Class X Physics - Chapter 4
Magnetic Effects of Electric Current
4.1 Introduction to Magnetism
Magnetism: The property of certain materials to attract iron, nickel, cobalt and their alloys is called magnetism.
Magnet: A substance that possesses the property of magnetism is called a magnet.
Properties of Magnets:
- Every magnet has two poles - North pole (N) and South pole (S)
- Like poles repel each other, unlike poles attract each other
- Magnetic poles always exist in pairs
- The force between magnetic poles follows inverse square law
- A freely suspended magnet always aligns in North-South direction
Types of Magnets:
| Type |
Origin |
Examples |
| Natural Magnets |
Found in nature |
Magnetite (Fe₃O₄) |
| Artificial Magnets |
Made by humans |
Bar magnet, Horseshoe magnet |
| Permanent Magnets |
Retain magnetism |
Steel magnets |
| Temporary Magnets |
Lose magnetism easily |
Soft iron magnets |
Example: When you bring two bar magnets close to each other, if you align N-S poles, they attract. If you align N-N or S-S poles, they repel each other.
4.2 Magnetic Field
Magnetic Field: The region around a magnet where its magnetic force can be experienced by another magnet or magnetic material is called magnetic field.
Magnetic Field Lines: Imaginary lines that represent the magnetic field around a magnet are called magnetic field lines or magnetic lines of force.
Properties of Magnetic Field Lines:
- They emerge from North pole and enter into South pole
- They form closed loops
- They never intersect each other
- Closer the lines, stronger the magnetic field
- They are parallel and equidistant in uniform field
- Direction at any point is given by tangent to the field line
Magnetic Field Strength:
Magnetic Field Strength (B): The force experienced by a unit north pole placed at that point in the magnetic field.
SI Unit: Tesla (T) or Weber/m²
CGS Unit: Gauss (G)
Relation: 1 Tesla = 10⁴ Gauss
Magnetic Field Lines of Bar Magnet
╭─────────────╮
╱ ╲
╱ N S ╲
╱ ╲
╱_____________________╲
╲ ╱
╲ ╱
╲ ╱
╲_______________╱
Field lines emerge from N and enter S
MCQ 1: Magnetic field lines emerge from:
(A) South pole
(B) North pole
(C) Both poles
(D) Neither pole
Answer: (B) North pole
4.3 Magnetic Effect of Electric Current
Oersted's Discovery: Hans Christian Oersted discovered that electric current produces magnetic field. A current-carrying conductor behaves like a magnet.
Magnetic Field due to Straight Current-Carrying Conductor:
- Magnetic field lines are concentric circles around the conductor
- Direction given by Right Hand Thumb Rule
- Field strength decreases with distance from conductor
- Field strength increases with increase in current
Right Hand Thumb Rule:
Right Hand Thumb Rule: If you hold a current-carrying straight conductor in your right hand such that thumb points in direction of current, then fingers encircle the conductor in the direction of magnetic field lines.
Magnetic Field around Straight Conductor
⊗ ← Current going into page
╱ ╲
╱ ╲
╱ ⊗ ╲ ← Circular field lines
╱ ╲
╱_________╲
⊙ ← Current coming out of page
Mathematical Expression:
B = (μ₀I)/(2πr)
where B = magnetic field, I = current, r = distance, μ₀ = permeability of free space
Example: When electric current flows through a wire near a compass needle, the needle deflects, showing that current produces magnetic field.
MCQ 2: The magnetic field around a straight current-carrying conductor forms:
(A) Straight lines
(B) Concentric circles
(C) Ellipses
(D) Parabolas
Answer: (B) Concentric circles
4.4 Magnetic Field due to Circular Current Loop
Circular Current Loop: When current flows through a circular conductor, it produces magnetic field with specific pattern.
Characteristics:
- Field lines are circular near the wire
- At the center, field lines are straight
- One face acts as North pole, other as South pole
- Field is stronger at the center
Magnetic Field at Center:
B = (μ₀I)/(2R)
where R = radius of circular loop
Right Hand Rule for Circular Loop:
Right Hand Rule: If you curl the fingers of right hand in the direction of current flow in the loop, the thumb points in the direction of magnetic field (North pole).
