Sciences · Topic 2.3
Waves and Heat Transfer
Waves transfer energy from one place to another without transferring matter. Heat energy is transferred by three mechanisms: conduction, convection, and radiation. Understanding these concepts explains phenomena from sound and light to home insulation and cooking.
What You'll Learn
- Distinguish between transverse and longitudinal waves with examples
- Describe and calculate wave properties: amplitude, frequency, wavelength, wave speed
- Explain conduction, convection, and radiation with everyday examples
- Compare the three methods of heat transfer in terms of mechanism and medium
- Apply knowledge to explain real-world thermal insulation scenarios
- Understand why sound cannot travel through a vacuum
IB Assessment Focus
Criterion A: Define wave types, properties, and heat transfer mechanisms correctly.
Criterion B: Design and analyse experiments on wave properties or thermal insulation.
Criterion C: Use correct vocabulary; explain phenomena using scientific reasoning.
Criterion D: Apply understanding to real-life contexts (building insulation, cooking, medical imaging).
Sciences · Topic 2.3
Types of Waves
Waves are disturbances that transfer energy from one location to another. There are two fundamental types based on the direction of vibration relative to the direction of travel.
Transverse Waves
In a transverse wave, particle vibration is perpendicular (at 90°) to the direction of wave travel.
Examples: Light, radio waves, microwaves, water waves, seismic S-waves
Imagine shaking a rope up and down while it moves forward — the rope moves up-down (perpendicular), but the wave travels forward.
Examples: Light, radio waves, microwaves, water waves, seismic S-waves
Imagine shaking a rope up and down while it moves forward — the rope moves up-down (perpendicular), but the wave travels forward.
Longitudinal Waves
In a longitudinal wave, particle vibration is parallel to the direction of wave travel. The wave is made of compressions (regions of high pressure) and rarefactions (regions of low pressure).
Examples: Sound, seismic P-waves
Imagine pushing a slinky back and forth — the coils compress and expand in the same direction as the wave travels.
Examples: Sound, seismic P-waves
Imagine pushing a slinky back and forth — the coils compress and expand in the same direction as the wave travels.
Critical Rule: Sound is a longitudinal wave — it cannot travel through a vacuum because it needs particles to compress and expand. Light is a transverse electromagnetic wave — it can travel through a vacuum at 300,000 km/s. This is why you can see but not hear an explosion in space.
Comparison Table
| Feature | Transverse | Longitudinal |
|---|---|---|
| Vibration direction | Perpendicular to travel | Parallel to travel |
| Needs a medium? | No (light travels in vacuum) | Yes (sound needs particles) |
| Examples | Light, radio, water waves | Sound, ultrasound |
| Wave features | Crests and troughs | Compressions and rarefactions |
Sciences · Topic 2.3
Wave Properties
All waves can be described using the same set of measurable properties.
Key Wave Properties
| Property | Definition | Unit |
|---|---|---|
| Amplitude | Maximum displacement of a particle from its rest position; related to the energy of the wave | metres (m) |
| Wavelength (λ) | Distance between two adjacent identical points (e.g., crest to crest) | metres (m) |
| Frequency (f) | Number of complete waves passing a fixed point per second | Hertz (Hz) |
| Time period (T) | Time for one complete wave to pass a fixed point | seconds (s) |
| Wave speed (v) | Speed at which the wave travels through the medium | metres per second (m/s) |
Wave Speed Equation
v = f × λ (wave speed = frequency × wavelength)
Frequency and Period
f = 1/T (frequency = 1 ÷ time period)
Amplitude and Energy
- Greater amplitude = more energy carried by the wave
- A louder sound has greater amplitude than a quiet sound
- Higher frequency (for light) corresponds to shorter wavelength and more energy per photon
Sciences · Topic 2.3
Heat Transfer
Heat energy always flows from hotter regions to cooler regions. It can be transferred by three distinct mechanisms.
1. Conduction
Definition: Transfer of heat through direct particle-to-particle vibration in a solid.
Mechanism: Hot particles vibrate faster. They bump into neighbouring slower particles, transferring kinetic energy along the material without the particles themselves moving.
