnosi (1802) first observed the magnetic effect of current
• Hans Christian Oersted (1820) conducted the first systematic experimental demonstration
• Michael Faraday (1831) discovered electromagnetic induction
• These discoveries laid the foundation for modern electrical technology

Fundamental Concepts of Magnetism

What is Magnetism?

Magnetism is the property by which certain substances attract pieces of iron, nickel, and cobalt. This property was first observed in a naturally occurring iron ore called magnetite (Fe₃O₄), found in Magnesia, Greece from which the term "magnetism" originates.

Magnetic Substances and Magnets

• Magnetic substance: A material that exhibits magnetic properties (iron, nickel, cobalt, steel)
• Magnet: A body made of magnetic material that can attract magnetic substances
• Types: Natural magnets (magnetite) and artificial magnets (manufactured in various shapes)

Magnetic Poles

When a bar magnet is dipped in iron filings, maximum concentration occurs at the ends these regions are called magnetic poles.

• North Pole (N): The end that points toward geographic north when freely suspended
• South Pole (S): The end that points toward geographic south
• Magnetic dipole: Every magnet has two poles; isolated magnetic poles cannot exist

Fundamental Properties of Magnets

1. Like poles repel: N-N or S-S poles push away from each other
2. Unlike poles attract: N-S poles pull toward each other
3. Magnetic poles always exist in pairs: Breaking a magnet creates two new magnets, each with both poles

Earth's Magnetism

The Earth behaves as a giant magnet due to electric currents flowing in its molten metallic core. 

• Core radius: Approximately 3,500 km (Earth's radius is 6,400 km)
• Magnetic field strength: Order of one gauss (10⁻⁴ Tesla)
• Field pattern: Resembles a bar magnet one-fifth of Earth's diameter buried at its center
• Geographic vs Magnetic poles: Earth's magnetic south pole is near the geographic north pole

Magnetic Field and Field Lines

Magnetic Field Definition

A magnetic field is the space around a magnet or current-carrying conductor where magnetic forces can be detected. It is a vector quantity with both magnitude and direction.

Magnetic Lines of Force (Field Lines)

Magnetic field lines are imaginary lines that represent the direction and relative strength of a magnetic field.

Definition

A magnetic line of force is a line, straight or curved, whose tangent at any point gives the direction of the magnetic field at that point.

Properties of Magnetic Field Lines

1. Closed loops: Lines emerge from the North pole, enter the South pole externally, and continue from S to N inside the magnet
2. Never intersect: Two field lines cannot cross; otherwise, the compass would point in two directions simultaneously
3. Perpendicular to surface: Field lines meet the magnet's surface at right angles
4. Density indicates strength: Closely spaced lines indicate stronger fields (concentrated near poles)
5. Direction convention: The direction in which a free north pole would move

Visualizing Magnetic Fields

Around a bar magnet:

• Lines curve from North to South pole outside the magnet
• Lines are parallel and straight inside the magnet
• Highest concentration near the poles

Oersted's Experiment: Discovery of Electromagnetism

Experimental Setup

Hans Christian Oersted demonstrated that electric current produces a magnetic field:

Apparatus:

• Straight wire AB connected to battery and key
• Wire positioned horizontally in the north-south direction
• Magnetic compass needle placed beneath the wire

Observations

1. Current off: Compass needle aligns north-south (Earth's magnetic field)
2. Current on (northward flow): North pole deflects toward west
3. Current reversed (southward flow): North pole deflects toward east
4. Wire below needle: Deflection direction reverses
5. Distance matters: Deflection decreases with distance from wire

Conclusion

A current-carrying conductor produces a magnetic field around it. The direction of the field depends on the direction of current flow.

SNOW Rule

A mnemonic for remembering the relationship:

South → North current (conductor over needle) → West deflection (North pole)

Magnetic Field Due to Current-Carrying Conductors

Case 1: Straight Conductor → Circular Magnetic Field

When current flows through a straight wire, the magnetic field forms concentric circles around the wire.

Demonstration with Iron Filings

Experiment:

1. Pass a vertical wire through a horizontal cardboard
2. Sprinkle iron filings on the cardboard
3. Close the circuit and tap the cardboard gently
4. Filings arrange in circular patterns around the wire

Result: Magnetic field lines are circular, centered on the wire, in planes perpendicular to it.