Magnetic Field of Circular Loop
N ←─── Thumb direction
↑
╭─────┴─────╮
╱ ╲
╱ ⊙ I ╲ ← Current loop
╱ ╲
╱___________________╲
↓
S
⊙ = Current coming out ⊗ = Current going in
Example: A single circular loop carrying 5 A current with radius 0.1 m produces magnetic field B = (4π × 10⁻⁷ × 5)/(2 × 0.1) = π × 10⁻⁵ T at its center.
MCQ 3: At the center of a circular current loop, magnetic field lines are:
(A) Circular
(B) Straight
(C) Curved
(D) Elliptical
Answer: (B) Straight
4.5 Solenoid
Solenoid: A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called solenoid.
Magnetic Field of Solenoid:
- Inside solenoid: uniform magnetic field parallel to axis
- Outside solenoid: field is negligible
- Ends behave like poles of bar magnet
- Field strength depends on current and number of turns
Magnetic Field Inside Solenoid:
B = μ₀nI
where n = number of turns per unit length, I = current
Polarity of Solenoid:
Right Hand Rule for Solenoid: If you curl the fingers of right hand in the direction of current flow, the thumb points towards the North pole of the solenoid.
Magnetic Field of Solenoid
N ←─────────────────────→ S
╭─○○○○○○○○○○○○○○○○○○○○○─╮
╱ ║║║║║║║║║║║║║║║║║║║║║ ╲
╱ ║║║║║║║║║║║║║║║║║║║║║ ╲
╱____║║║║║║║║║║║║║║║║║║║║║____╲
Uniform field inside
Current direction determines polarity
Applications of Solenoid:
- Electromagnets: Temporary strong magnets
- Electric bells: Attraction mechanism
- Relays: Switching circuits
- Motors and generators: Field generation
Example: A solenoid with 1000 turns per meter carrying 2 A current produces magnetic field B = 4π × 10⁻⁷ × 1000 × 2 = 2.51 × 10⁻³ T inside it.
MCQ 4: Inside a solenoid, the magnetic field is:
(A) Zero
(B) Non-uniform
(C) Uniform
(D) Circular
Answer: (C) Uniform
4.6 Force on Current-Carrying Conductor
Motor Effect: When a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force. This is called motor effect.
Fleming's Left Hand Rule:
Fleming's Left Hand Rule: Stretch the thumb, forefinger and middle finger of left hand mutually perpendicular to each other. If forefinger points in direction of magnetic field, middle finger in direction of current, then thumb points in direction of force.
Fleming's Left Hand Rule
Thumb → Force (F)
↑
│
│
───────┼─────── Forefinger → Field (B)
│
│
↓
Middle finger → Current (I)
Mathematical Expression:
F = BIL sin θ
where F = force, B = magnetic field, I = current, L = length, θ = angle
Special Cases:
• When θ = 90° (perpendicular): F = BIL (maximum)
• When θ = 0° (parallel): F = 0 (minimum)
Unit of Force:
SI Unit: Newton (N)
Relation: 1 N = 1 kg⋅m/s²
Example: A conductor of length 0.5 m carrying 3 A current in magnetic field of 0.2 T experiences force F = 0.2 × 3 × 0.5 = 0.3 N (when perpendicular).
MCQ 5: The force on a current-carrying conductor in magnetic field is maximum when:
(A) Current is parallel to field
(B) Current is perpendicular to field
(C) Current is at 45° to field
(D) No current flows
Answer: (B) Current is perpendicular to field
4.7 Electric Motor
Electric Motor: A device that converts electrical energy into mechanical energy based on the motor effect is called electric motor.
Principle:
When a current-carrying coil is placed in a magnetic field, it experiences a force and rotates. This rotation can be used to do mechanical work.
