Chapter 10: Transfer of Heat Energy and its Effects
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 |
| SI unit | kelvin (K), where K = °C + 273; the degree Celsius (°C) is also used | joule (J) |
| What it depends on | How fast the particles are moving | The temperature and the mass of the object |
- A cup of water at 80 °C and a bathtub at 80 °C are at the same temperature.
- The bathtub holds far more thermal energy, because it contains a much larger mass of water and therefore many more particles.
- An object at a lower temperature can still hold more thermal energy than a hotter one, if its mass is large enough.
- When a hot object touches a cold one, thermal energy is transferred from the hotter object to the cooler object.
- The transfer stops once both reach the same temperature: they are in thermal equilibrium, with no net transfer of thermal energy between them.
- This one rule underlies everything in this chapter — conduction, convection and radiation are three ways of reaching it.
2 Effects of Gaining and Losing Thermal Energy
- Gaining thermal energy can either raise the temperature of a substance, or bring about a change of state.
- Losing thermal energy can either lower the temperature, or bring about the reverse change of state.
| Change of state | On gaining thermal energy | On losing thermal energy |
|---|---|---|
| Solid ⇄ liquid | Melting — the particles vibrate so strongly that they break free of their fixed positions and slide past one another | Freezing — the particles slow down and settle into an orderly arrangement |
| Liquid ⇄ gas | Boiling — the particles move fast and far apart, escaping from the liquid | Condensation — the particles slow down and are pulled close together again |
| Solid ⇄ gas | Sublimation — a solid turns straight into a gas | Deposition — a gas turns straight into a solid |
- While a substance is changing state its temperature stays the same: the thermal energy supplied is being used to pull the particles apart, not to speed them up.
- That is why a heating curve is flat at the melting point and again at the boiling point.
3 Expansion and Contraction
Expansion is the increase in the size of matter when thermal energy is absorbed. Contraction is the decrease in the size of matter when thermal energy is given out.
- Solids, liquids and gases all expand when heated and contract when cooled.
- Particles in a solid vibrate about fixed positions. Given thermal energy they vibrate faster and over larger distances, so they sit further apart and the body takes up more space.
- Cooling reverses this: the particles vibrate more slowly and through smaller distances, so they move closer together.
- The volume changes. The mass and the number of particles do not.
- Since density = mass ÷ volume, a larger volume with the same mass gives a lower density.
- For the same rise in temperature: gases expand the most, then liquids, then solids — the particles in a gas are already far apart and free to move.
4 When Expansion Is a Problem
- If matter is prevented from expanding or contracting it exerts a very large force — easily enough to distort a metal rail or crack concrete.
- So designers deliberately leave room for expansion and contraction, and the force never builds up.
| Structure | What is done about it |
|---|---|
| Railway tracks | Small gaps are left between the rails, so that expansion on a hot day cannot distort or crack the track. |
| Bridges and girders | One end is fixed and the other rests on rollers, so the structure can expand and contract freely. |
| Overhead wires | Hung loosely between the poles, so that contraction on a cold night cannot snap them. |
| Water and gas pipes | Laid with large bends, so the pipe can expand or contract without cracking. |
| Concrete roads | Cast in slabs with gaps between them, so the slabs do not push against each other and crack. |
5 When Expansion Does Useful Work
| Application | How expansion or contraction is used |
|---|---|
| Rivets | A rivet is made red-hot, pushed through the holes in the plates and hammered flat. As it cools it contracts, and the contracting force pulls the plates tightly together. |
| Fitting an axle | The axle is cooled in a cold mixture so that it contracts, then slotted into the hub. As it warms back to room temperature it expands and grips the hub tightly. |
| A stuck jar lid | Hot water is run over the metal lid. The metal expands more than the glass, so the lid loosens and can be unscrewed. |
| Hot air balloon | The burner heats the air inside the balloon. The air expands and becomes less dense than the air outside, and the difference in density gives an upward force that lifts the balloon. |
| Liquid-in-glass thermometer | The liquid inside expands up the narrow tube as it is heated, so the length of the column shows the temperature. The liquid expands uniformly, so equal rises in temperature give equal rises in the column. |
6 Negative Thermal Expansion: Water Below 4 °C
- Nearly all substances contract as they cool. Water is the exception: below about 4 °C it expands as it cools further.
- So water is most dense at 4 °C, and less dense both above and below that temperature.
- In winter the surface of a pond cools first. Once it reaches 4 °C, further cooling makes it expand and become less dense, so it stays at the top and freezes there.
- Meanwhile the water below stays at 4 °C — the densest water sinks — so the pond freezes from the top down, never from the bottom up.
- This is why fish and water plants survive the winter in the water beneath the ice.
Part B · The bimetallic strip
7 The Bimetallic Strip
- A bimetallic strip is made by riveting together two strips of different metals — for example copper and iron.
- The two metals expand by different amounts for the same rise in temperature.
- Heated: the strip bends with the metal that expands more on the outside of the curve (so copper on the outside, iron inside).
- Cooled: the strip bends the other way, with the metal that contracts more on the inside of the curve.