Magnetic Field Strength Formula

For a straight current-carrying conductor:

B = (μ₀I)/(2πr)

Where:

• B = Magnetic field strength (Tesla)
• μ₀ = Permeability of free space (4π × 10⁻⁷ T·m/A)
• I = Current (Amperes)
• r = Distance from wire (meters)

Key observations:

• B ∝ I (directly proportional to current)
• B ∝ 1/r (inversely proportional to distance)

Right-Hand Thumb Rule (Maxwell's Corkscrew Rule)

Statement

For Straight Conductors: Curl the fingers of your right hand around the conductor with the thumb pointing in the direction of current. Your curled fingers indicate the direction of the circular magnetic field lines.

For Circular Conductors (coils): Curl your fingers in the direction of current flow in the coil. Your thumb points in the direction of the magnetic field (straight lines at the center).

Maxwell's Corkscrew Rule (Alternative)

Imagine rotating a corkscrew in the direction of current flow. The direction of rotation gives the direction of the magnetic field.

Quick Summary:

• Current upward → Magnetic field anticlockwise
• Current downward → Magnetic field clockwise

Simple Classroom Experiments to Show Magnetic Effect of Current

Experiment 1: Compass Deflection Near a Current-Carrying Wire

Materials: Battery, connecting wires, switch, compass needle

Procedure:

1. Place a straight wire north-south above a compass
2. Connect wire to battery through a switch
3. Close switch and observe needle deflection
4. Reverse current direction and observe

Observation: Needle deflects perpendicular to wire; direction reverses with current reversal

Conclusion: Current produces magnetic field; field direction depends on current direction

Experiment 2: Iron Filings Pattern Around a Wire

Materials: Stiff cardboard, iron filings, thick wire, battery, switch

Procedure:

1. Pass wire vertically through cardboard center
2. Sprinkle iron filings uniformly on cardboard
3. Connect wire to battery
4. Gently tap cardboard while current flows

Observation: Iron filings form concentric circular patterns

Conclusion: Magnetic field around straight wire is circular

Experiment 3: Magnetic Field of a Circular Coil

Materials: Circular coil, battery, switch, compass, cardboard

Procedure:

1. Mount coil vertically
2. Sprinkle iron filings on cardboard placed near coil
3. Pass current through coil
4. Map field with compass at various points

Observation:

• Field lines straight at center
• Field lines form closed loops outside
• One face acts as North pole, other as South

Conclusion: Circular coil produces magnetic field similar to bar magnet

Experiment 4: Electromagnet Demonstration

Materials: Soft iron rod, insulated copper wire, battery, iron nails/pins

Procedure:

1. Wind 50-100 turns of wire around iron rod
2. Connect to battery through switch
3. Bring near iron nails with current on
4. Switch off and observe

Observation:

• Nails attracted when current flows
• Nails drop when current stops

Conclusion: Soft iron becomes temporary magnet with current (electromagnet)

Case 2: Circular Conductor → Straight Magnetic Field

Magnetic Field at the Center of a Circular Coil

When current flows through a circular loop, the magnetic field at the center is perpendicular to the plane of the coil and forms straight parallel lines.

Formula

B = (μ₀nI)/(2r)

Where:

• n = Number of turns in the coil
• r = Radius of the coil
• Other symbols as before

Key observations:

• B ∝ I (proportional to current)
• B ∝ n (proportional to number of turns)
• B ∝ 1/r (inversely proportional to radius)

Determining Polarity

Clock Face Rule:

• Current clockwise (viewed from one side) → That face is South pole
• Current anticlockwise → That face is North pole

Magnetic Field Due to a Solenoid

What is a Solenoid?

A solenoid is a long cylindrical coil of many turns of insulated wire, wound tightly in a helical pattern. It acts as an electromagnet when current flows through it.

Characteristics:

• Length >> Diameter
• Acts like a bar magnet when current flows
• Produces uniform magnetic field inside

Magnetic Field Pattern

Inside the solenoid:

• Field is uniform (constant strength)
• Field lines are parallel and straight
• Direction: From south pole to north pole internally

Outside the solenoid:

• Field resembles that of a bar magnet
• Lines curve from north to south pole
• Field is non-uniform and weaker

Formula for Magnetic Field Inside a Solenoid

B = μ₀μᵣnI

Where:

• μᵣ = Relative permeability of core material
• n = Number of turns per unit length (N/L)
• For air core: μᵣ = 1

Factors affecting strength:

1. Number of turns per unit length (n): B ∝ n
2. Current (I): B ∝ I
3. Core material permeability (μᵣ): B ∝ μᵣ

Polarity Determination

Rule:

• Current clockwise → Face is South pole
• Current anticlockwise → Face is North pole

(When viewed from the end of the solenoid)

Difference Between Solenoid and Bar Magnet Magnetic Fields

Similarity: Both produce similar external field patterns with poles at ends.

