Electromagnetic Effects
⚡ ELECTROMAGNETIC EFFECTS ⚡
š Section 18.1: Electromagnetic Induction
Definition
Electromagnetic Induction is the process by which an induced electromotive force (e.m.f.) is produced in a conductor due to a changing magnetic field.
Faraday's Discovery (1831)
Michael Faraday discovered that a changing magnetic field produces an induced current in a conductor. This was the opposite of what was already known - that current produces a magnetic field.
Faraday's Experiment
Observations from Faraday's Experiment
| Action | Galvanometer Reading |
|---|---|
| Magnet moves INTO coil | Needle deflects in ONE direction |
| Magnet is STATIONARY inside coil | NO deflection (zero) |
| Magnet moves OUT OF coil | Needle deflects in OPPOSITE direction |
⚠️ Important Conclusion: An e.m.f. is induced ONLY when there is relative movement between the magnet and the solenoid. No movement = No induced e.m.f.
Factors Affecting Induced E.M.F.
The magnitude of induced e.m.f. can be increased by:
- Increasing the number of turns in the solenoid
- Using a stronger magnet
- Increasing the speed of relative motion between magnet and coil
The Two Laws of Electromagnetic Induction
Faraday's Law of Induction
Īµ = -N (dĪ¦/dt)
The magnitude of induced e.m.f. is directly proportional to the rate of change of magnetic flux.
Where:
Īµ = induced e.m.f. (V)
N = number of turns in coil
Ī¦ = magnetic flux (Wb)
t = time (s)
Lenz's Law
The direction of the induced e.m.f. (and hence induced current) is always such that its magnetic effect opposes the motion or change producing it.
"Nature opposes change!"
Understanding Lenz's Law
š” Conservation of Energy: Lenz's Law is connected to the conservation of energy. If the induced current helped the motion instead of opposing it, we could create a perpetual motion machine - which is impossible!
š Section 18.2: The A.C. Generator
Definition
A.C. Generator (Alternator): A device that transforms mechanical energy (motion) into electrical energy using electromagnetic induction.
Structure of A.C. Generator
Main Components
- Rectangular coil (Armature): Wire wound in rectangular shape
- Permanent magnets: Provide magnetic field (N and S poles)
- Axle: Allows the coil to rotate
- Slip rings: Two metal rings attached to coil ends
- Carbon brushes: Press against slip rings to transfer current
- External circuit: Where generated electricity flows
How A.C. Generator Works
- The coil is rotated (by turning the handle)
- As coil rotates, wires cut through magnetic field lines
- This creates changing magnetic flux
- By Faraday's Law, e.m.f. is induced
- Induced current flows through slip rings and brushes
- The lamp lights up!
Fleming's Right-Hand Rule (for Generators)
Fleming's Right-Hand Rule
How to use:
- Point THUMB in direction of wire's motion
- Point FOREFINGER in direction of magnetic field (N → S)
- Your MIDDLE FINGER shows direction of induced current
Output Voltage Graph
Five Key Positions
| Position | Coil Orientation | Induced E.M.F. |
|---|---|---|
| 1 | Parallel to field | Maximum (arms cut field lines fastest) |
| 2 | Perpendicular to field | Zero (arms don't cut field lines) |
| 3 | Parallel again | Maximum (opposite direction) |
| 4 | Perpendicular again | Zero |
| 5 | Back to start | Cycle repeats |
Increasing Output E.M.F.
- Increase the number of turns in coil (double turns → double voltage)
- Use stronger permanent magnets
- Increase the rotation frequency (spin faster)
- Add a soft iron core inside coil
š Section 18.3: Magnetic Effect of a Current
Oersted's Discovery (1820)
Key Discovery: A current-carrying conductor produces a magnetic field around it.
Danish professor Hans Christian Oersted discovered that when current flows through a wire, it creates a magnetic field that can deflect a compass needle.
Magnetic Field Pattern Around a Straight Wire
⚠️ Important Symbols:
- ⊙ (dot) = Current flowing OUT of page (towards you)
- ⊗ (cross) = Current flowing INTO page (away from you)
Right-Hand Grip Rule
Right-Hand Grip Rule (for current-carrying wire)
Steps:
- GRIP the wire with your right hand
- THUMB points in direction of current flow
- FINGERS curl in direction of magnetic field
Magnetic Field of a Solenoid
Key Observation: A solenoid (coil of wire) carrying current behaves like a bar magnet with North and South poles!
Factors Affecting Solenoid's Magnetic Field
- Increase the current flowing through it
- Increase the number of turns per unit length
- Insert a soft iron core inside (concentrates field lines)
Application: Electromagnetic Relay
š” How Relay Works:
- Close switch in primary (low voltage) circuit
- Electromagnet activates
- Iron lever is attracted to electromagnet
- Moveable contact touches fixed contact
- Secondary (high voltage) circuit completes
- Motor runs!
Advantage: You can safely control a high-power device using a low-voltage switch!
š Section 18.4: Force on a Current-Carrying Conductor
The Motor Effect
Motor Effect: When a current-carrying conductor is placed in a magnetic field, it experiences a force.
