⚡ ELECTROMAGNETIC EFFECTS ⚡

Complete Theory and Formulas - Grade 9 Physics

1. ELECTROMAGNETIC INDUCTION

📖 Definition

Electromagnetic Induction is the process through which an induced e.m.f. (electromotive force) is produced in a conductor due to a changing magnetic field.

Key Principle

Moving a magnet near a coil of wire (or moving the coil near a magnet) generates electricity. The magnetic field MUST be changing - no change means no induced e.m.f.

Faraday's Experiment:


    [Galvanometer]

          │

    ══════●══════  Solenoid (coil)

          │

     ┌────┴────┐

     │ N ─── S │  Bar Magnet

     └─────────┘

          ↓

      (Moving)

Faraday's Observations

  • Magnet moving IN → Galvanometer needle deflects in one direction
  • Magnet STATIONARY → No deflection (zero current)
  • Magnet moving OUT → Galvanometer needle deflects in opposite direction

Factors Affecting Induced E.M.F.

The magnitude of induced e.m.f. can be increased by:

  1. Increasing the number of turns in the coil
  2. Using a stronger magnet
  3. Moving the magnet faster relative to the coil
  4. Adding a soft iron core inside the coil

Laws of Electromagnetic Induction

Faraday's Law

"The magnitude of the induced e.m.f. in a circuit is directly proportional to the rate of change of magnetic flux in the circuit."

Induced e.m.f. ∝ Rate of change of magnetic flux

Lenz's Law

"The direction of the induced e.m.f., and hence the induced current in a closed circuit, is always such that its magnetic effect opposes the motion or change producing it."

Induced current opposes the change
💡 Important Note: Lenz's Law is a consequence of the law of conservation of energy. The induced current creates a magnetic field that opposes the original change, requiring work to be done, which is converted into electrical energy.

2. A.C. GENERATOR

📖 Definition

Alternating Current (A.C.) Generator is a device that converts mechanical energy into electrical energy using electromagnetic induction.

Main Components

  • Rectangular Coil (Armature): Rotates in the magnetic field
  • Permanent Magnets: Provide steady magnetic field (N and S poles)
  • Axle: Allows coil to rotate freely
  • Slip Rings: Two separate rings that rotate with the coil
  • Carbon Brushes: Press against slip rings to transfer current to external circuit

A.C. Generator Structure:


     Carbon Brush

          │

      Slip Ring ────┐

          │         │

    N ═══●═══●═══ S│

        Coil       │

          │         │

      Slip Ring ────┘

          │

     Carbon Brush

          │

        [Load]

How A.C. Generator Works

Position 1: Coil PARALLEL to field

• Coil sides cut through maximum magnetic field lines
- Maximum induced e.m.f. produced

Position 2: Coil PERPENDICULAR to field

• Coil sides move along field lines (not cutting)
- Zero induced e.m.f.

Position 3: Coil PARALLEL again (after 180° rotation)

• Maximum cutting of field lines
- Maximum e.m.f. but opposite direction

Output Voltage Graph


Voltage

   │

+Vmax │    ╱╲        ╱╲

   │   ╱  ╲      ╱  ╲

 0 ├──╱────╲────╱────╲─── Time

   │       ╲  ╱      ╲  ╱

-Vmax │        ╲╱        ╲╱

   

   ← One complete rotation →
📝 Note: The output is a sine wave, producing alternating current (a.c.) that changes direction periodically.

Fleming's Right-Hand Rule (Generator)

Use your RIGHT HAND to find the direction of induced current:


     Thumb = Motion (Force)

         ↑

         │

    ←────●────→

    │         │

First Finger  Second Finger

= Field       = Current

(N to S)      (Induced)
First finger = Field | Thumb = Motion | Second finger = Current

Increasing Induced E.M.F. in Generator

  1. Increase number of turns in coil
  2. Use stronger permanent magnets
  3. Increase rotation frequency (spin faster)
  4. Wind coil around soft iron core

3. MAGNETIC EFFECT OF CURRENT

📖 Key Principle

A current-carrying conductor produces a magnetic field around it. This was discovered by Hans Christian Oersted in 1820.