Electromagnets: Principles and Applications

What is an Electromagnet?

An electromagnet consists of:

• Many turns of insulated copper wire wound on a soft iron core
• Soft iron core (increases magnetic field strength)
• Connected to a power source

Factors Affecting Strength

1. Number of turns: More turns → Stronger field
2. Current magnitude: More current → Stronger field
3. Air gap between poles: Smaller gap → Stronger field at gap
4. Core material: Soft iron core significantly increases strength (μᵣ >> 1)

Advantages Over Permanent Magnets

Applications

• Electric bells and buzzers
• Relay switches
• Electric motors and generators
• Magnetic cranes (lifting heavy iron objects)
• MRI machines (medical imaging)
• Maglev trains (magnetic levitation)
• Loudspeakers and microphones
• Magnetic locks

Force on a Current-Carrying Conductor in a Magnetic Field

Fundamental Principle

When a current-carrying conductor is placed in an external magnetic field, it experiences a mechanical force due to interaction between the two magnetic fields (field due to current and external field).

Force Formula

F = BIL sin θ

Where:

• F = Force on conductor (Newtons)
• B = Magnetic field strength (Tesla)
• I = Current (Amperes)
• L = Length of conductor in field (meters)
• θ = Angle between conductor and magnetic field

Special cases:

1. Perpendicular (θ = 90°): F = BIL (maximum force)
2. Parallel (θ = 0°): F = 0 (no force)

Direction of Force: Fleming's Left-Hand Rule

This is one of the most important rules in electromagnetism.

Statement

Stretch the thumb, forefinger, and middle finger of your left hand mutually perpendicular to each other such that:

• Forefinger (First finger) → Direction of magnetic Field (N to S)
• Middle finger (seCond finger) → Direction of Current (+ to −)
• Thumb → Direction of Force (or motion/thrust)

Mnemonic: First finger Field, seCond finger Current, thuMb Motion

Fleming's Left-Hand Rule: Detailed Examples

Example 1: Basic Application

Given:

• Current flows from west to east (horizontal)
• Magnetic field directed north to south (horizontal)

Solution:

1. Point forefinger toward south (field direction)
2. Point middle finger toward east (current direction)
3. Thumb points downward (force direction)

Result: Conductor experiences downward force

Example 2: Vertical Wire in Horizontal Field

Given:

• Current flows upward through vertical wire
• Magnetic field directed horizontally eastward

Solution:

1. Forefinger points east (field)
2. Middle finger points up (current)
3. Thumb points north (force)

Result: Wire experiences force toward north

Example 3: Electric Motor Application

Given:

• Rectangular coil in magnetic field (N-pole on left, S-pole on right)
• Current in arm AB flows from A to B (downward)
• Current in arm CD flows from C to D (upward)

Analysis for AB:

• Field: Left to right (N → S)
• Current: Top to bottom (A → B)
• Force: Out of page (toward viewer)

Analysis for CD:

• Field: Left to right (N → S)
• Current: Bottom to top (C → D)
• Force: Into page (away from viewer)

Result: Coil rotates anticlockwise (when viewed from right side)

Example 4: Zero Force Condition

Given:

• Current flows north to south
• Magnetic field also directed north to south

Solution: Since current and field are parallel, θ = 0° F = BIL sin(0°) = 0

Result: No force on conductor

Example 5: Charged Particle in Magnetic Field

Given:

• Proton moving eastward
• Magnetic field directed vertically upward

Solution:

1. Proton (+ve charge) moving east = current toward east
2. Middle finger: East (current direction)
3. Forefinger: Upward (field direction)
4. Thumb: South (force direction)

Result: Proton deflects toward south

Note: For electrons (−ve charge), current direction is opposite to motion.

Numerical Problems on Force on Current-Carrying Conductor

Problem 1: Basic Force Calculation

Question: A conductor of length 50 cm carrying a current of 5 A is placed perpendicular to a magnetic field of 0.2 T. Calculate the force experienced.

Given:

• L = 50 cm = 0.5 m
• I = 5 A
• B = 0.2 T
• θ = 90° (perpendicular)

Solution: F = BIL sin θ
F = 0.2 × 5 × 0.5 × sin(90°)
F = 0.2 × 5 × 0.5 × 1
F = 0.5 N

Problem 2: Parallel Conductor

Question: A wire of length 2 m carrying 3 A current is placed parallel to a magnetic field of 0.5 T. What is the force?