Fleming's Left-Hand Rule (for Motors)
Fleming's Left-Hand Rule
How to use:
- Point FOREFINGER in direction of magnetic field (N → S)
- Point MIDDLE FINGER in direction of current
- THUMB shows direction of force (motion)
All three are at RIGHT ANGLES to each other!
Reversing the Force
⚠️ The force direction is REVERSED when you reverse either:
- The current direction, OR
- The magnetic field direction
Reversing BOTH: Force stays in SAME direction!
Forces Between Parallel Wires
| Current Direction | Result | Diagram |
|---|---|---|
| Same direction (both ⊙⊙) | ATTRACT each other | ← Wire A | Wire B → |
| Opposite direction (⊙⊗) | REPEL each other | → Wire A | Wire B ← |
Force on Charged Particles in Magnetic Field
š” Key Points:
- Since current = moving charges, magnetic fields also affect individual charged particles
- A magnetic field can only exert force on a MOVING charge, not stationary
- Positive charges: Use current direction as given
- Negative charges (electrons): Current direction is OPPOSITE to electron motion
š Section 18.5: The D.C. Motor
Definition
D.C. Motor: A device that converts electrical energy (from direct current) into mechanical energy (motion/rotation).
Energy Conversion: Electrical Energy → Mechanical Energy
Structure of D.C. Motor
Main Components
- Rectangular coil (ABCD): Mounted on an axle
- Permanent magnets: Provide the magnetic field (N and S poles)
- Axle (PQ): Allows rotation
- Split-ring commutator (XY): Two half-rings that rotate with coil
- Carbon brushes: Press against commutator
- Battery: Provides current
How D.C. Motor Works
⚠️ Critical Function of Split-Ring Commutator:
The commutator reverses the current direction in the coil every half rotation. This ensures the forces always act in the same rotational direction, producing continuous rotation instead of oscillation.
Increasing Motor Power
- Insert soft iron core inside coil (concentrates magnetic field)
- Increase number of turns in coil
- Increase the current (larger forces)
- Use stronger magnets
- Use curved pole pieces for uniform field
š Section 18.6: The Transformer
Definition
Transformer: A device that changes the voltage of an alternating current (a.c.).
⚠️ Important: Transformers work ONLY with A.C., NOT with D.C.!
Types of Transformers
| Type | Function | Turns Ratio | Effect |
|---|---|---|---|
| Step-Up | Increases voltage | Ns > Np | Vs > Vp, Is < Ip |
| Step-Down | Decreases voltage | Ns < Np | Vs < Vp, Is > Ip |
Structure of a Transformer
Main Components
- Primary coil (Np turns): Connected to input voltage
- Secondary coil (Ns turns): Connected to output/load
- Laminated soft iron core: Links the two coils magnetically
How a Transformer Works
- Alternating voltage (Vp) applied to primary coil
- Alternating current flows in primary coil
- Varying magnetic field created in iron core
- Varying field links secondary coil through iron core
- Changing magnetic flux induces e.m.f. in secondary coil
- Induced e.m.f. (Vs) drives current through load
⚠️ Why Transformers Don't Work with D.C.:
D.C. produces a constant (steady) magnetic field. Since there's no change in magnetic flux, there's NO induced e.m.f. in the secondary coil. Transformers require a VARYING magnetic field, which only A.C. can provide!
Transformer Equations
Voltage-Turns Ratio
Vp / Vs = Np / Ns
Where:
Vp = Primary voltage (V)
Vs = Secondary voltage (V)
Np = Number of turns in primary coil
Ns = Number of turns in secondary coil
Power Equation (Ideal Transformer)
Ip × Vp = Is × Vs
Power Input = Power Output
(No energy loss)
Where:
Ip = Primary current (A)
Is = Secondary current (A)
Combined Equation
Vp / Vs = Np / Ns = Is / Ip
Notice: Current ratio is INVERTED!
Efficiency
Efficiency = (Is × Vs) / (Ip × Vp) × 100%
Efficiency = (Output Power / Input Power) × 100%
Typical efficiency: 95-99%
Worked Example
Problem: A transformer has 1000 turns in primary, 50 turns in secondary. Primary voltage is 240 V with 2 mA current. Find:
a) Secondary voltage
b) Secondary current (ideal transformer)
c) Type of transformer
Solution:
a) Using Vp/Vs = Np/Ns:
Vs = Vp × (Ns/Np) = 240 V × (50/1000) = 240 V × 0.05 = 12 V
b) Using Ip × Vp = Is × Vs:
Is = (Ip × Vp) / Vs = (2 mA × 240 V) / 12 V = 480 / 12 = 40 mA
c) Type:
Since Ns < Np and Vs < Vp, this is a STEP-DOWN transformer
Transformers in Power Transmission
The Problem: When electricity travels through cables, power is lost as heat according to the formula:
Ploss = I² R
The Solution: Transmit electricity at HIGH VOLTAGE (which means LOW CURRENT) to minimize I²R losses!
