IB MYP Sciences – Electromagnetism and waves Revision Notes

For full explanations and worked examples, read the electromagnetism and waves study guide first. These notes are the condensed version for the final weeks.

They cover electromagnetism and waves for IB MYP Sciences, aligned to the International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014. That brief lists “electromagnetism” and “waves” among the topics explored in the MYP sciences on-screen examinations. There is no SL/HL split in MYP, and these notes suit MYP years 4 and 5, including students sitting the on-screen examination at the end of year 5.

MYP has no prescribed content list: schools design their own units. Treat these notes as the standard core of the topic and check the edges of your own unit with your teacher, who will share the task-specific clarifications.

Links: practice questions · course hub · printable checklist · how the eAssessment is structured

How this topic is examined

The brief sets out three on-screen examination tasks. Knowing and understanding (criterion A, 25 marks) is where definitions, rules and v = fλ calculations sit. Investigation skills (criteria B and C, 50 marks) could use an electromagnet or ripple-tank investigation as its context. Applying science (criterion D, 25 marks) asks you to reflect on the impact of science, for example the benefits and risks of a use of the EM spectrum. Each criterion has eight achievement levels (1–8) in four bands. For how each level is judged, see criteria in practice.

Definitions

Term	Definition
Magnetic field	Region where a magnet or magnetic material feels a force
Magnetic materials	Iron, nickel, cobalt and alloys such as steel
Induced magnet	Becomes a magnet only while in a field; always attracted
Solenoid	A long coil of wire; carries a bar-magnet-shaped field
Electromagnet	Solenoid on a soft iron core; can be switched on and off
Motor effect	Force on a current-carrying wire in a magnetic field
Electromagnetic induction	A voltage induced when a conductor cuts field lines or the field through a coil changes
Amplitude	Maximum displacement from the rest position
Wavelength (λ)	Distance between two matching points on neighbouring waves
Frequency (f)	Waves per second, in hertz (Hz)
Period (T)	Time for one wave; T = 1/f
Normal	Line at 90° to a surface, used to measure angles

Equations and constants

Equation	Rearranged	Units
v = fλ	f = v/λ, λ = v/f	m/s, Hz, m
T = 1/f	f = 1/T	s, Hz
speed = distance / time	distance = speed × time	m/s, m, s
echo distance = (v × t) / 2	t = 2d / v	m, m/s, s

Speed of EM waves in a vacuum: 3.0 × 10⁸ m/s. Speed of sound in air: about 340 m/s (use the value a question gives).

Prefixes: k = 10³, M = 10⁶, G = 10⁹; c = 10⁻², m = 10⁻³, n = 10⁻⁹.

Magnetism in six lines

1. Like poles repel; unlike poles attract.
2. Field lines run N → S outside a magnet, never cross, and are closest where the field is strongest.
3. A straight wire’s field is a set of circles; the right-hand grip rule gives the direction.
4. Electromagnet strength rises with current, turns and a soft iron core.
5. Soft iron, not steel: it loses its magnetism when the current stops.
6. Uses: relays, scrapyard cranes, electric bells, door locks.

Method in steps: Fleming’s left-hand rule

1. Point your First finger along the field (N to S).
2. Point your seCond finger along the conventional current (+ to −).
3. Your thuMb now shows the direction of the force (motion).
4. Reverse either field or current and the force reverses. Reverse both and it does not change.

Motor: forces on opposite sides of the coil act in opposite directions, so the coil turns. The split-ring commutator reverses the current every half turn to keep it turning one way.

Induction checklist

Situation	Induced voltage?
Magnet moving into coil	Yes, one direction
Magnet moving out of coil	Yes, opposite direction
Magnet stationary inside coil	No
Coil moving over stationary magnet	Yes (relative movement is what counts)
Faster movement, stronger magnet, more turns	Bigger voltage
a.c. in transformer primary	Yes, a.c. in secondary
Steady d.c. in transformer primary	No (the field is not changing)

Generator: rotating coil + slip rings → a.c. Transformer: secondary turns > primary turns → step-up; fewer → step-down.

Method in steps: any v = fλ question

1. List the values with units.
2. Convert to Hz, m and m/s (MHz → × 10⁶; cm → ÷ 100).
3. Write the equation you will use, rearranged if needed.
4. Substitute and calculate.
5. Give the answer with a unit, to a sensible number of significant figures (match the data, usually 2 or 3).

Worked reminder: a wave has f = 4.0 Hz and λ = 0.25 m. v = 4.0 × 0.25 = 1.0 m/s.