Given:

• L = 2 m
• I = 3 A
• B = 0.5 T
• θ = 0° (parallel)

Solution: F = BIL sin θ
F = 0.5 × 3 × 2 × sin(0°)
F = 0.5 × 3 × 2 × 0
F = 0 N

Conclusion: No force when conductor is parallel to field

Problem 3: Angle Between Field and Conductor

Question: A 0.8 m conductor carrying 4 A makes an angle of 30° with a 0.6 T magnetic field. Find the force.

Given:

• L = 0.8 m
• I = 4 A
• B = 0.6 T
• θ = 30°

Solution: F = BIL sin θ
F = 0.6 × 4 × 0.8 × sin(30°)
F = 0.6 × 4 × 0.8 × 0.5
F = 0.96 N

Problem 4: Finding Magnetic Field Strength

Question: A 1.5 m conductor carrying 2 A experiences a force of 0.9 N when perpendicular to a magnetic field. Find B.

Given:

• F = 0.9 N
• I = 2 A
• L = 1.5 m
• θ = 90°

Solution: F = BIL
B = F/(IL)
B = 0.9/(2 × 1.5)
B = 0.3 T

Problem 5: Charged Particle Force

Question: An electron (q = 1.6 × 10⁻¹⁹ C) moves at 3 × 10⁶ m/s perpendicular to a 0.4 T field. Find the force.

Given:

• q = 1.6 × 10⁻¹⁹ C
• v = 3 × 10⁶ m/s
• B = 0.4 T
• θ = 90°

Solution: For moving charge: F = qvB sin θ
F = 1.6 × 10⁻¹⁹ × 3 × 10⁶ × 0.4 × 1
F = 1.92 × 10⁻¹³ N
F ≈ 1.9 × 10⁻¹³ N

How to Draw Field Lines for a Current-Carrying Circular Coil

Step-by-Step Procedure

Step 1: Draw the Coil

• Draw a circle representing the circular loop
• Mark the direction of current flow with arrows
• Label the plane of the coil

Step 2: Determine Polarity

• Use the right-hand thumb rule
• Curl fingers in direction of current
• Thumb points toward North pole face
• Opposite face is South pole

Step 3: Draw Internal Field Lines

• At the center, draw straight parallel lines perpendicular to coil plane
• Lines should be evenly spaced
• All pointing in the direction of the thumb (from coil center)

Step 4: Draw External Field Lines

• Lines emerge from North pole face
• Curve around outside
• Enter through South pole face
• Form closed loops
• Lines should never cross

Step 5: Add Details

• Lines closer together near coil (stronger field)
• Lines spread apart far from coil (weaker field)
• Use arrows to show direction (N → S externally)

Example Drawing

For anticlockwise current (viewed from right):

• Right face: North pole (lines emerge)
• Left face: South pole (lines enter)
• Center: Straight lines pointing right
• Outside: Curved lines from right face to left face

Electromagnetic Induction

Discovery by Faraday (1831)

Michael Faraday discovered that a changing magnetic field induces an electric current in a conductor the reverse of Oersted's observation.

Fundamental Principle

Electromagnetic Induction: The phenomenon by which an electromotive force (EMF) and current are induced in a conductor when the magnetic flux linked with it changes.

Faraday's Experimental Setup

Experiment 1: Moving Magnet

Setup:

• Coil connected to galvanometer
• Bar magnet

Observations:

1. Magnet at rest near coil → No deflection
2. Magnet moved toward coil → Deflection in one direction
3. Magnet held stationary inside → No deflection
4. Magnet withdrawn from coil → Deflection in opposite direction
5. Faster motion → Larger deflection

Conclusion: Changing magnetic flux induces current

Experiment 2: Two Coils

Setup:

• Primary coil P connected to battery and key
• Secondary coil S (nearby) connected to galvanometer

Observations:

1. Key open → No deflection
2. Key pressed (circuit closed) → Momentary deflection
3. Key held closed → No deflection
4. Key released (circuit opened) → Deflection in opposite direction

Conclusion: Changing current in P changes magnetic flux through S, inducing current

Factors Affecting Induced Current

1. Number of turns in coil: More turns → Larger induced EMF
2. Strength of magnet: Stronger magnet → Larger induced EMF
3. Speed of motion: Faster motion → Larger induced EMF
4. Area of coil: Larger area → More flux change → Larger EMF

Fleming's Right-Hand Rule

Application: Direction of Induced Current

While Fleming's Left-Hand Rule applies to motors (force on current), Fleming's Right-Hand Rule applies to generators (induced current).