Construction:
| Part |
Function |
| Armature |
Current-carrying coil that rotates |
| Field Magnet |
Provides magnetic field |
| Commutator |
Reverses current direction |
| Brushes |
Maintain electrical contact |
| Axle |
Transfers mechanical energy |
Working:
Step-by-step Working:
1. Current flows through armature coil
2. Coil experiences force due to magnetic field
3. Force causes rotation of coil
4. Commutator reverses current every half rotation
5. Continuous rotation is maintained
Role of Commutator:
Commutator: A device that reverses the direction of current through the armature every half rotation, ensuring continuous rotation in the same direction.
Types of Motors:
- DC Motors: Work on direct current
- AC Motors: Work on alternating current
- Universal Motors: Work on both AC and DC
Applications:
- Electric fans and pumps
- Washing machines and mixers
- Electric vehicles and trains
- Computer hard drives
MCQ 6: The function of commutator in DC motor is to:
(A) Increase current
(B) Reverse current direction
(C) Reduce resistance
(D) Increase speed
Answer: (B) Reverse current direction
4.8 Electromagnetic Induction
Electromagnetic Induction: The phenomenon of producing electrical energy from mechanical energy using magnetism is called electromagnetic induction.
Faraday's Discovery:
Michael Faraday discovered that when magnetic flux through a coil changes, an EMF (electromotive force) is induced in the coil.
Conditions for Electromagnetic Induction:
- Relative motion between magnet and coil
- Change in magnetic field strength
- Change in area of coil in magnetic field
- Change in orientation of coil in magnetic field
Faraday's Laws:
First Law: Whenever magnetic flux linked with a coil changes, an EMF is induced in the coil.
Second Law: The magnitude of induced EMF is directly proportional to the rate of change of magnetic flux.
EMF = -dΦ/dt
where Φ = magnetic flux = B⋅A⋅cos θ
Lenz's Law:
Lenz's Law: The direction of induced current is such that it opposes the change that caused it. (Negative sign in Faraday's law)
Example: When north pole of magnet approaches a coil, induced current creates magnetic field opposing the approach (acting like north pole facing the magnet).
MCQ 7: Electromagnetic induction was discovered by:
(A) Oersted
(B) Faraday
(C) Fleming
(D) Maxwell
Answer: (B) Faraday
4.9 Electric Generator
Electric Generator: A device that converts mechanical energy into electrical energy based on electromagnetic induction is called electric generator.
Principle:
When a coil rotates in a magnetic field, the magnetic flux through it changes continuously, inducing EMF and current in the coil.
Construction:
| Part |
Function |
| Armature |
Rotating coil in which EMF is induced |
| Field Magnet |
Provides magnetic field |
| Slip Rings (AC) |
Maintain electrical contact for AC |
| Commutator (DC) |
Converts AC to DC |
| Brushes |
Collect current from rotating parts |
Types of Generators:
1. AC Generator (Alternator):
- Uses slip rings
- Produces alternating current
- Current changes direction periodically
- Used in power stations
2. DC Generator:
- Uses commutator
- Produces direct current
- Current flows in one direction
- Used for battery charging
Fleming's Right Hand Rule:
Fleming's Right Hand Rule: For generators - Thumb (motion), Forefinger (field), Middle finger (induced current).
AC Generator Working
N ────────────── S
│ ╭─────╮ │
│ ╱ ╲ │
│ ╱ ● ╲ │ ← Rotating coil
│╱ ╲ │
│╲ ╱ │
│ ╲ ● ╱ │
│ ╲ ╱ │
│ ╰─────╯ │
│ │
EMF varies as coil rotates
Example: In hydroelectric power plants, flowing water rotates turbines connected to generators, converting kinetic energy to electrical energy.
MCQ 8: AC generator uses:
(A) Commutator
(B) Slip rings
(C) Both A and B
(D) Neither A nor B
Answer: (B) Slip rings
4.10 Domestic Electric Circuits
Domestic Electric Circuit: The electrical wiring system in houses that supplies electricity to various appliances is called domestic electric circuit.