Magnetic Field Around Straight Wire

Pattern: Concentric Circles


View from above:

     ⊙ (current toward you)

    ╱│╲

   ╱ │ ╲  ← Circular

  │  │  │    magnetic field

   ╲ │ ╱

    ╲│╱

⊙ = current OUT of page

⊗ = current INTO page
💡 Key Points:
  • Field lines are closer together near the wire (stronger field)
  • Field strength decreases with distance from wire
  • Direction depends on current direction

Right-Hand Grip Rule

To find magnetic field direction around a straight wire:


    Thumb points in

    current direction

         ↑

         │

    ═════●═════  Wire

         

Fingers curl in

field direction
Thumb = Current direction | Fingers = Field direction

Magnetic Field Around Solenoid

Pattern: Like a Bar Magnet


  N ←═══════════════════════→ S

    ║║║║║║║║║║║║║║║║║║║║║

  ←═══════════════════════════→

    ║║║║║║║║║║║║║║║║║║║║║

  ←═══════════════════════→

  

  Strong uniform field INSIDE

  Weak field OUTSIDE

Factors Affecting Magnetic Field Strength

Factor Effect
Increase Current Stronger magnetic field
Reverse Current Reverse field direction
More Turns per Length Stronger field (solenoid)
Add Soft Iron Core Much stronger field (electromagnet)

Applications

A. Relay (Remote Switch)

• Small current activates electromagnet
- Electromagnet attracts iron lever
- Lever closes high-voltage circuit
- Safe control of dangerous circuits

B. Loudspeaker

• A.C. current through coil
- Coil becomes alternating electromagnet
- Attracted/repelled by permanent magnet
- Coil vibrates → Creates sound waves

4. FORCE ON CURRENT-CARRYING CONDUCTOR

📖 Motor Effect

When a current-carrying conductor is placed in a magnetic field, it experiences a force. This is called the motor effect.

Fleming's Left-Hand Rule (Motor)

Use your LEFT HAND to find the direction of force:


First Finger = Field (N→S)

        ↑

        │

   ←────●────→ Thumb = Force

        │

Second Finger = Current
First finger = Field | Second finger = Current | Thumb = Force
⚠️ Important: Use RIGHT hand for generators (finding current), LEFT hand for motors (finding force). Don't mix them up!

Why Does the Force Exist?

The magnetic field from the current-carrying wire combines with the external magnetic field:

  • On one side: fields reinforce (stronger)
  • On other side: fields oppose (weaker)
  • Wire is pushed from stronger field toward weaker field

Forces Between Parallel Current-Carrying Wires

Current Direction Force Diagram
Same Direction ATTRACT 
↑I    ↑I
│     │
●←───→●
Opposite Direction REPEL 
↑I    ↓I
│     │
●→───←●

Charged Particles in Magnetic Field

Positive Charge:

• Current direction = particle motion direction
- Use Fleming's left-hand rule
- Particle path curves

Negative Charge (Electron):

• Current direction = OPPOSITE to electron motion
- Use Fleming's left-hand rule (with opposite current)
- Particle curves in opposite direction to positive charge

Reverse Magnetic Field:

• Force direction reverses
- Particle curves in opposite direction

5. D.C. MOTOR

📖 Definition

Direct Current (D.C.) Motor is a device that converts electrical energy into mechanical energy (rotational motion).

Main Components

  • Rectangular Coil: Mounted on axle, can rotate
  • Permanent Magnets: Provide steady magnetic field
  • Split-Ring Commutator: ONE ring split into TWO halves (X and Y)
  • Carbon Brushes: Press on commutator, transfer current
  • D.C. Source: Battery or power supply

D.C. Motor Structure:


    Carbon Brush

         │

    ╔════╗════╗ Split-ring

    ║ X  │  Y ║ Commutator

    ╚════╩════╝

         │  │

   N  ═══●══●═══  S

       A║  ║D

        B══C

         │

     [Battery]

       + -

How D.C. Motor Works

Stage 1: Horizontal Position


Force ↑           Force ↓

    │               │

  ══A═══════════D══

    ║           ║

    B═══════════C

• Current: A→B→C→D
- Force on AB: UPWARD
- Force on CD: DOWNWARD
- Coil rotates anticlockwise

Stage 2: Vertical Position


        A

        ║

   N    ║    S

        ║

        D

• Commutator loses contact with brushes
- Current momentarily cut off
- Momentum carries coil through

Stage 3: Past Vertical


Force ↓           Force ↑

    │               │

  ══D═══════════A══

    ║           ║

    C═══════════B

• Commutator switches connection
- Current REVERSED in coil
- But coil already rotated 180°
- Forces maintain same rotation direction
- Continues rotating anticlockwise

💡 Key Function of Split-Ring Commutator: Reverses current direction in the coil every half rotation, ensuring continuous rotation in ONE direction.

