# Energy Transfer by Heating — GCSE Physics
Energy is always transferred from hotter objects to cooler ones. This can happen through conduction, convection, and radiation. Understanding these mechanisms is crucial for explaining insulation, energy efficiency, and reducing energy waste in homes.
1. Three Methods of Heat Transfer
1.1 Conduction
Conduction is the transfer of thermal energy through a material by the vibration of particles, without the particles themselves moving from place to place.
How it works:
- Particles at the hot end vibrate faster (more kinetic energy)
- They collide with neighbouring particles, transferring energy
- Energy passes along from particle to particle through the material
In metals: Conduction is especially fast because metals have free (delocalised) electrons that can move through the structure, carrying energy quickly. This is why metals are good conductors.
Key facts:
- Occurs mainly in solids (particles are close together)
- Metals are excellent conductors (free electrons)
- Non-metals, liquids, and gases are generally poor conductors (insulators)
- Air is a very poor conductor — this is why trapped air is used for insulation
1.2 Convection
Convection is the transfer of thermal energy by the movement of particles in a fluid (liquid or gas).
How it works (convection current):
- Fluid near the heat source is heated → particles move faster → fluid expands → becomes less dense
- The less dense hot fluid rises
- Cooler, denser fluid moves in to replace it
- The cooler fluid is then heated, and the cycle continues
- This creates a convection current
Key facts:
- Only occurs in fluids (liquids and gases)
- Does NOT occur in solids (particles are fixed)
- Does NOT occur in a vacuum (no particles)
- Examples: sea breezes, central heating radiators, weather systems
1.3 Radiation (Infrared)
Thermal radiation is the transfer of energy by infrared (IR) electromagnetic waves.
Key facts:
- Does NOT need particles — can travel through a vacuum
- This is how the Sun's energy reaches Earth
- All objects emit and absorb thermal radiation
- Hotter objects emit more radiation
- Dark, matt surfaces are the best emitters and absorbers of IR
- Light, shiny surfaces are the best reflectors (worst absorbers) of IR
| Surface | Absorption | Emission | Reflection |
|---|---|---|---|
| Dark, matt (black) | Best | Best | Worst |
| Light, shiny (white/silver) | Worst | Worst | Best |
2. Reducing Unwanted Energy Transfers
In Homes
| Method | How It Reduces Heat Loss | Transfer Prevented |
|---|---|---|
| Cavity wall insulation | Foam or mineral wool fills the gap, trapping air and preventing convection | Convection, conduction |
| Loft insulation | Fibreglass or wool traps air between fibres | Conduction, convection |
| Double glazing | Two glass panes with trapped air/gas between them | Conduction, convection |
| Draught excluders | Seals gaps around doors and windows | Convection |
| Foil behind radiators | Reflects infrared radiation back into the room | Radiation |
| Thick curtains | Trap a layer of air; reduce radiation through windows | Conduction, radiation |
| Carpets/rugs | Insulate the floor; trap air in fibres | Conduction |
The Role of Trapped Air
Air is an excellent insulator (poor conductor) because its particles are far apart and don't transfer energy efficiently. Many insulation methods work by trapping air in small pockets to prevent convection currents from forming.
3. Rate of Energy Transfer
The rate of cooling (energy transfer) depends on:
- Temperature difference — the greater the difference between the object and surroundings, the faster the energy transfer
- Surface area — larger surface area means faster energy transfer
- Volume — smaller objects cool faster (higher surface-area-to-volume ratio)
- Material/surface — dark matt surfaces emit faster; insulating materials slow transfer
Newton's Law of Cooling: The rate of heat loss is approximately proportional to the temperature difference between the object and its surroundings.
Worked Example: Example 1
Question: Explain why a metal spoon left in a hot drink becomes hot but a wooden spoon does not.
Metal is a good conductor of heat because it contains free electrons that transfer kinetic energy quickly. Wood is a poor conductor (insulator) — energy transfers slowly through vibrating particles, so the handle stays cool.
Worked Example: Example 2
Question: Explain why a radiator heats a room by convection.
The radiator heats the air nearby. This air expands, becomes less dense, and rises. Cooler, denser air moves in to replace it and is heated in turn. This creates a convection current that circulates warm air around the room.
Worked Example: Example 3
Question: Explain why cavity wall insulation reduces heat loss from a house.
Cavity wall insulation fills the gap between the inner and outer walls with foam or mineral wool. This:
- Traps air in small pockets, preventing convection currents from forming in the cavity
- The insulating material and trapped air are poor conductors, reducing conduction through the wall
- Overall, less thermal energy is transferred from the warm inside to the cold outside
Worked Example: Example 4
Question: A thermos flask keeps drinks hot (or cold) for a long time. Explain how its design reduces each type of heat transfer.
- Vacuum between walls: Prevents conduction and convection (no particles to transfer energy)
- Silvered (shiny) surfaces: Reduce radiation (reflect IR radiation back towards the drink)
- Insulated stopper: Reduces conduction and convection through the top
- Plastic outer casing: Poor conductor, reduces conduction
5. Practice Questions
- Explain the difference between conduction and convection. (4 marks)
- Why is copper used for the base of saucepans? (2 marks)
- Explain why double glazing reduces heat loss through windows. (3 marks)
- A black car and a white car of the same model are parked in the sun. Which heats up faster? Explain why. (3 marks)
- Explain how a convection current forms in a pan of water being heated from below. (4 marks)
Answers
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Frequently Asked Questions
Does a radiator mainly heat by radiation?
Despite the name, domestic radiators mainly heat rooms by convection (warming air that rises and circulates). Some heat is transferred by radiation, but convection is the dominant mechanism.
Can conduction occur in liquids and gases?
Yes, but it's very slow because particles are further apart. In fluids, convection is usually the dominant mechanism.
Why does a vacuum prevent conduction and convection but not radiation?
Conduction and convection require particles to transfer energy. A vacuum has no particles. Radiation travels as electromagnetic waves, which don't need a medium — they can travel through a vacuum.
Summary
- Conduction: Energy transfer through vibrating particles; fastest in metals (free electrons)
- Convection: Energy transfer by movement of heated fluid; creates convection currents
- Radiation: Energy transfer by infrared electromagnetic waves; doesn't need particles
- Dark, matt surfaces absorb/emit best; light, shiny surfaces reflect best
- Insulation reduces heat loss by trapping air and preventing convection
- Rate of cooling depends on temperature difference, surface area, and material