Statement

Stretch the thumb, forefinger, and middle finger of your RIGHT hand mutually perpendicular such that:

• Forefinger → Direction of magnetic Field (N → S)
• Thumb → Direction of Motion of conductor
• Middle finger → Direction of induced Current

Mnemonic: First Field, thuMb Motion, seCond Current

Example Application

Given:

• Conductor moving upward
• Magnetic field from left to right (west to east)

Solution:

1. Forefinger: Point right (field direction)
2. Thumb: Point up (motion direction)
3. Middle finger: Points toward viewer (out of page)

Result: Induced current flows out of the page (or north if in horizontal plane)

Lenz's Law

Statement

The direction of the induced current is such that it opposes the change in magnetic flux that produced it.

This is a consequence of the law of conservation of energy.

Understanding with Examples

Example 1: North pole of magnet approaches a coil

• Flux through coil increases
• To oppose this increase, induced current creates a magnetic field that repels the approaching magnet
• Coil face acts as North pole (like pole repels)
• By right-hand rule, current flows anticlockwise (viewed from magnet side)

Example 2: North pole withdrawn from coil

• Flux through coil decreases
• To oppose this decrease, induced current creates field that attracts the magnet
• Coil face acts as South pole (unlike pole attracts)
• Current flows clockwise

Point: "Opposes" means the induced effect tries to counteract the change, not necessarily oppose the motion itself.

Direct Current (DC) vs Alternating Current (AC)

Direct Current (DC)

Definition: Electric current that flows in one direction only and has constant magnitude.

Sources:

• Batteries
• Solar cells
• DC generators (with commutator)

Graphical representation: Horizontal straight line (constant value)

Applications:

• Electronic devices
• Battery-powered equipment
• Electroplating
• Charging batteries

Alternating Current (AC)

Definition: Electric current that periodically reverses direction and changes magnitude continuously.

Sources:

• AC generators (alternators)
• Power stations
• Household electrical supply

Graphical representation: Sinusoidal wave (sine curve)

Frequency:

• India: 50 Hz (50 cycles per second)
• USA: 60 Hz
• Current changes direction 100 times per second in India (twice per cycle)

Comparison Table

Advantages of AC over DC

1. Long-distance transmission: AC can be stepped up to high voltage, reducing current and power loss (P = I²R)
2. Easy transformation: Transformers work only with AC
3. Cheaper generation: AC generators simpler than DC generators
4. Easy voltage conversion: Step up/down easily with transformers
5. Simpler motors: AC motors have no commutator

Disadvantages of AC

1. More dangerous: Attractive shock is harder to release from
2. Skin effect: Current concentrates on conductor surface at high frequency
3. Cannot charge batteries directly: Needs rectification
4. Sudden large shock: Can be fatal

Electric Motor (DC Motor)

Principle

An electric motor converts electrical energy into mechanical energy based on the principle that a current-carrying conductor experiences force in a magnetic field.

Construction

Main Components:

1. Armature:
   • Rectangular coil wound on soft iron core
   • Rotates between magnetic poles
   • Mounted on axle/shaft
2. Magnet:
   • Strong permanent magnet or electromagnet
   • Creates uniform magnetic field
   • N and S poles face each other
3. Split-ring Commutator:
   • Cylindrical ring split into two halves (C₁ and C₂)
   • Each coil end connected to one half
   • Rotates with armature
   • Crucial function: Reverses current direction every half rotation
4. Brushes:
   • Two carbon/graphite blocks (B₁ and B₂)
   • Press against commutator
   • Maintain electrical contact during rotation
   • Connected to battery terminals

Working Principle

Step 1: Current enters through brush B₁ → Commutator C₁ → Coil arm AB

Step 2: Apply Fleming's Left-Hand Rule:

• Arm AB: Current direction → Field direction → Force downward
• Arm CD: Current direction → Field direction → Force upward

Step 3: These opposing forces create torque, rotating coil anticlockwise

Step 4: After half rotation, commutator halves switch brushes:

• Current direction in coil reverses
• But force directions remain same relative to magnet poles
• Rotation continues in same direction

Step 5: Momentum carries coil through vertical position (where torque is zero)

Result: Continuous rotation in one direction

Why Commutator is Essential

Without commutator:

• Current direction would remain constant
• After half rotation, force directions would reverse
• Coil would oscillate, not rotate continuously

With commutator:

• Current reverses every half rotation
• Force always acts to rotate coil in same direction
• Continuous unidirectional rotation achieved

Factors Increasing Motor Power

1. Increase number of turns in armature coil
2. Increase current flowing through coil
3. Use stronger magnets
4. Increase area of coil
5. Use laminated soft iron core (reduces eddy currents)

Electric Generator (Dynamo)

Principle

An electric generator converts mechanical energy into electrical energy based on electromagnetic induction a changing magnetic flux induces EMF in a conductor.

