Chapter 10: Transfer of Heat Energy and its Effects (Summary Sheet)
Created by Miss Clarissa Ng | www.clartutors.com
Part A · Temperature, thermal energy and expansion
1 Temperature and Thermal Energy
| Temperature | Thermal energy | |
|---|---|---|
| What it measures | The average kinetic energy of the particles in an object | The total kinetic energy of all the particles in an object |
| Unit | kelvin (K), the SI unit; also °C. K = °C + 273 | joule (J) |
| Depends on | How fast the particles move | Temperature and mass (number of particles) |
| Compare | A beaker and a basin of water at 80 °C have the same temperature | The basin has more thermal energy — it holds more water particles |
- Thermal energy always flows from a hotter region to a cooler one.
- It stops when both objects reach the same temperature — thermal equilibrium, with no net transfer of thermal energy.
2 Expansion and Contraction
- Expansion: matter increases in size when thermal energy is supplied.
- Contraction: matter decreases in size when thermal energy is lost.
- Heating makes the particles vibrate or move faster and over larger distances, so they sit further apart.
- The volume changes; the mass and the number of particles do not.
- Bigger volume with the same mass means lower density (density = mass ÷ volume).
- For the same temperature change: gas expands most, then liquid, then solid.
3 Expansion in Daily Life: Problems and Uses
| What is done, and why | |
|---|---|
| Railway tracks | Gaps are left between rails so the rails can expand in hot weather without buckling or cracking. |
| Bridges and girders | One end is fixed and the other rests on rollers, so the structure can expand and contract freely. |
| Telephone wires | Hung loosely between poles so they do not snap when they contract in cold weather. |
| Water and gas pipes | Built with large bends so the pipes can expand or contract without cracking. |
| Rivets | A red-hot rivet is hammered flat, then contracts as it cools and pulls the metal plates tightly together. |
| Fitting an axle | The axle is cooled so it contracts, then slotted into the hub; it expands again at room temperature and fits tightly. |
| Hot air balloon | Heated air expands and becomes less dense than the air outside, giving an upward force that lifts the balloon. |
- When matter is prevented from expanding or contracting, it exerts a very large force — which is why these designs matter.
4 The Bimetallic Strip
- Two strips of different metals riveted together (for example copper and iron).
- They expand by different amounts when heated.
- Heated: the strip bends with the metal that expands more on the outside of the curve (copper outside, iron inside).
- Cooled: it bends the other way, with that same metal on the inside of the curve.
- Used in thermostats (ovens, electric irons, refrigerators) and fire alarms.
| Device | How the strip works it |
|---|---|
| Oven thermostat | Too hot → the strip bends and breaks the circuit, so heating stops. As it cools the strip straightens, closes the circuit and heating starts again. |
| Fire alarm | The strip is straight at room temperature, so the contacts are apart. Heat from a fire makes it bend until the contacts touch and complete the circuit — the alarm sounds. |
| Cooling device | The circuit is closed when warm. As the temperature falls the strip bends away, opening the circuit and switching the cooling off. |
Part B · The three ways thermal energy travels
5 Conduction
- Thermal energy passes through a medium from particle to particle, with no bulk movement of the medium.
- Heated particles vibrate more vigorously and collide with their cooler neighbours, handing on the energy.
- Solids conduct best (particles are packed close, so collisions are frequent); gases conduct worst (particles are far apart).
- Metals are the best conductors of all — energy travels by vibrating ions and by fast-moving free electrons.
| Good conductors | Poor conductors (insulators) | Why it matters |
|---|---|---|
| Copper, aluminium, silver, iron (metals) | Wood, plastic, rubber, glass, water, air | Metal pots and pans transfer energy quickly to the food; wooden or plastic handles stay cool in your hand; fur traps air, which insulates an animal. |
6 Convection
- Thermal energy is carried by the bulk movement of a fluid (liquid or gas) — a convection current.
- It is driven by density differences: heated fluid expands, becomes less dense and rises; cooler, denser fluid sinks to take its place, and the cycle repeats.