Main Components:
| Component |
Function |
Specifications |
| Supply Lines |
Bring electricity from grid |
230 V, 50 Hz in India |
| Live Wire |
Carries current to appliances |
Red/Brown insulation |
| Neutral Wire |
Completes the circuit |
Black/Blue insulation |
| Earth Wire |
Safety grounding |
Green insulation |
| Main Switch |
Controls entire house supply |
Double pole switch |
| Distribution Box |
Distributes power to circuits |
Contains MCBs/fuses |
Wire Color Coding:
- Live Wire: Red or Brown (Hot wire)
- Neutral Wire: Black or Blue (Return path)
- Earth Wire: Green or Green-Yellow (Safety)
Electrical Supply in India:
Standard Supply:
• Voltage: 230 V (between live and neutral)
• Frequency: 50 Hz
• Type: Single phase AC for homes
• Three phase: 440 V for industries
Circuit Protection Devices:
1. Fuse:
- Contains fusible wire that melts on overcurrent
- Connected in series with live wire
- Ratings: 5A, 15A, 30A etc.
- Once blown, needs replacement
2. MCB (Miniature Circuit Breaker):
- Automatic switch that trips on overcurrent
- Can be reset after tripping
- More convenient than fuse
- Faster response time
Domestic Circuit Layout
Electric Meter
│
│
Main Switch (DP)
│
│
┌─────┴─────┐
│Distribution│
│ Box │
└─┬───┬───┬─┘
│ │ │
MCB MCB MCB ← Circuit breakers
│ │ │
Light Fan Socket ← Different circuits
Earthing System:
Earthing: Connecting the metal body of electrical appliances to the earth through earth wire for safety is called earthing.
Purpose of Earthing:
- Prevents electric shock
- Protects appliances from damage
- Provides path for leakage current
- Maintains appliance body at zero potential
Example: In a house with 230V supply, if a 100W bulb operates for 5 hours daily, monthly energy consumption = 0.1 kW × 5 h × 30 days = 15 kWh.
MCQ 9: The earth wire is connected to:
(A) Live wire
(B) Neutral wire
(C) Metal body of appliance
(D) Main switch
Answer: (C) Metal body of appliance
4.11 Electrical Safety
Common Electrical Hazards:
| Hazard |
Cause |
Prevention |
| Electric Shock |
Contact with live wire |
Proper earthing, insulation |
| Short Circuit |
Live and neutral touch |
Good insulation, MCB |
| Overloading |
Excess current draw |
Proper fuse rating |
| Fire |
Overheating of wires |
Correct wire gauge |
Safety Measures:
- Always switch off mains before working on circuits
- Use proper tools with insulated handles
- Never touch electrical appliances with wet hands
- Install ELCB (Earth Leakage Circuit Breaker)
- Regular checking of electrical connections
- Use ISI marked electrical goods
Short Circuit:
Short Circuit: Direct contact between live and neutral wires, causing large current flow and potential fire hazard.
Overloading:
Overloading: Drawing more current than the safe capacity of wires, causing overheating and fire risk.
Example: If a 5A fuse is used in a circuit drawing 8A current, the fuse will blow to protect the circuit from damage.