Construction (DC Generator)

Main Components:

1. Armature:
   • Rectangular coil (ABCD) wound on soft iron core
   • Rotated mechanically in magnetic field
2. Magnet:
   • Strong permanent magnet or electromagnet
   • Provides uniform field between N-S poles
3. Split-ring Commutator:
   • Two halves C₁ and C₂
   • Each connected to one coil end
   • Function: Converts AC to DC output
4. Brushes:
   • Carbon blocks B₁ and B₂
   • Touch commutator
   • Connected to external circuit (e.g., bulb)

Working Principle

Step 1: Coil rotated mechanically (by engine, turbine, hand crank, etc.)

Step 2: Position 1 (arms AB and CD cutting field lines):

• Apply Fleming's Right-Hand Rule
• Arm AB moves down → Induced current in direction → A to B
• Arm CD moves up → Induced current in direction → C to D
• Current flows: A→B→external circuit→C→D→A
• Terminal P at higher potential, Q at lower

Step 3: Position 2 (after half rotation):

• Arm AB now moves up → Current direction reverses (B to A)
• Arm CD now moves down → Current reverses (D to C)
• But commutator halves have switched contact with brushes
• Current in external circuit still flows P to Q (same direction)

Step 4: Continuous rotation maintains unidirectional (DC) current in external circuit

AC Generator (Alternator)

Difference from DC Generator:

• Uses slip rings instead of split-ring commutator
• Slip ring: Full ring, each connected to one coil end
• Brush B₁ always touches ring C₁, B₂ always touches C₂

Result:

• Current direction in external circuit alternates every half rotation
• Output is alternating current (AC)

Comparison: DC vs AC Generator

Transformer

Definition

A transformer is a device that transfers electrical energy from one circuit to another through electromagnetic induction, typically changing the voltage level.

Principle

Based on mutual induction: Changing current in one coil induces EMF in a nearby coil due to changing mutual magnetic flux.

Construction

Components:

1. Primary Coil:
   • Input coil with Nₚ turns
   • Connected to AC source
2. Secondary Coil:
   • Output coil with Nₛ turns
   • Connected to load
3. Laminated Iron Core:
   • Soft iron sheets stacked together
   • Insulated from each other
   • Reduces eddy current losses
   • Provides path for magnetic flux

Working

Step 1: AC flows through primary coil

Step 2: Changing current creates changing magnetic flux in iron core

Step 3: This changing flux links with secondary coil

Step 4: Induced EMF in secondary by Faraday's law

Step 5: If circuit closed, induced current flows in secondary

Transformer Equation

Vₛ/Vₚ = Nₛ/Nₚ = Iₚ/Iₛ

Where:

• Vₚ, Vₛ = Primary and secondary voltages
• Nₚ, Nₛ = Number of turns in primary and secondary
• Iₚ, Iₛ = Currents in primary and secondary

Ideal transformer (no losses): VₚIₚ = VₛIₛ (power conservation)

Types

1. Step-Up Transformer:

• Nₛ > Nₚ (more turns in secondary)
• Vₛ > Vₚ (voltage increases)
• Iₛ < Iₚ (current decreases)
• Used in power transmission

2. Step-Down Transformer:

• Nₛ < Nₚ (fewer turns in secondary)
• Vₛ < Vₚ (voltage decreases)
• Iₛ > Iₚ (current increases)
• Used in household supply (11 kV → 220 V)

Energy Losses in Transformers

1. Copper losses: I²R heating in coil wires (minimized by thick wires)
2. Iron losses: Hysteresis loss in core (minimized by soft iron)
3. Eddy current losses: Induced currents in core (minimized by lamination)
4. Flux leakage: Not all flux links both coils (minimized by core design)

Note: Transformers work only with AC, not DC (DC produces constant flux, no induction)

Domestic Electric Circuits

Power Supply System

From Power Station to Home:

1. Generation: Electricity generated at power stations (11 kV or 33 kV)
2. Transmission: Stepped up to 220 kV or 440 kV for long-distance transmission
3. Distribution: Stepped down to 11 kV at substations
4. Final step-down: Reduced to 220 V for household supply

Components of House Wiring

1. Service Mains (Overhead/Underground)

Three wires enter premises:

