LEARNING OBJECTIVES
What you will be able to do
- describe permanent and induced magnetism
- predict forces on current-carrying conductors
- explain electromagnetic induction
- apply transformer and power-transmission relationships
AT A GLANCE
INTRODUCTION · THE BIG IDEA
Explain magnetic fields, electromagnets, motors, induction and transformers.
Electricity and magnetism are linked: current produces a magnetic field, a magnetic field can exert a force on current, and a changing magnetic field can induce a voltage.
These three ideas underpin relays, loudspeakers, motors, generators and the transformers used in national grids.
SECTION 01
Magnetic fields and electromagnets
Outside a bar magnet, field lines run from north to south. Magnetic field direction is the direction a north pole would move; closer field lines show a stronger field.
Soft iron magnetises and demagnetises easily, so it suits electromagnets. Steel retains magnetism and suits permanent magnets. A solenoid's field becomes stronger with greater current, more turns per unit length or a soft-iron core.
SECTION 02
Force on a current and the motor effect
A current-carrying conductor in a magnetic field experiences a force because its field interacts with the external field. Increasing field strength, current or conductor length in the field increases force.
In a d.c. motor, opposite forces on the coil create a turning effect. A split-ring commutator reverses current every half-turn so torque stays in the same rotational direction. Loudspeakers use a varying current to vibrate a coil and cone.
| Quantity changed | Effect |
|---|---|
| current increases | force increases |
| field strength increases | force increases |
| current reverses | force reverses |
| field reverses | force reverses |
SECTION 03
Electromagnetic induction and generators
Moving a conductor across a field, moving a magnet relative to a coil or changing current in a nearby coil changes magnetic flux and induces an e.m.f. A complete circuit allows induced current.
A faster change, stronger magnet or more coil turns gives a larger e.m.f. Lenz's law states that the induced effect opposes the change causing it. An a.c. generator uses slip rings; a d.c. generator uses a split-ring commutator.
SECTION 04
Transformers and power transmission
A transformer uses alternating current in a primary coil to create changing magnetic flux in an iron core, inducing alternating voltage in a secondary coil. It does not operate continuously from steady d.c.
For the same transmitted power, a higher voltage means a lower current. Since cable heating is I²R, step-up transformers greatly reduce energy loss; step-down transformers later provide safer local voltages.
Step-down transformer
- The primary has 2000 turns at 240 V.
- The secondary has 100 turns.
- Vₛ = 240 × 100/2000.
Answer: Secondary voltage = 12 V.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Magnetic fields show force direction and strength.
- The motor effect converts electrical input to motion.
- Induction requires changing magnetic flux.
- Transformers allow efficient high-voltage power transmission.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- describe permanent and induced magnetism
- predict forces on current-carrying conductors
- explain electromagnetic induction
- apply transformer and power-transmission relationships