- The bending makes or breaks an electrical contact, which is what makes the strip useful in automatic devices.
| Device | How the strip works it |
|---|---|
| Oven thermostat | The circuit is closed while the oven is below the set temperature. Once it gets too hot the strip bends and opens the circuit, so heating stops; as the oven cools the strip straightens and closes the circuit again. The temperature stays steady. |
| Fire alarm | The contacts are apart at room temperature, so the circuit is open. The heat of a fire bends the strip until the contacts touch, completing the circuit, and the bell rings. |
| Refrigerator and air-conditioner | The circuit is closed while it is warm. As the temperature falls the strip bends away and opens the circuit, switching the cooling off before things freeze. |
Exam tip: always name the metal on the outside of the curve and say why — it is the metal that expands more than the other. A working strip also needs two metals with different rates of expansion.
Part C · The three ways thermal energy travels
8 Conduction
Conduction is the transfer of thermal energy through a medium, from particle to particle, without any bulk movement of the medium itself.
- Particles at the heated end gain thermal energy and vibrate more vigorously. They collide with their cooler neighbours and hand the vibrational energy on, and the process repeats all along the material.
- No particle travels from the hot end to the cold end: the energy moves, the particles stay put.
- Eventually the whole object reaches the same temperature — thermal equilibrium.
- Solids conduct best, because their particles are packed closely and collide frequently. Gases conduct worst, because their particles are far apart.
- Metals conduct far better than other solids, because energy travels two ways at once: the vibrating ions pass it along, and the metal's free electrons carry it rapidly through the lattice.
- Thermal conductivity measures how well a material conducts thermal energy. The higher the value, the better the conductor.
| Material | Thermal conductivity (W m−1 K−1) | Conductor or insulator |
|---|---|---|
| Copper | about 400 | Very good conductor |
| Aluminium | about 205 | Good conductor |
| Iron | about 80 | Good conductor |
| Glass | about 0.8 | Poor conductor |
| Water | about 0.6 | Poor conductor |
| Air | about 0.025 | Very poor conductor — the best insulator of all |
- Different materials transfer thermal energy at different rates, which is why a metal spoon in hot soup burns your hand but a wooden one does not.
- Good conductors are chosen when thermal energy must move quickly: copper or aluminium bases on pots and pans, metal parts of an engine that must lose heat.
- Poor conductors are chosen to slow thermal energy down: wooden or plastic handles on pans and tools, a double-glazed window with an air gap, loft insulation, and the fur or feathers of animals that trap air.
9 Convection
Convection is the transfer of thermal energy from one place to another by the bulk movement of a fluid (a liquid or a gas).
- The moving fluid is called a convection current, and it is driven by differences in density.
- Fluid near the heat source gains thermal energy, expands, becomes less dense and rises.
- Cooler, denser fluid is pushed aside and sinks to take its place, where it is heated in turn. The cycle repeats for as long as the heating continues.
- Convection cannot happen in solids: their particles can only vibrate about fixed positions, so the medium cannot flow.
- Gases convect more readily than liquids, because a gas expands more for the same rise in temperature and so loses more density.
- In a beaker warmed from below, a few coloured crystals show the current: coloured water rises from the bottom, spreads out, and sinks down the sides until the whole beaker is evenly coloured.
- In a diagram, draw the arrows rising vertically from the heat source, then curving outwards and sinking down the sides.
| Application | How it works |
|---|---|
| Air-conditioner | Mounted high on the wall, near the ceiling: the air it cools becomes denser and sinks, pushing warmer air up towards the unit to be cooled in turn. |
| Heater | Placed low, near the floor: the air it heats becomes less dense and rises, while cooler air sinks down to be heated in turn. |
| Freezer compartment | At the top of a refrigerator, so that the air it cools sinks down over the food below. |
| Sea breeze and land breeze | Land heats up and cools down faster than the sea. By day, air rises over the warmer land and cooler air moves in from the sea; at night the reverse happens. |
| Hot water system | Water heated in the boiler expands, becomes less dense and rises into the tank; colder water from the tank sinks down to the boiler to be heated. |
10 Radiation: Emission and Absorption
Radiation is the transfer of thermal energy by electromagnetic waves, mainly infrared.
- It is the only one of the three processes that needs no medium. It crosses a vacuum, which is how thermal energy reaches the Earth from the Sun.
- Every object emits and absorbs infrared radiation all the time. An object that emits radiation loses thermal energy and its temperature falls; one that absorbs radiation gains thermal energy and its temperature rises.
- The hotter the object, the faster it emits.
| Factor | Effect on the rate of heat loss or gain |
|---|---|
| Surface temperature | The higher the temperature of the surface, the faster it emits radiation. A hot iron soleplate radiates faster than a warm mug. |
| Surface area | The larger the surface area, the faster the object emits and absorbs radiation. Cooling fins on an engine are ridged for exactly this reason, and a shallow tray of water cools faster than the same water in a tall glass. |
| Colour and texture | Dull, rough, dark surfaces are the best emitters and the best absorbers. Shiny, smooth, light surfaces are poor emitters and poor absorbers — they reflect infrared instead. |
| Surface | Emitter | Absorber | Reflector |
|---|---|---|---|
| Dull black | Best | Best | Poorest |
| Shiny black | Good | Good | Poor |
| White | Poor | Poor | Good |
| Silvery | Poorest | Poorest | Best |
- Test it: two identical cans, one painted dull black and one shiny silver, filled with equally hot water. The black can cools faster, because its surface emits infrared faster.