1. Live Wire (L):
   • Red or Brown insulation
   • Potential: 220 V relative to ground
   • Carries current from power station
2. Neutral Wire (N):
   • Black or Light Blue insulation
   • Potential: 0 V (earthed at supply end)
   • Completes circuit
3. Earth Wire (E):
   • Green or Green-Yellow insulation
   • Potential: 0 V (connected to earth)
   • Safety wire (conducts leakage current to ground)

2. Main Board (Outside Building)

Contains:

• Energy Meter: Records electrical energy consumed (in kWh)
• Main Switch: Double-pole switch for L and N wires
• Main Fuse: In series with live wire (safety device)

3. Distribution Inside Building

Parallel connection: All appliances connected in parallel across L and N

Advantages of parallel connection:

• Each appliance gets full 220 V
• Appliances work independently
• Failure of one doesn't affect others
• Different power appliances can be used simultaneously

Three-Pin Plug and Socket System

Three-Pin Socket

Pin arrangement:

1. Top (longest pin): Earth connection
2. Bottom left: Live connection
3. Bottom right: Neutral connection

Longer earth pin: Ensures earth connection made first (safety)

Three-Pin Plug

Color code (International standard):

• Brown wire → Live pin → Right hole
• Blue wire → Neutral pin → Left hole
• Green/Yellow wire → Earth pin → Top hole

Earthing: Purpose and Function

What is Earthing?

Connecting the metallic body of an appliance to the earth wire through the three-pin system.

Why is Earthing Necessary?

Problem without earthing:

• Insulation failure → Live wire touches metal body
• Metal body becomes live (220 V)
• Person touching body gets electric shock

Solution with earthing:

• Metal body connected to earth wire (0 V)
• Leakage current flows to earth through earth wire
• Body potential remains at 0 V
• No shock to user

Earth wire characteristics:

• Very low resistance path
• Diverts leakage current safely to ground
• Triggers fuse/MCB if leakage current is high

Safety Devices in Electrical Circuits

Electric Fuse

What is a Fuse?

A safety device consisting of a thin wire that melts when excess current flows, thereby breaking the circuit.

Construction

Material: Alloy of tin, lead, or copper

• Low melting point
• High resistivity
• Thin cross-section

Placement: In series with live wire only (near main switch or appliance)

Working Principle

Normal operation: Current < fuse rating → Fuse intact

Overload/short circuit: Current >> fuse rating → Excessive heat (I²R) → Fuse wire melts → Circuit breaks → Appliances protected

Fuse Rating

Fuse rating: Maximum current (in amperes) a fuse can carry continuously without melting

Example:

• 5 A fuse for appliances up to 1100 W
• 15 A fuse for heavy appliances like geysers

Selection: Fuse rating should be slightly higher than normal operating current of appliance

Miniature Circuit Breaker (MCB)

Modern alternative to fuse:

• Electromagnetic switch
• Automatically trips when current exceeds limit
• Can be reset (unlike fuse which must be replaced)
• Faster response time
• More reliable

Short Circuit and Overloading

Short Circuit

Definition

Direct contact between live and neutral wires, bypassing the load.

Causes

1. Damaged insulation exposing wires
2. Rodent damage to cables
3. Appliance internal fault
4. Loose connections

Consequences

• Resistance ≈ 0 → Current becomes extremely large (I = V/R)
• Massive heat generation (H = I²Rt)
• Fire hazard
• Sparks and equipment damage

Prevention

• Good quality insulation
• Regular wiring inspection
• Proper fuses/MCBs
• Keep wires away from sharp edges

Overloading

Definition

Exceeding the safe current limit of a circuit by connecting too many appliances simultaneously.

Example

• Circuit designed for 15 A (3300 W at 220 V)
• Connecting: AC (1500 W) + Geyser (2000 W) + Iron (1000 W) = 4500 W
• Current drawn: 4500/220 ≈ 20.5 A > 15 A limit

Consequences

• Wires heat up excessively
• Insulation melts
• Fire hazard
• Voltage drop (lights dim)

Prevention

• Distribute load across multiple circuits
• Avoid connecting heavy appliances on single socket
• Use appliances with appropriate rating
• Install adequate fuses/MCBs

Formulas: Quick Reference Table

Constants:

• μ₀ (Permeability of free space) = 4π × 10⁻⁷ T·m/A
• Household voltage (India) = 220 V
• Household frequency (India) = 50 Hz

Important Definitions:

Practical Applications in Daily Life

1. Electric Bell

Working:

• Electromagnet attracts iron armature when current flows
• Hammer strikes gong
• Circuit breaks, electromagnet demagnetizes
• Armature returns, circuit reconnects
• Rapid repetition creates ringing sound

