4-5 Physics Chapter 4 Part 2: Electromagnetic Induction
Introduction
Instructor: Teacher 令
Topic: 4.2 Electromagnetic Induction (Part of Physics Chapter 4)
Focus: Principle of electricity generation
Relevance: Extension of Form 3 material to Form 5
4.2 Electromagnetic Induction
Key Concepts
Definition: Inducing an EMF (electromotive force) across a wire by either cutting the magnetic field or changing the magnetic flux.
Explanation: When a wire cuts through a magnetic field or when the magnetic flux through the wire changes, it generates an induced current in the wire.
Form 3 Recap: Example with two magnets and a wire moving up and down to cut the magnetic field, inducing a current.
Key Examples
Straight Wire
Setup: Two poles of a magnet and a copper wire moving downwards.
Mechanism: Wire cuts the magnetic field, changing magnetic flux, inducing EMF, and generating induced current.
Detection: A galvanometer shows needle deflection when current is induced.
Measuring device: Galvanometer (more sensitive than ammeter).
Direction Determination: Fleming Right Hand Rule (for generators).
Solenoid (Coil)
Setup: Solenoid and bar magnet.
Mechanism: Moving bar magnet into the solenoid induces current due to changing magnetic flux.
Observations: Galvanometer needle deflects.
Direction Determination: Use of Lenz's Law and Right Hand Grip Rule to determine the direction of induced current.
Laws and Rules
Fleming's Rules
Right Hand Rule: For generators, determines the direction of induced current.
Left Hand Rule: For motors, determines the force direction.
Lenz's Law
Definition: Induced current flows in a direction to oppose the change causing it.
Analogy: Comparing magnetic fields to human interactions (e.g., tool people and fickle partners).
Right Hand Grip Rule
Use: To determine the direction of current in solenoids.
Detailed Examples with Direction Determination
Bar Magnet Entering Solenoid
North Pole enters: Induces a North Pole at entry point to create repulsion.
Current direction: Determined using Fleming's Right Hand Rule and Right Hand Grip Rule.
Bar Magnet Leaving Solenoid
Creates South Pole at exit point to pull the magnet back.
Current direction: Opposite to the entering case.
Factors Affecting Induced EMF (Faraday's Laws)
Rate of Change of Magnetic Flux: Direct relationship with generated EMF.
Increasing Magnetic Field Strength: Stronger magnets increase the induced EMF.
Speed of Relative Motion: Faster movement increases the rate of cutting magnetic lines and thus the EMF.
Number of Turns in Solenoid: More turns result in higher EMF.
Continuous Change Requirement: To generate a steady EMF, the magnetic field must continuously change.
Generators
DC Generator
Similar structure to DC Motor (without battery).
Principle: Using rotational force to generate EMF.
Phase Analysis: Explains changing positions of coils and resultant EMF generation.
Carbon Brush and Commutator: Establishes contact to generate steady direct current.
AC Generator
Difference: Uses slip rings instead of commutators.
Output: Alternating current due to changing direction of generated EMF.
Phase Analysis: Similar to DC generator but includes negative cycle for inverse EMF.
Comparison: DC generators produce a constant direction in current whereas AC generators produce alternating direction.