Example 1: rate of cooling
Tea cools from 85 °C to 70 °C in 6 minutes. Calculate the mean rate of cooling.
- Temperature fall = 85 − 70 = 15 °C.
- Rate = 15 ÷ 6 = 2.5 °C per minute.
- The rate will fall as the tea approaches room temperature.
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IB DemystifiedMYP Sciences
Thermal energy transfer
A metal bench feels colder than a wooden one on the same morning. A sea breeze arrives on a hot afternoon. A white roof keeps a room cooler. Each is thermal energy moving from hotter to colder places, by conduction, convection or radiation.
Recommended for MYP 4 · eAssessment priority · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding thermal transfer
- Conduction, convection and radiation
- Thermal transfer in the real world
- Worked examples
- In the eAssessment
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- explain conduction in metals and non-metals using particles and free electrons
- explain convection currents using changes in density
- explain how surfaces absorb, reflect and emit infrared radiation
- explain how insulation and everyday designs reduce energy transfer
- analyse cooling data and calculate rates and payback times
- evaluate energy-saving measures and their impacts
Before you start
You will use these skills. If any feel shaky, review them first.
- the particle model of solids, liquids and gases (see States of matter)
- density (see Density)
- the electromagnetic spectrum, including infrared (see EM spectrum)
Key vocabulary
- Conduction
- Transfer of thermal energy through a material by particle vibrations and, in metals, free electrons.
- Convection
- Transfer of thermal energy by the movement of a fluid, as warmer, less dense parts rise and cooler, denser parts sink.
- Infrared radiation
- Electromagnetic waves emitted by all objects; hotter objects emit more. It needs no medium.
- Insulator
- A material that transfers thermal energy slowly, often because it traps still air.
- Rate of cooling
- How quickly temperature falls, in °C per minute; it depends on the temperature difference with the surroundings.
- Payback time
- The time for savings to cover the cost of a measure: cost ÷ saving per year.
Understanding thermal transfer
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What is it?
Whenever there is a temperature difference, thermal energy is transferred from the hotter place to the colder one, until both reach the same temperature. The bigger the difference, the faster the transfer.
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How does it happen?
In conduction, faster-vibrating particles pass energy to their neighbours; free electrons make metals much faster. In convection, a heated fluid expands, becomes less dense and rises, while cooler fluid sinks, setting up a current. In radiation, every surface emits infrared; matt black surfaces are the best absorbers and emitters, and shiny surfaces the worst.
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How do we know?
Cooling curves, infrared sensors and thermal cameras let us measure energy transfer directly. In the 1800s, John Leslie used a cube with differently finished sides to show that surfaces at the same temperature emit different amounts of radiation.
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Why does it matter?
Heating and cooling buildings uses a large share of the world's energy. Understanding thermal transfer helps us design insulated homes, efficient engines and cooking equipment, and clothing for extreme climates.
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What does it connect to?
Thermal transfer links to specific heat capacity, the particle model and infrared in physics; to temperature control in animals and plants in biology; and to weather, ocean currents and climate in Earth science.
Conduction, convection and radiation
- Conduction is fastest in metals and slow in non-metals and gases. Still air is an excellent insulator, which is why wool, foam, feathers and double glazing all trap air.
- Convection happens only in liquids and gases. Heaters go low in a room; cooling units go high in a fridge.
- Radiation needs no medium. Matt black: best absorber and emitter. Shiny silver: worst absorber and emitter, and best reflector.
A vacuum flask combines all three ideas: a vacuum stops conduction and convection, silvered walls reduce radiation, and a stopper stops convection from the top.
Thermal transfer in the real world
In hot cities, people use thick walls, courtyards, wind towers and light-coloured surfaces to keep homes cool without electricity. In cold countries, loft and wall insulation are among the cheapest ways to cut household energy use. Thermal cameras reveal where buildings leak energy, guiding repairs.
Worked examples
Example 2: explaining a design
Explain why solar water heaters on roofs have matt black pipes behind glass.
- Matt black surfaces absorb the most radiation from the Sun.
- The glass traps a layer of air, reducing convection and conduction losses to the outside.
- So more of the absorbed energy heats the water.
In the eAssessment
Thermal transfer questions usually ask you to name the method and explain it with particles. Expect:
- Explain designs (flasks, pans, homes, clothing) by matching each feature to a method of transfer.
- Analyse cooling data: temperature falls, rates, comparisons and anomalies.
- Calculate payback times and energy costs.
- Discuss energy-saving measures, weighing cost, comfort and emissions.
Common ways to lose marks: writing "heat rises" instead of "warm air rises because it is less dense"; saying particles get lighter or expand when heated; saying insulators produce heat; and saying metals are "naturally colder".
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: how many layers of insulation?
Partially guided investigation · about 50 minutes · pairs
- Research question
- How does the number of layers of paper towel (0, 1, 2, 3, 4) wrapped round a cup affect the temperature fall of 100 cm³ of hot water in 10 minutes?
- Scientific background
- Layers of paper trap air, a poor conductor, reducing energy transfer by conduction and convection. The rate of energy loss also depends on the temperature difference with the room.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Five identical paper or plastic cups, paper towels, sticky tape, card lids with a hole, thermometers or temperature probes, measuring cylinder, kettle, stopwatch.
- Method
- Wrap each cup in a different number of layers and fix with tape.
- Pour 100 cm³ of hot water into each cup, add a lid and thermometer, and record the starting temperature.
- Record the temperature every 2 minutes for 10 minutes.
- Repeat and calculate the mean temperature fall for each number of layers.
Safety. Use water no hotter than about 70 °C, carry it carefully, and stand cups on a tray to catch spills. Cool any scald under running cold water for at least 10 minutes and tell your teacher.
Then evaluate: does each extra layer make the same difference?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
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