IB MYP Sciences – Electromagnetism and waves Study Guide

This study guide teaches electromagnetism and waves for IB MYP Sciences from scratch. It is aligned to the International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014, which names “electromagnetism” and “waves” among the topics explored in the MYP sciences on-screen examinations. MYP has no SL/HL split, so everything here applies to every student. The page suits MYP years 4 and 5, including anyone preparing for the on-screen examination at the end of year 5.

One point first. MYP has no prescribed content list: schools design their own units, so your teacher decides exactly what your unit includes. This page covers a topic the IB’s brief lists, at a typical year 4–5 depth. Where your school’s unit differs, your teacher will share the task-specific clarifications.

Use it with the revision notes and the practice questions. The course hub and the printable checklist show where this unit sits in the whole course.

What this unit covers

Area	What you should be able to do	Criterion it mostly trains
Magnets and fields	Describe forces between poles, draw field lines, name magnetic materials	A
Electromagnets	Describe the field of a wire and a coil; state what makes an electromagnet stronger	A, B
Motor effect	Predict the direction of the force on a wire; explain how a simple motor turns	A
Induction (qualitative)	Explain when a voltage is induced and what changes its size and direction	A
Wave properties	Use amplitude, wavelength, frequency, period and speed; compare transverse and longitudinal waves	A, C
v = fλ	Calculate speed, frequency or wavelength with correct units	A, C
Reflection and refraction	Apply the law of reflection; explain refraction as a change of speed	A
Electromagnetic spectrum	Order the seven regions; give a use and a hazard for each	A, D

The brief gives energy, movement, transformation and models as examples of related concepts in MYP sciences. All four fit here: waves transfer energy; a motor produces movement; a generator transforms kinetic energy into electrical energy; field lines and wave diagrams are models.

Magnets and magnetic fields

A magnet has a north-seeking pole (N) and a south-seeking pole (S). Like poles repel. Unlike poles attract. Only a few materials are magnetic: iron, nickel, cobalt and alloys such as steel. Copper, aluminium, brass and plastic are not.

A magnetic field is the region around a magnet where a magnetic material or another magnet feels a force. We model it with field lines:

• Lines run from N to S outside the magnet.
• Lines never cross.
• The closer the lines, the stronger the field. The field is strongest at the poles.

You can plot a field line by moving a small compass step by step, marking where its needle points each time.

A permanent magnet keeps its magnetism. An induced magnet is a magnetic material that becomes a magnet only while it sits in a field, which is why it is always attracted, never repelled.

Electromagnets

A current in a wire produces a magnetic field. For a straight wire the field lines are circles around the wire. The right-hand grip rule gives their direction: grip the wire with your right hand, thumb along the conventional current, and your fingers curl the way the field lines go. Reverse the current and the field reverses.

Wind the wire into a coil (a solenoid) and the fields of each turn add together. Outside, the field looks like a bar magnet’s; inside, it is strong and nearly uniform.

An electromagnet is a solenoid wound on an iron core. You can make it stronger by:

1. increasing the current,
2. adding more turns of wire,
3. using a soft iron core.

Soft iron magnetises strongly and loses its magnetism as soon as the current stops. That on–off control is the point: a scrapyard crane drops its load when switched off, and a relay lets a small current switch a larger circuit.

Worked example: reading an electromagnet test

An electromagnet lifts 4 paperclips with 20 turns at 1.0 A, 9 clips with 40 turns at 1.0 A, and 17 clips with 40 turns at 2.0 A. Rows 1 and 2 change only the turns, so they show more turns give a stronger magnet. Rows 2 and 3 change only the current, so they show more current gives a stronger magnet. Rows 1 and 3 change two things at once, so they are not a fair test of either.

The motor effect

A wire carrying a current in a magnetic field feels a force. This is the motor effect: the wire’s own field interacts with the magnet’s field.

• No force acts on a wire parallel to the field; the force is largest at 90°.
• Reversing the current, or reversing the field, reverses the force.
• A bigger current, a stronger magnet, or a longer length of wire in the field gives a bigger force.

Fleming’s left-hand rule gives the direction. Hold your first finger, second finger and thumb at right angles. First finger = Field (N to S), seCond finger = Current, thuMb = Motion (force).

How a simple d.c. motor works

A rectangular coil sits between two poles. Current flows one way along one side of the coil and the opposite way along the other side, so the two sides feel forces in opposite directions. That pair of forces turns the coil. A split-ring commutator reverses the current in the coil every half turn, so the forces keep turning the coil the same way. Brushes carry current in and out. More current, more turns or a stronger magnet make it turn faster.

Electromagnetic induction (qualitative)

Induction is the motor effect in reverse. If a wire moves through a magnetic field, cutting field lines, a voltage is induced across it. If the wire is part of a complete circuit, a current flows. The same happens when a magnet moves into or out of a coil.

What matters is change:

• Magnet held still inside the coil: no induced voltage.
• Magnet pushed in: the meter deflects one way.
• Magnet pulled out: the meter deflects the other way.
• Pushed in faster, a stronger magnet, or more turns on the coil: a bigger induced voltage.

A generator uses this. A coil turns in a magnetic field and slip rings and brushes carry the induced current out. Each side of the coil moves up then down through the field, so the output is alternating current (a.c.).