Why High Voltage Transmission?
š” Mathematical Explanation:
For the same power transmitted: P = V × I
Therefore: I = P / V
If voltage is 10× higher → current is 10× lower
Power loss = I²R becomes (1/10)² = 1/100 → 100× smaller loss!
Complete Transmission System Values
| Stage | Typical Voltage | Purpose |
|---|---|---|
| Power Station | 25 kV | Generation |
| After Step-Up | 400 kV | Long-distance transmission |
| Local Distribution | 33 kV | Regional distribution |
| After Step-Down | 240 V | Household use |
Advantages of High-Voltage Transmission
- Lower power losses - more efficient (less energy wasted as heat)
- Lower cable costs - can use thinner cables
- Lower construction costs - lighter support structures needed
- More economical - saves money over time
š KEY FORMULAS SUMMARY
| Topic | Formula | Description |
|---|---|---|
| Faraday's Law | Īµ = -N(dĪ¦/dt) | Induced e.m.f. proportional to rate of change of flux |
| Transformer Voltage | Vp/Vs = Np/Ns | Voltage ratio equals turns ratio |
| Transformer Power | Ip × Vp = Is × Vs | Power in = Power out (ideal) |
| Transformer Combined | Vp/Vs = Np/Ns = Is/Ip | All ratios related |
| Efficiency | Eff = (Is×Vs)/(Ip×Vp) × 100% | Output power / Input power |
| Power Loss | Ploss = I²R | Heat loss in transmission cables |
| Current from Power | I = P/V | Current calculation from power and voltage |
š IMPORTANT RULES SUMMARY
Fleming's Right-Hand Rule (Generators)
- Thumb: Motion (Force)
- Forefinger: Magnetic Field (N→S)
- Middle Finger: Induced Current
- Used for: Finding direction of induced current in generators
Fleming's Left-Hand Rule (Motors)
- Thumb: Force (Motion)
- Forefinger: Magnetic Field (N→S)
- Middle Finger: Current direction
- Used for: Finding direction of force on current-carrying conductor
Right-Hand Grip Rule (Magnetic Field)
- Thumb: Current direction
- Fingers: Magnetic field direction (circular)
- Used for: Finding magnetic field direction around wire or solenoid
š MOTOR VS GENERATOR COMPARISON
| Feature | D.C. Motor | A.C. Generator |
|---|---|---|
| Energy Conversion | Electrical → Mechanical | Mechanical → Electrical |
| Fleming's Rule | Left-Hand Rule | Right-Hand Rule |
| Current Direction | Supplied by battery | Produced by induction |
| Commutator Type | Split-ring (reverses current) | Slip rings (maintains contact) |
| Function | Produces rotation/motion | Produces electricity |
| Input | Electric current | Mechanical rotation |
| Output | Mechanical rotation | Electric current (A.C.) |
š PRACTICE QUESTIONS
Section 18.1: Electromagnetic Induction
- State TWO ways to increase the magnitude of induced e.m.f. in a solenoid.
- Explain why no current is induced when a magnet is held stationary inside a coil.
- State Lenz's Law and explain its connection to conservation of energy.
Section 18.2: A.C. Generator
- Draw and label the main components of a simple a.c. generator.
- Explain the function of slip rings in an a.c. generator.
- Sketch a graph showing how output voltage varies with time for one complete rotation.
Section 18.3: Magnetic Effect of Current
- Draw the magnetic field pattern around a straight current-carrying wire.
- State THREE ways to increase the magnetic field strength of a solenoid.
- Explain how an electromagnetic relay works and state one advantage of using it.
Section 18.4: Force on Conductor
- A wire carries current from left to right in a magnetic field directed into the page. Using Fleming's Left-Hand Rule, find the direction of force.
- Explain what happens to two parallel wires carrying currents in the same direction.
Section 18.5: D.C. Motor
- Explain the function of the split-ring commutator in a d.c. motor.
- State the energy conversion that takes place in a d.c. motor.
- List THREE ways to increase the turning effect of a d.c. motor.
Section 18.6: Transformer
- A transformer has 500 turns on primary and 50 turns on secondary. If primary voltage is 240 V, calculate secondary voltage.
- Explain why electricity is transmitted at high voltage.
- A transformer has efficiency of 95%. If input power is 1000 W, calculate output power.
š” EXAM TIPS
- Always show your working - even if answer is wrong, you get method marks!
- Include units in all calculations (V, A, W, turns, etc.)
- Draw clear diagrams - label ALL parts properly
- Use correct scientific terms - e.g., "e.m.f." not "voltage produced"
- Practice Fleming's Rules with your hands - it helps in exams!
- Understand energy conversions - very common exam question
- Remember: Transformers work with A.C. ONLY, not D.C.
- Check your calculations - does the answer make sense?
- Read questions carefully - "step-up" or "step-down"?
- Time management - don't spend too long on one question!
š End of Notes š
Understanding Electromagnetic Effects opens doors to understanding how most modern electrical devices work - from your phone charger to electric vehicles to power grids!