- Radiant heaters: the hot bars emit infrared straight to the people in front of them, so the air between is not what warms you.
- Solar radiation: solar water heaters and solar panels have black surfaces to absorb radiation from the Sun, with shiny reflectors behind to catch more of it.
- To stay cool, surfaces are made reflective: roofs, water tanks and food coolers are painted white or silver. To lose heat fast, surfaces are made black: the radiator fins on the back of a refrigerator are painted black so that they emit faster.
11 The Three Processes Side by Side
| Conduction | Convection | Radiation | |
|---|---|---|---|
| How energy moves | Particle to particle, by collisions; no bulk movement | Bulk movement of a fluid, driven by density differences | Electromagnetic (infrared) waves |
| Needs a medium? | Yes | Yes — a liquid or a gas | No — it crosses a vacuum |
| Best in | Solids, especially metals (free electrons) | Gases, which expand the most | Dull, dark, rough surfaces |
| Everyday example | A metal spoon left in hot soup | A sea breeze, or water heated from below | Warmth reaching us from the Sun |
Part D · Applications and effects
12 All Three at Once: The Vacuum Flask
- In real situations the three processes happen together. A mug of hot soup on a wooden table loses thermal energy to the table by conduction, to the air above by convection, and in all directions by radiation.
- Some processes are less efficient than others, but all three are happening at once.
- A vacuum flask is designed to block all three, so a hot drink stays hot and a cold drink stays cold.
| Feature | Which process it stops, and why |
|---|---|
| Vacuum between the glass walls | Conduction and convection. With no particles in the gap there is nothing to pass energy along and nothing that can flow. |
| Silvered walls | Radiation. The shiny coating reflects infrared back towards the drink instead of letting it escape. |
| Plastic cap or stopper | Cuts down conduction and convection through the open top, because plastic is a poor conductor and it stops air moving. |
| Plastic supports | Hold the inner glass vessel steady while conducting as little thermal energy as possible. |
Exam tip: a vacuum flask question wants all three processes named, each matched to the feature that blocks it. Same idea for insulation: fur, feathers and Styrofoam all work by trapping air, since air is a very poor conductor.
13 Climate Change
- Greenhouse gases in the atmosphere, such as carbon dioxide, water vapour and methane, absorb the infrared radiation given out by the Earth and re-radiate some of it back towards the ground.
- This keeps the Earth warm enough for life — the greenhouse effect.
- Human activity adds more of these gases, so more infrared is trapped and the Earth's average temperature rises: global warming.
- The wider changes to temperature, rainfall, wind and sea level that follow are called climate change.
- The oceans absorb over 90% of the trapped thermal energy. As they warm they expand, and along with melting ice this raises sea level and speeds up coastal erosion.
| Man-made causes | Natural causes |
|---|---|
| Burning fossil fuels in power stations, factories and vehicles; deforestation; industrial processes; methane from livestock and landfill | Volcanic eruptions releasing carbon dioxide and dust; variations in the Sun's activity; slow natural changes in the Earth's orbit |
- Effects: polar ice and glaciers melt; sea levels rise; weather becomes more extreme; and the oceans take in more carbon dioxide and become more acidic, which damages coral reefs.
- Slowing it down means releasing less carbon dioxide: renewable energy, public transport, and planting trees that take carbon dioxide out of the air.
Exam tip: the question asks for causes that are man-made and natural — give at least one of each, and link every cause back to greenhouse gases trapping more infrared radiation.
MAPConcept Map
Chapter 10: Transfer of Heat Energy — the ideas that hold the chapter together
Why does anything warm up?
because
Thermal energy always moves from the hotter body to the cooler one, and stops when both are at the same temperature — thermal equilibrium.
Temperature or thermal energy?
different things
Temperature is the average kinetic energy of the particles (kelvin); thermal energy is the total, so it depends on mass as well.
Three routes, one destination
→
Conduction: particle to particle, no bulk movement — best in metals
Convection: the fluid itself flows, driven by density
Radiation: infrared waves, needs no medium at all
Convection: the fluid itself flows, driven by density
Radiation: infrared waves, needs no medium at all
What expansion does
→
Particles vibrate further apart → volume up → density down. Gases expand most, solids least. It can wreck structures, join them, or — in a bimetallic strip — switch a circuit.
How to slow heat transfer down
block the route
A vacuum stops conduction and convection; a shiny silver surface reflects radiation; trapped air (fur, foam, double glazing) insulates. That is the whole vacuum flask.
How to speed it up
open the route
Metal contact for conduction, and for radiation: dull dark rough surfaces, a high surface temperature and a large area — working for us in radiator fins and solar panels, against us in the climate.
Radiation in, radiation out
→ climate
More greenhouse gases → less infrared escapes → global warming → ice melts, oceans expand, sea level rises, weather turns extreme. Causes are both man-made and natural.