- It cannot happen in solids — their particles can only vibrate about fixed positions.
- Gases convect more readily than liquids, because a gas expands more for the same rise in temperature.
| Application | How it works |
|---|---|
| Air-conditioner | Mounted near the ceiling: air near it is cooled, becomes denser and sinks, pushing warm air up to be cooled in turn. |
| Heater | Placed near the floor: air near it is heated, becomes less dense and rises, while cooler air sinks to be heated in turn. |
| Sea breeze (day) | Land heats up faster than the sea, so air rises over the warmer land and cooler air moves in from the sea. |
| Land breeze (night) | Land cools down faster than the sea, so air rises over the warmer sea and cooler air moves in from the land. |
7 Radiation
- Thermal energy travels as electromagnetic waves, mainly infrared.
- It is the only process that needs no medium — it crosses a vacuum, which is how energy reaches us from the Sun.
- Every object emits and absorbs radiation; the hotter the object, the faster it emits.
| Factor | Effect on the rate of heat loss or gain |
|---|---|
| Surface temperature | The hotter the surface, the faster it emits radiation. |
| Surface area | The larger the surface area, the faster the emission. |
| Colour and texture | Dull, rough, dark surfaces are the best emitters and absorbers. Shiny, smooth, light surfaces are poor emitters and absorbers — they reflect radiation. |
- Applications: radiant heaters (hot bars emit infrared), solar water heaters and panels (black surfaces absorb best), black cooling fins on engines, white or silver surfaces on roofs and tanks to stay cool.
8 The Three Processes Side by Side
| Conduction | Convection | Radiation | |
|---|---|---|---|
| How energy moves | Particle to particle, by collisions | Bulk movement of a fluid | Electromagnetic (infrared) waves |
| Needs a medium? | Yes | Yes — liquid or gas only | No — crosses a vacuum |
| Best in | Solids, especially metals | Gases | Dull, dark surfaces |
| Everyday example | Metal spoon left in hot soup | Sea breeze; water heated from below | Warmth from the Sun |
Part C · Applications and effects
9 The Vacuum Flask
- It slows down all three processes at once.
| Feature | What it stops |
|---|---|
| Vacuum between the walls | Conduction and convection — with no particles in the gap there is nothing to pass energy along or to flow. |
| Silvered walls | Radiation — the shiny coating reflects infrared back towards the liquid. |
| Plastic cap | Cuts conduction and convection through the open top, because plastic conducts poorly. |
| Plastic supports | Hold the inner glass steady while conducting as little energy as possible. |
Exam tip: name all three processes and match each one to the feature that blocks it. That is where the marks are.
10 Climate Change
- Greenhouse gases such as carbon dioxide and methane absorb infrared radiation leaving the Earth and re-radiate some of it back, keeping the planet warm — the greenhouse effect.
- Adding more of these gases strengthens the effect, and the extra trapped energy shows up as global warming and climate change.
| Man-made causes | Natural causes |
|---|---|
| Burning fossil fuels; deforestation; industrial activity; livestock and landfills releasing methane | Volcanic eruptions; changes in the Sun's activity; natural changes in the Earth's orbit |
- Effects: melting ice caps and glaciers, rising sea levels (seawater also expands as it warms), more extreme weather, and damage to coral reefs.
11 Say It Like This
| Question asks about | Phrase that earns the mark |
|---|---|
| Heat transfer generally | Thermal energy is transferred from a region of higher temperature to a region of lower temperature until both reach the same temperature. |
| Expansion | The particles vibrate faster over larger distances about their fixed positions, so they are further apart; the volume increases but the mass stays the same. |
| Conduction | Particles at the heated end vibrate more vigorously and collide with neighbouring particles, passing the energy along. |
| Convection | The fluid gains thermal energy, expands, becomes less dense and rises; cooler, denser fluid sinks to take its place, setting up a convection current. |
| Radiation | Dull, dark, rough surfaces are the best absorbers and emitters of infrared radiation; shiny, light surfaces reflect it. |
| Vacuum flask | The vacuum stops conduction and convection, and the silvered walls stop radiation. |