MCQ 10: Short circuit occurs when:
(A) Live and neutral wires touch
(B) Appliance is switched off
(C) Earth wire is connected
(D) MCB trips
Answer: (A) Live and neutral wires touch
4.12 Important Formulas Summary
Magnetic Field:
Straight conductor: B = μ₀I/(2πr)
Circular loop (center): B = μ₀I/(2R)
Solenoid: B = μ₀nI
where μ₀ = 4π × 10⁻⁷ H/m
Force and Motion:
Force on conductor: F = BIL sin θ
EMF in generator: EMF = BLv sin θ
Induced EMF: EMF = -dΦ/dt
Power Relations:
Power: P = VI = I²R = V²/R
Energy: E = Pt = VIt
Domestic supply: V = 230V, f = 50Hz
Constants and Units:
| Quantity |
Symbol |
SI Unit |
Value/Note |
| Magnetic Field |
B |
Tesla (T) |
1 T = 10⁴ Gauss |
| Permeability |
μ₀ |
H/m |
4π × 10⁻⁷ H/m |
| Force |
F |
Newton (N) |
kg⋅m/s² |
| EMF |
ε |
Volt (V) |
J/C |
4.13 Practical Applications
Electromagnetic Devices:
| Device |
Principle |
Application |
| Electric Bell |
Electromagnet attraction |
Door bells, alarms |
| Relay |
Electromagnetic switching |
Remote control circuits |
| Loudspeaker |
Force on current in magnetic field |
Audio systems |
| Microphone |
Electromagnetic induction |
Sound recording |
| Transformer |
Mutual induction |
Voltage conversion |
Power Generation:
- Thermal Power Plants: Steam turbines drive generators
- Hydroelectric Plants: Water turbines drive generators
- Wind Power: Wind turbines drive generators
- Nuclear Power: Steam from nuclear heat drives generators
Transportation:
- Electric trains: Use motors for propulsion
- Maglev trains: Use magnetic levitation
- Electric cars: Use battery-powered motors
Example: In an electric train, pantograph collects current from overhead wire, motors convert electrical energy to mechanical energy for motion.
Practice Questions
MCQ 11: The magnetic field inside a solenoid is:
(A) Zero
(B) Uniform
(C) Variable
(D) Circular
Answer: (B) Uniform
MCQ 12: Fleming's left hand rule is used to find:
(A) Direction of induced current
(B) Direction of magnetic field
(C) Direction of force on conductor
(D) Direction of motion
Answer: (C) Direction of force on conductor
MCQ 13: In India, domestic electric supply is:
(A) 110 V, 60 Hz
(B) 230 V, 50 Hz
(C) 440 V, 50 Hz
(D) 230 V, 60 Hz
Answer: (B) 230 V, 50 Hz
MCQ 14: The function of brushes in motor is to:
(A) Provide magnetic field
(B) Maintain electrical contact
(C) Support the coil
(D) Reverse current
Answer: (B) Maintain electrical contact
MCQ 15: Lenz's law is related to:
(A) Conservation of energy
(B) Conservation of charge
(C) Conservation of momentum
(D) Conservation of mass
Answer: (A) Conservation of energy
MCQ 16: The device used to convert AC to DC is:
(A) Transformer
(B) Commutator
(C) Slip rings
(D) Generator
Answer: (B) Commutator
MCQ 17: The color of live wire in domestic circuit is:
(A) Green
(B) Blue
(C) Red
(D) Black
Answer: (C) Red
MCQ 18: Electromagnetic induction is used in:
(A) Electric motor
(B) Electric generator
(C) Electric heater
(D) Electric bulb
Answer: (B) Electric generator
MCQ 19: The magnetic field due to current in straight wire is:
(A) Parallel to wire
(B) Perpendicular to wire
(C) Circular around wire
(D) Radial from wire
Answer: (C) Circular around wire
MCQ 20: Which device works on heating effect of current?
(A) Electric motor
(B) Electric fuse
(C) Electric generator
(D) Galvanometer
Answer: (B) Electric fuse
4.14 Important Points to Remember
Key Concepts:
- Current produces magnetic field (Oersted's discovery)
- Moving magnet induces current (Faraday's discovery)
- Motors convert electrical to mechanical energy
- Generators convert mechanical to electrical energy
- Proper earthing prevents electric shock
Right Hand Rules:
- Thumb rule: For magnetic field around straight conductor
- Curl rule: For polarity of solenoid
- Fleming's rules: For force and induced current directions
Safety in Electricity:
- Never work on live circuits
- Use proper protective equipment
- Ensure proper earthing of appliances
- Install appropriate fuses/MCBs
- Regular maintenance of electrical systems
Practical Applications:
- Electric motors in fans, pumps, vehicles
- Generators in power plants
- Transformers for voltage conversion
- Electromagnetic relays in control circuits
- Solenoids in automatic systems
End of Chapter 4
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