Worked reminder: a microwave signal has λ = 3.0 cm = 0.030 m. f = (3.0 × 10⁸)/0.030 = 1.0 × 10¹⁰ Hz (10 GHz).

Reflection and refraction in brief

• Reflection: angle of incidence = angle of reflection, both from the normal.
• Refraction: caused by a change of speed at a boundary.
• Slower medium → bends towards the normal. Faster medium → bends away.
• Along the normal: speed changes, direction does not.
• Frequency stays the same; wavelength changes in proportion to speed.
• Water waves travel slower in shallow water, so they refract at a depth change.

Electromagnetic spectrum

Long wavelength, low frequency → short wavelength, high frequency:

Radio → Microwaves → Infrared → Visible → Ultraviolet → X-rays → Gamma rays

Memory aid: Rich Men In Vegas Use X-ray Glasses.

Region	One use	One hazard
Radio	Broadcasting	Low risk
Microwaves	Mobile phones, cooking	Internal heating at high power
Infrared	Remote controls, thermal cameras	Burns
Visible	Sight, optical fibres	Retina damage from intense light
Ultraviolet	Sterilising water, security marks	Sunburn, skin cancer
X-rays	Bone images, baggage scanners	Ionising: cell damage, cancer
Gamma	Cancer treatment, sterilising equipment	Ionising: cell damage, cancer

All EM waves: transverse, travel in a vacuum at 3.0 × 10⁸ m/s, transfer energy.

Must-know distinctions

• Transverse vs longitudinal: vibrations at 90° to energy transfer (light, water) vs parallel to it (sound).
• Motor vs generator: current in → movement out vs movement in → current out.
• Commutator vs slip rings: d.c. motor reverses current each half turn vs generator keeps a.c. output connected.
• Permanent vs induced magnet: keeps its magnetism vs only magnetic in a field.
• Sound vs EM waves: needs a medium, about 340 m/s in air vs travels in a vacuum at 3.0 × 10⁸ m/s.
• Amplitude vs wavelength: a height measured from the rest line vs a length measured along the wave.

Quick self-test

1. Name three magnetic materials.
2. State two ways of reversing the force on a wire in a motor.
3. A magnet sits still inside a coil connected to a meter. What does the meter read, and why?
4. Why does a transformer need a.c.?
5. Find the period of a 50 Hz wave.
6. Sound travels at 340 m/s. Find the wavelength of a 170 Hz note.
7. Twelve waves pass a point in 4.0 s. Find the frequency.
8. A ship’s sonar pulse returns 0.20 s after it is sent. Sound travels at 1500 m/s in seawater. How deep is the seabed?
9. Light passes from air into water and slows down. Which of speed, frequency and wavelength stay the same?
10. Which EM region has the longest wavelength, and which two regions are ionising in the table above?
11. Give one difference between X-rays and ultrasound.

Answers

1. Any three of iron, nickel, cobalt, steel.
2. Reverse the current; reverse the magnetic field (swap the poles).
3. Zero. No relative movement, so no field lines are cut and no voltage is induced.
4. Only a changing current makes the changing field in the core needed to induce a voltage in the secondary.
5. T = 1/50 = 0.02 s.
6. λ = 340/170 = 2.0 m.
7. f = 12/4.0 = 3.0 Hz.
8. d = (1500 × 0.20)/2 = 150 m.
9. Only the frequency. Speed and wavelength both decrease.
10. Radio waves. X-rays and gamma rays (ultraviolet at its high-frequency end can also ionise).
11. X-rays are electromagnetic, transverse and ionising; ultrasound is a longitudinal sound wave above human hearing and is not ionising.

Where marks are usually lost

• Drawing field lines that start at S, cross each other, or have no arrows.
• Listing “use a bigger magnet” as a way to strengthen an electromagnet. The answer needs current, turns or core.
• Stating that a stationary magnet in a coil induces a current.
• Using the right hand for the motor effect, or labelling the thumb as current.
• Writing 100 MHz as 100 Hz, or a wavelength in cm, then getting an answer 10⁶ or 100 times out.
• Missing the “÷ 2” in echo and sonar questions.
• Saying light “bends because it enters glass” without mentioning a change in speed.
• Swapping the order of ultraviolet and X-rays, or putting infrared after visible.
• Giving a hazard with no mechanism in a criterion D answer (“X-rays are dangerous”) instead of “ionising radiation can damage cells and cause cancer”.
• Final numerical answers with no unit.

Official syllabus

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