Occurs in: Mainly solids; also liquids (slowly)
Example: A metal spoon in hot soup quickly becomes hot. Touching a metal surface feels cold because metal conducts heat away from your hand rapidly.
Mechanism: Hot particles vibrate faster. They bump into neighbouring slower particles, transferring kinetic energy along the material without the particles themselves moving.
Occurs in: Mainly solids; also liquids (slowly)
Example: A metal spoon in hot soup quickly becomes hot. Touching a metal surface feels cold because metal conducts heat away from your hand rapidly.
2. Convection
Definition: Transfer of heat through the bulk movement of fluids (liquids and gases).
Mechanism: Hot fluid expands, becomes less dense, and rises. Cooler, denser fluid sinks to replace it, creating a convection current.
Occurs in: Liquids and gases (fluids)
Example: Hot air rises in a room and cool air sinks, circulating around the room. Boiling water in a pot creates convection currents.
Mechanism: Hot fluid expands, becomes less dense, and rises. Cooler, denser fluid sinks to replace it, creating a convection current.
Occurs in: Liquids and gases (fluids)
Example: Hot air rises in a room and cool air sinks, circulating around the room. Boiling water in a pot creates convection currents.
3. Radiation
Definition: Transfer of heat through electromagnetic waves (infrared radiation). Does not require a medium.
Mechanism: All objects emit electromagnetic radiation. Hotter objects emit more energy. Radiation can travel through a vacuum.
Occurs in: Any medium, including vacuum
Example: The Sun heats the Earth across 150 million km of empty space. You feel heat from a campfire even if you don't touch it.
Mechanism: All objects emit electromagnetic radiation. Hotter objects emit more energy. Radiation can travel through a vacuum.
Occurs in: Any medium, including vacuum
Example: The Sun heats the Earth across 150 million km of empty space. You feel heat from a campfire even if you don't touch it.
Comparison Summary
| Method | Medium Required? | State of Matter | Mechanism |
|---|---|---|---|
| Conduction | Yes | Mainly solids | Particle vibration |
| Convection | Yes | Liquids and gases | Bulk fluid movement |
| Radiation | No | Any (including vacuum) | Electromagnetic waves |
Sciences · Topic 2.3
Real-World Applications
Understanding waves and heat transfer explains many everyday phenomena and technologies.
Thermal Insulation
Good insulation reduces all three types of heat transfer:
- Reducing conduction: Use materials with low thermal conductivity (e.g., fibreglass, foam, wood)
- Reducing convection: Trap air in small pockets so it cannot circulate (e.g., double glazing, bubblewrap)
- Reducing radiation: Use shiny surfaces to reflect infrared radiation (e.g., emergency blankets, thermos flask silvered walls)
The Electromagnetic Spectrum
| Wave Type | Frequency | Wavelength | Use |
|---|---|---|---|
| Radio waves | Lowest | Longest | Broadcasting, communication |
| Microwaves | ↑ | ↓ | Cooking, mobile phones |
| Infrared | ↑ | ↓ | Remote controls, thermal cameras |
| Visible light | ↑ | ↓ | Vision |
| Ultraviolet | ↑ | ↓ | Sterilisation, tanning |
| X-rays | ↑ | ↓ | Medical imaging |
| Gamma rays | Highest | Shortest | Cancer treatment, sterilisation |
All electromagnetic waves travel at the same speed in a vacuum: 3 × 10&sup8; m/s (300,000 km/s).
Key Point: All electromagnetic waves are transverse waves and travel at the speed of light in a vacuum. They transfer energy without transferring matter, and can travel through a vacuum (unlike sound).
Sciences · Topic 2.3
Worked Examples
These examples show the depth of scientific reasoning expected at Grade 7.
EXAMPLE 1Compare conduction and convection as methods of heat transfer.
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Full Solution
Both conduction and convection require a medium and transfer heat energy. Conduction transfers heat through direct particle-to-particle vibration — the faster vibrating particles of the hot substance bump into slower particles of the cooler substance, transferring kinetic energy. It works mainly in solids. Convection involves the actual bulk movement of fluids — hot liquid or gas becomes less dense and rises, carrying energy with it, while cooler, denser fluid sinks to replace it, creating a convection current. Convection works in liquids and gases only.