A transformer has two coils on one iron core. Alternating current in the primary coil makes a changing field in the core, which induces an alternating voltage in the secondary. More turns on the secondary gives a higher output voltage (step-up); fewer gives a lower one (step-down). Steady d.c. gives an unchanging field, so a transformer does not work on d.c.

Wave properties

A wave transfers energy without transferring matter. The particles of the medium only vibrate about a fixed position.

• Transverse waves: the vibrations are at right angles to the direction the energy travels. Examples: waves on water, waves on a rope, all electromagnetic waves.
• Longitudinal waves: the vibrations are parallel to the direction the energy travels, making compressions and rarefactions. Example: sound. Sound needs a medium and cannot travel through a vacuum.

Quantity	Meaning	Unit
Amplitude	Largest displacement from the rest position	m
Wavelength, λ	Distance from one point on a wave to the same point on the next (crest to crest)	m
Frequency, f	Number of waves passing a point each second	Hz
Period, T	Time for one complete wave; T = 1/f	s
Wave speed, v	Distance the wave travels each second	m/s

The wave equation v = fλ

wave speed = frequency × wavelength, or v = fλ.

Rearranged: f = v/λ and λ = v/f. Always convert to base units first: 1 kHz = 10³ Hz, 1 MHz = 10⁶ Hz, 1 GHz = 10⁹ Hz, 1 cm = 0.01 m, 1 nm = 10⁻⁹ m.

Worked example 1: water waves

Twenty crests pass a post in 8.0 s. The crests are 0.60 m apart. Find the frequency, the period and the wave speed.

f = number of waves / time = 20 / 8.0 = 2.5 Hz
T = 1 / f = 1 / 2.5 = 0.40 s
v = f λ = 2.5 × 0.60 = 1.5 m/s

Worked example 2: a radio wave

A radio transmitter broadcasts at 1.2 MHz. Radio waves travel at 3.0 × 10⁸ m/s. Find the wavelength.

f = 1.2 MHz = 1.2 × 10⁶ Hz
λ = v / f = (3.0 × 10⁸) / (1.2 × 10⁶) = 250 m

Worked example 3: an echo

You clap 0.50 s before you hear the echo from a wall. Sound travels at 340 m/s in air. How far away is the wall?

distance travelled by the sound = v × t = 340 × 0.50 = 170 m
the sound goes there and back, so distance to wall = 170 / 2 = 85 m

Reflection and refraction

Reflection. The angle of incidence equals the angle of reflection. Both angles are measured from the normal, a construction line at 90° to the surface. Refraction. When a wave passes into a different medium its speed changes. If it meets the boundary at an angle, the change of speed makes it change direction:

• slows down (e.g. air into glass, deep into shallow water): bends towards the normal;
• speeds up (e.g. glass into air): bends away from the normal;
• along the normal: changes speed but not direction.

The frequency does not change during refraction, because it is set by the source. So if v falls, λ must fall too (λ = v/f).

Worked example 4: light entering glass

Light of frequency 5.0 × 10¹⁴ Hz passes from air (3.0 × 10⁸ m/s) into glass, where it travels at 2.0 × 10⁸ m/s. Find its wavelength in each.

air:   λ = (3.0 × 10⁸) / (5.0 × 10¹⁴) = 6.0 × 10⁻⁷ m  (600 nm)
glass: λ = (2.0 × 10⁸) / (5.0 × 10¹⁴) = 4.0 × 10⁻⁷ m  (400 nm)

The frequency is the same in both; only speed and wavelength change.

The electromagnetic spectrum

Electromagnetic (EM) waves are transverse, need no medium, and all travel at 3.0 × 10⁸ m/s in a vacuum. They transfer energy. In order of decreasing wavelength (and increasing frequency):

Region	Typical use	Main hazard
Radio waves	Radio and TV broadcasting	Low risk at normal levels
Microwaves	Mobile phones, satellite links, microwave ovens	Heating of body tissue at high power
Infrared	Remote controls, thermal imaging, grills, optical fibre data	Skin burns
Visible light	Seeing, photography, optical fibres	Very bright light damages the retina
Ultraviolet	Detecting forged banknotes, sterilising water	Sunburn, skin cancer, eye damage
X-rays	Medical images of bones, airport baggage checks	Ionising: damages cells, can cause cancer
Gamma rays	Killing cancer cells, sterilising medical equipment	Ionising: damages cells, can cause cancer

Higher frequency means more energy per wave, so the high-frequency end is the most harmful. Optical fibres carry signals by total internal reflection: light strikes the inside edge of the glass at a large angle and reflects back in.

Common errors

• Drawing field lines from S to N, or letting them cross.
• Saying an electromagnet with a steel core works better. Steel keeps its magnetism, so the magnet will not switch off cleanly.
• Saying a magnet resting inside a coil induces a current. Induction needs relative movement or a changing field.
• Forgetting to halve the distance in echo questions.
• Leaving MHz or cm in v = fλ instead of converting to Hz and m.
• Saying frequency changes on refraction. Frequency stays the same; speed and wavelength change.

Where next

Go to the revision notes for the condensed version and a self-test, then the practice questions for criterion-labelled questions with worked answers. For how the criteria are judged, read criteria in practice and investigation skills exam preparation. The subject guide gives the course overview.

Official syllabus

International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014.