Increasing Turning Effect

  1. Insert soft iron core inside coil
  2. Increase number of turns in coil
  3. Increase current in coil
  4. Use stronger permanent magnets

Reversing Motor Direction

Method Action
Method 1 Reverse CURRENT (swap battery terminals)
Method 2 Reverse MAGNETIC FIELD (flip magnets)
⚠️ Warning: If you reverse BOTH current AND field, the motor will still rotate in the SAME direction!

6. TRANSFORMER

📖 Definition

Transformer is a device that changes (transforms) alternating voltage from one level to another.

⚠️ Critical: Transformers work ONLY with A.C. (alternating current), NOT with D.C. (direct current)!

Main Components

Transformer Structure:


Primary Coil      Secondary Coil

  (Np turns)         (Ns turns)

     │││               │││

  ───●●●───[IRON]───●●●───

   Vp       CORE       Vs

  Input              Output
  • Primary Coil: Connected to input voltage (Vp), has Np turns
  • Secondary Coil: Produces output voltage (Vs), has Ns turns
  • Laminated Soft Iron Core: Links magnetic fields between coils

How Transformer Works

  1. A.C. flows through primary coil
  2. Creates changing magnetic field in iron core
  3. Changing field passes through secondary coil
  4. Induces e.m.f. in secondary coil (Faraday's Law)
  5. Output voltage produced
💡 Why A.C. only?
- D.C. = constant current = constant field = no change = no induced e.m.f. ✗
- A.C. = changing current = changing field = induced e.m.f. ✓

Transformer Equations

Main Formula: Voltage Ratio

Vp / Vs = Np / Ns

Where:
Vp = Primary (input) voltage (V)
Vs = Secondary (output) voltage (V)
Np = Number of primary turns
Ns = Number of secondary turns

For 100% Efficient Transformer

Ip × Vp = Is × Vs

Power in = Power out

Where:
Ip = Primary current (A)
Is = Secondary current (A)

Combined Equation

Vp / Vs = Np / Ns = Is / Ip

Notice: Current ratio is INVERTED!

Transformer Efficiency

Efficiency = (Is × Vs) / (Ip × Vp) × 100%
Efficiency = (Output Power) / (Input Power) × 100%

Typical efficiency: 95-99%

Types of Transformers

Type Turns Voltage Current
Step-UP Ns > Np Vs > Vp (increases) Is < Ip (decreases)
Step-DOWN Ns < Np Vs < Vp (decreases) Is > Ip (increases)

Worked Example

Problem:

A transformer has 1000 turns in primary, 100 turns in secondary. Primary voltage is 240 V. Find secondary voltage.

Solution:

Given: Np = 1000, Ns = 100, Vp = 240 V

Using: Vp/Vs = Np/Ns

240/Vs = 1000/100
Vs = (240 × 100) / 1000 = 24 V

Answer: 24 V (Step-down transformer)

High Voltage Transmission

Power Loss in Cables

Ploss = I² × R

Where:
Ploss = Power lost as heat (W)
I = Current in cables (A)
R = Resistance of cables (Ω)

Current in Transmission

I = Pout / V

Therefore:

Ploss = (Pout / V)² × R
💡 Key Insight: Power loss is inversely proportional to voltage squared!
- Double voltage → Current halves → Power loss becomes 1/4
- 10× voltage → Current becomes 1/10 → Power loss becomes 1/100

Power Grid System


Power      Step-Up      Transmission    Step-Down    Homes

Station    Transform    Lines           Transform    

25,000V    ────────→    400,000V    ────────→        240V

  │            │            │              │           │

[═══]──→[Transformer]──→[~~~~~]──→[Transformer]──→[House]

         Increase V                     Decrease V

Advantages of High Voltage Transmission

  1. Reduced Power Loss: Lower current → Less I²R heating
  2. Lower Cost: Can use thinner cables (less copper)
  3. More Efficient: Less energy wasted

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