EXAMPLE 2Why can sound not travel through outer space?
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Full Solution
Sound is a longitudinal mechanical wave — it requires particles to compress and expand to transmit energy. Outer space is essentially a vacuum, with no particles present. With no particles to vibrate, there is no medium through which sound can propagate. This is why explosions in space are silent. Light, however, is an electromagnetic transverse wave that can travel through a vacuum, which is why you can see but not hear distant stars.
EXAMPLE 3A wave has a frequency of 500 Hz and a wavelength of 0.68 m. Calculate its wave speed.
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Full Solution
Formula: v = f × λ
Substituting: v = 500 Hz × 0.68 m
Result: v = 340 m/s
This is approximately the speed of sound in air at room temperature.
Substituting: v = 500 Hz × 0.68 m
Result: v = 340 m/s
This is approximately the speed of sound in air at room temperature.
EXAMPLE 4Explain how a thermos flask uses all three methods of heat reduction to keep a drink hot.
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Full Solution
A thermos flask uses three strategies:
1. Reduces conduction: The double-walled glass is a poor conductor. The vacuum between walls prevents conduction entirely.
2. Reduces convection: The vacuum between the walls prevents convection (no fluid to move).
3. Reduces radiation: The inside surfaces are silvered (mirror-like) to reflect infrared radiation back into the flask, preventing it from escaping as radiation.
Together, these features minimise heat loss from the hot drink.
1. Reduces conduction: The double-walled glass is a poor conductor. The vacuum between walls prevents conduction entirely.
2. Reduces convection: The vacuum between the walls prevents convection (no fluid to move).
3. Reduces radiation: The inside surfaces are silvered (mirror-like) to reflect infrared radiation back into the flask, preventing it from escaping as radiation.
Together, these features minimise heat loss from the hot drink.
EXAMPLE 5Explain why a metal floor feels colder than a wooden floor at the same temperature.
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Full Solution
Both floors are at the same temperature (e.g., 20°C — room temperature). However, metal is a much better conductor of heat than wood. When you stand on the metal floor, heat conducts rapidly from your foot (at 37°C) into the metal, making your foot cool down quickly. You perceive this rapid heat loss as "coldness." With the wooden floor, heat conducts away much more slowly, so your foot cools less rapidly and the floor feels warmer. The floors are the same temperature — it is the rate of heat transfer that differs.
EXAMPLE 6A wave has a time period of 0.01 seconds. Calculate its frequency.
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Full Solution
Formula: f = 1/T
Substituting: f = 1 ÷ 0.01
Result: f = 100 Hz
Substituting: f = 1 ÷ 0.01
Result: f = 100 Hz
EXAMPLE 7A student says: "Water waves and sound waves are the same type of wave." Is this correct? Explain.
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Full Solution
The student is incorrect. Water waves are transverse waves — the water molecules move up and down (perpendicular) while the wave travels horizontally. Sound waves are longitudinal waves — air molecules compress and expand in the same direction as the wave travels. While both require a medium and both transfer energy, they are fundamentally different types of wave based on the direction of particle vibration relative to wave travel.
Sciences · Topic 2.3
Practice Q&A
Attempt each question before revealing the model answer.
IDENTIFYState whether sound is transverse or longitudinal and explain why.
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Model Answer
Sound is a longitudinal wave. Air particles vibrate in the same direction (parallel) as the wave travels, creating compressions (high pressure) and rarefactions (low pressure). This is different from transverse waves where vibration is perpendicular to the direction of travel.
EXPLAINExplain why radiation is the only method of heat transfer that can occur through a vacuum.
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Model Answer
Conduction and convection both require particles to transfer heat energy. In a vacuum there are no particles, so neither can occur. Radiation transfers heat through electromagnetic waves, which are self-propagating and do not require particles — they can travel through a vacuum at the speed of light.
CALCULATEA wave has frequency 200 Hz and wavelength 1.7 m. Calculate the wave speed.
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Model Answer
v = f × λ = 200 × 1.7 = 340 m/s
DESCRIBEDescribe the convection current that forms when a room is heated by a radiator placed on one wall.
