P4.5 Electromagnetic Effects

Interactive IGCSE-style lesson covering electromagnetic induction, a.c. generators, magnetic fields, motor effect, d.c. motors and transformers.

Starter

Lesson Objectives

By the end of this lesson, students should be able to describe and apply the main electromagnetic effects from P4.5.

Core Ideas

  • Recognise that changing magnetic fields can induce an e.m.f.
  • Describe the turning effect in motors and generators.
  • Explain how transformers change voltage.

Key Terms

induced e.m.f., magnetic field, current, solenoid, slip rings, split-ring commutator, primary coil, secondary coil, step-up, step-down

Quick prompt: A wire is moved through a magnetic field. What must change for a larger induced e.m.f. to be produced?
P4.5.1

Electromagnetic Induction

An e.m.f. is induced when a conductor moves across a magnetic field or when a changing magnetic field links with a conductor.

Interactive: What affects induced e.m.f.?

4
5
3

Induced e.m.f.: medium

P4.5.2

The A.C. Generator

A simple a.c. generator uses a rotating coil in a magnetic field. Slip rings and brushes allow the induced alternating current to be transferred to an external circuit.

Main parts: rotating coil, magnet, slip rings, brushes and external circuit.
As the coil rotates, the sides of the coil cut through magnetic field lines. The direction of the induced e.m.f. reverses every half-turn, producing an alternating output.
The e.m.f.-time graph for a simple a.c. generator is a repeated wave. The e.m.f. changes direction, so the graph moves above and below zero.
N S rotating coil slip rings + brushes
P4.5.3

Magnetic Effect of Current

A current in a wire produces a magnetic field. Around a straight wire the field is circular. Around a solenoid the field is similar to a bar magnet.

Straight Wire

Increasing current increases the magnetic field strength.

Solenoid

Field strength increases when:

  • current increases
  • number of turns increases
  • an iron core is added
P4.5.4

Force on a Current-Carrying Conductor

A current-carrying conductor in a magnetic field experiences a force. Reversing the current or reversing the magnetic field reverses the direction of the force.

Interactive: Reverse the current or field

Current direction: left to right

Field direction: north to south

Force direction: upwards

P4.5.5

The D.C. Motor

A current-carrying coil in a magnetic field experiences a turning effect. A split-ring commutator reverses the current every half-turn so the coil continues to rotate in the same direction.

What increases the turning effect?
Increasing the number of turns on the coil, increasing the current, or increasing the strength of the magnetic field.
What does the split-ring commutator do?
It reverses the current in the coil every half-turn so the motor keeps rotating in the same direction.
What do brushes do?
They maintain electrical contact between the rotating commutator and the external circuit.
P4.5.6

The Transformer

A transformer uses electromagnetic induction to change an alternating voltage. It has a primary coil, a secondary coil and a soft iron core.

Transformer Formula

Vp / Vs = Np / Ns

Step-up transformers increase voltage. Step-down transformers decrease voltage.

100% Efficiency

IpVp = IsVs

If voltage increases, current decreases. This helps reduce power loss in transmission cables.

Interactive Transformer Calculator

120
100
200

Secondary voltage, Vs: 240 V

Transformer type: step-up

Check

Quick Knowledge Quiz

Exit Ticket

Student Reflection