2. Magnetic Crane

Application: Lifting heavy magnetic materials (scrap iron, steel)

Working:

• Large electromagnet suspended from crane
• Current switched on → Strong magnetic field → Attracts metal
• Moved to desired location
• Current switched off → Metal drops

3. MRI Machine

Application: Medical imaging (detailed body scans)

Principle:

• Very strong electromagnets (1.5 to 3 Tesla)
• Align hydrogen atoms in body
• Radio waves disturb alignment
• Return signal creates detailed images

4. Maglev Trains

Application: High-speed transportation

Principle:

• Magnetic levitation
• Electromagnets on train and track
• Repulsive forces lift train (no friction)
• Speeds over 500 km/h possible

5. Induction Cooktop

Principle: Electromagnetic induction

Working:

• AC through coil creates changing magnetic field
• Induces eddy currents in metal cookware
• Eddy currents heat cookware directly
• Efficient and fast cooking

Common Misconceptions and Clarifications

Misconception 1: "Magnetic field lines are real"

Clarification: Field lines are imaginary representations to visualize invisible magnetic fields. Only the magnetic force is real.

Misconception 2: "Isolated magnetic poles exist"

Clarification: Every magnet always has both poles. Cutting a magnet creates two new dipoles, never isolated poles.

Misconception 3: "Current flows from negative to positive"

Clarification:

• Conventional current: Positive to negative (used in diagrams)
• Electron flow: Negative to positive (actual electron motion)
• Both conventions work; we typically use conventional current

Misconception 4: "Earth's geographic north pole is magnetic north"

Clarification: Earth's magnetic south pole is near geographic north (that's why compass north points there opposite poles attract).

Misconception 5: "Transformer can work with DC"

Clarification: Transformers require changing magnetic flux. DC produces constant flux, so no induction occurs. Transformers work only with AC.

Misconception 6: "Earth wire carries current normally"

Clarification: Under normal conditions, earth wire carries zero current. It conducts current only during fault conditions (insulation failure).

Misconception 7: "Stronger current = stronger shock"

Clarification: While current intensity matters, voltage determines whether current can flow through body. Even small currents (>30 mA) through heart can be fatal. Safety depends on proper insulation and earthing, not just limiting current.

Tips for Examination

Quick Revision Points

1. Right-hand rules: Practice both thumb rules with different scenarios
2. Fleming's rules: Remember which hand for which (Left = motor/force, Right = generator/induced current)
3. Formulas: Memorize the formula table; understand when each applies
4. Diagrams: Practice drawing magnetic field patterns for different configurations
5. Units: B (Tesla), I (Ampere), F (Newton), V (Volt), W (Watt)

Common Diagram Questions

Practice drawing:

• Magnetic field around straight wire (concentric circles)
• Magnetic field around circular coil (straight at center)
• Magnetic field of solenoid (similar to bar magnet)
• Magnetic field of bar magnet
• DC motor labeled diagram
• AC generator labeled diagram
• Domestic circuit wiring diagram

Numerical Problem Strategy

1. Identify what is given and what is to be found
2. Select appropriate formula
3. Convert units if necessary (cm → m, mA → A)
4. Substitute values carefully
5. Calculate and write answer with correct units
6. Check if answer is reasonable

Conclusion

The magnetic effects of electric current form the foundation of modern electrical technology. From the simple compass deflection observed by Oersted to sophisticated MRI machines and maglev trains, the interplay between electricity and magnetism has revolutionized human civilization.

Important Notes:

1. Current produces magnetic field (Oersted) - basis of electromagnets and motors
2. Changing magnetic field induces current (Faraday) - basis of generators and transformers
3. Force on current in magnetic field (Fleming's Left-Hand Rule) - basis of motors and measuring instruments
4. Direction matters: Right-hand rules for field direction, Fleming's rules for force/current direction
5. Safety first: Proper earthing, fuses, and circuit design prevent electrical hazards

These principles not only helps in academic success but also develops awareness about the technology that powers our daily lives. Whether it's the generator producing electricity, the motor turning a fan, or the transformer stepping down voltage for safe household use all operate on these fundamental electromagnetic principles discovered centuries ago and still serving humanity today.

Other Resources

1. NCERT Science Textbook Class 10, Chapter 13
2. Electromagnetic Induction experiments (YouTube)
3. Practical demonstrations of Oersted's and Faraday's experiments
4. Interactive simulations (PhET, MERLOT)
5. Previous years' CBSE question papers
6. CBSE class 10 sample papers