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Model Answer
The radiator heats the air nearby. This warm air expands, becomes less dense, and rises toward the ceiling. As it spreads across the room and loses heat, it becomes cooler and denser, sinking back down to the floor. The cooler air near the floor then flows back toward the radiator to be reheated. This circular movement is a convection current that distributes heat throughout the room.
COMPARECompare the amplitude of a loud sound and a quiet sound.
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Model Answer
A loud sound has a larger amplitude than a quiet sound. Amplitude represents the maximum displacement of particles from their rest position. A larger amplitude means more energy is being transferred by the wave — sound with more energy is perceived as louder. Frequency (pitch) is different from amplitude (loudness).
APPLYWhy do penguins huddle together in groups in cold Antarctic winters?
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Model Answer
Penguins huddle to reduce heat loss. By pressing together, they reduce the surface area exposed to the cold air, which reduces heat loss by convection and conduction. The penguins' bodies warm the trapped air between them. Each penguin in the group loses less heat than it would alone, making the group more energy-efficient in extreme cold.
IDENTIFYName two examples of transverse waves and two examples of longitudinal waves.
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Model Answer
Transverse waves: Light (electromagnetic), water waves (surface waves), microwaves, radio waves.
Longitudinal waves: Sound, ultrasound, seismic P-waves.
Longitudinal waves: Sound, ultrasound, seismic P-waves.
EXPLAINExplain why a cavity wall in a house (two walls with a gap) reduces heat loss.
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Model Answer
A cavity wall reduces heat loss in two ways. First, the trapped air in the gap is a poor conductor of heat. Second, keeping the air still (or filling with insulating foam) prevents convection currents from carrying heat through the gap. Filling the cavity with fibreglass or foam increases the insulating effect further by preventing air movement and using materials that conduct heat poorly.
Sciences · Review
Flashcard Review
Tap each card to reveal the answer. Try to answer from memory first.
What is a transverse wave? Give an example.
A wave where particle vibration is perpendicular (90°) to the direction of travel. Example: light, water waves.
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What is a longitudinal wave? Give an example.
A wave where particle vibration is parallel to the direction of travel, with compressions and rarefactions. Example: sound.
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Why can sound not travel through space?
Sound is a longitudinal mechanical wave requiring particles to vibrate. Space is a vacuum — no particles — so sound cannot propagate.
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What is amplitude?
The maximum displacement of a particle from its rest position. Greater amplitude = more energy. For sound: greater amplitude = louder.
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What is frequency? What is its unit?
The number of complete waves passing a fixed point per second. Measured in Hertz (Hz).
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What is the wave speed equation?
v = f × λ (wave speed = frequency × wavelength)
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What is conduction?
Heat transfer through direct particle-to-particle vibration. Mainly occurs in solids. Metals are good conductors.
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What is convection?
Heat transfer through bulk movement of fluids. Hot fluid rises (less dense), cool fluid sinks, creating a convection current. Occurs in liquids and gases.
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What is radiation?
Heat transfer through electromagnetic waves (infrared). Does not require a medium — can travel through a vacuum. Example: heat from the Sun.
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Which heat transfer method does NOT need a medium?
Radiation. It travels as electromagnetic waves through empty space. Conduction and convection both require particles.
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What is wavelength?
The distance between two adjacent identical points on a wave (e.g., crest to crest, or compression to compression). Measured in metres.
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Name three electromagnetic waves in order of increasing frequency.
Radio → Microwave → Infrared → Visible Light → UV → X-ray → Gamma. All travel at 3×10&sup8; m/s in vacuum.
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Why does metal feel colder than wood at the same temperature?
Metal conducts heat away from your hand much faster than wood. The rapid heat loss is perceived as coldness, even though both are at the same temperature.
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What is a convection current?
A circular flow of fluid: hot fluid rises (less dense), transfers heat, cools and sinks, then flows back to be reheated. This circulates heat throughout the fluid.
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How does a thermos flask reduce heat loss?
Vacuum between walls eliminates conduction and convection. Silvered surfaces reflect infrared radiation back in, reducing radiation loss.
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Sciences · Practice Test
Practice Test — 20 Questions
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