Electromagnetic Effects

2026 Syllabus Objectives

By the end of this topic, you should be able to:

4.5.1 Electromagnetic Induction

  • Core: Understand that moving a conductor across a magnetic field or changing the magnetic field around a conductor can induce an e.m.f.; describe an experiment to demonstrate this; state the factors affecting the size of induced e.m.f.
  • Supplement: Know that induced e.m.f. opposes the change causing it; use the relative directions of force, field, and induced current.

4.5.2 The A.C. Generator

  • Supplement: Describe a simple a.c. generator; sketch and interpret graphs of e.m.f. against time.

4.5.3 Magnetic Effect of a Current

  • Core: Describe the magnetic field pattern around straight wires and solenoids; describe experiments to identify these patterns; describe how this effect is used in relays and loudspeakers.
  • Supplement: State how field strength varies around wires and solenoids; describe effects of changing current magnitude and direction.

4.5.4 Force on a Current-Carrying Conductor

  • Core: Describe an experiment showing force on a current-carrying conductor in a magnetic field, including effects of reversing current and field direction.
  • Supplement: Use relative directions of force, field, and current; determine force direction on charged particle beams.

4.5.5 The D.C. Motor

  • Core: Know that a current-carrying coil in a magnetic field experiences a turning effect, which increases with more turns, more current, or stronger field.
  • Supplement: Describe how an electric motor operates, including the split-ring commutator and brushes.

4.5.6 The Transformer

  • Core: Describe transformer construction; use terms primary, secondary, step-up, step-down; use the equation Vp/Vs = Np/Ns; describe use in high-voltage transmission and state its advantages.
  • Supplement: Explain transformer operation; use IpVp = IsVs for 100% efficiency; use P = I²R to explain why high voltage reduces power losses.

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4.5.1 Electromagnetic Induction

What is Electromagnetic Induction?

Electromagnetic induction is the process of creating an electric voltage (called an electromotive force or e.m.f.) in a conductor by changing the magnetic field around it.

An e.m.f. is induced (created) in a conductor when:

  • The conductor moves through a magnetic field, OR
  • The magnetic field around the conductor changes

Think of it this way: when you move a wire through a magnetic field, you're "cutting" through invisible magnetic field lines. This cutting action generates electricity in the wire.

Demonstrating Electromagnetic Induction

Experiment 1: Moving a Magnet Through a Coil

Equipment needed:

  • A coil of wire (many loops of wire wound together)
  • A bar magnet
  • A sensitive voltmeter (to measure the e.m.f.) or galvanometer (to detect current)

Method:

  1. Connect the coil to the voltmeter
  2. Move the bar magnet into the coil
  3. Hold the magnet still inside the coil
  4. Move the magnet back out of the coil
  5. Observe the voltmeter reading at each stage

Results:

  • Magnet outside and stationary: Voltmeter reads zero (no e.m.f. induced)
  • Magnet moving into coil: Voltmeter shows a reading (e.m.f. is induced) - needle deflects to the right
  • Magnet stationary inside coil: Voltmeter reads zero (no e.m.f. induced)
  • Magnet moving out of coil: Voltmeter shows a reading with opposite sign (e.m.f. induced in opposite direction) - needle deflects to the left

Key observation: An e.m.f. is only induced when there is relative movement between the magnet and the coil. When both are stationary, no e.m.f. is produced.

If the coil is part of a complete circuit, the induced e.m.f. will cause a current to flow, which can be detected using an ammeter.

Experiment 2: Moving a Wire Through a Magnetic Field

Equipment needed:

  • A long straight wire
  • Two magnets (creating a magnetic field between them)
  • A sensitive voltmeter

Method:

  1. Connect the wire to a voltmeter
  2. Place the magnets so they create a magnetic field (North pole facing South pole)
  3. Move the wire up and down between the magnetic poles
  4. Observe the voltmeter reading

Results:

  • Wire stationary: Voltmeter reads zero
  • Wire moving perpendicular to field: Voltmeter shows maximum reading
  • Wire moving parallel to field: Voltmeter reads zero
  • Wire moving in opposite direction: Voltmeter shows reading with opposite sign

Factors Affecting the Magnitude of Induced E.M.F.

The size (magnitude) of the induced e.m.f. can be increased by:

  1. Moving the magnet or wire faster

    • Faster movement = more field lines cut per second = larger e.m.f.

  2. Using a stronger magnet

    • Stronger magnet = more field lines = larger e.m.f.

  3. Increasing the number of turns on the coil

    • More turns = more wire cutting field lines = larger e.m.f.
    • Each turn contributes to the total e.m.f.

  4. Increasing the area of the coil

    • Larger coil = more field lines cut = larger e.m.f.

  5. For a solenoid: inserting a soft iron core

    • Iron concentrates the magnetic field lines, making them stronger through the coil

Direction of Induced E.M.F. - Lenz's Law (Supplement)

Lenz's Law states: The direction of an induced e.m.f. always opposes the change that causes it.

This means the induced e.m.f. creates a magnetic field that tries to stop whatever is happening.

Example 1 - Pushing magnet into coil:

  • When you push the North pole of a magnet into a coil, the coil becomes a North pole at the end facing the magnet
  • Why? Because North poles repel each other - the coil is trying to push the magnet away to oppose the motion

Example 2 - Pulling magnet out of coil:

  • When you pull the North pole out of a coil, the coil becomes a South pole at the end facing the magnet
  • Why? Because North and South poles attract - the coil is trying to pull the magnet back to oppose the removal

Fleming's Right-Hand Dynamo Rule (Supplement)

This rule helps you find the direction of the induced e.m.f. when a wire moves through a magnetic field.

Hold your right hand with thumb, first finger, and second finger all at right angles:

  • First Finger = Direction of magnetic Field (North to South)
  • ThuMb = Direction of Motion (how the wire moves)
  • SeCond finger = Direction of induced Current (or e.m.f.)

Remember: Current flows from positive to negative (conventional current direction).

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