Thermal processes explain everyday physics: why a metal spoon left in hot soup becomes too hot to hold, why a sea breeze blows onshore in the afternoon, and how the Sun warms the Earth across about 150 million kilometres of empty space. The whole topic rests on one rule. Net thermal energy always flows from the region of higher temperature to the region of lower temperature until thermal equilibrium is reached.
These notes cover the three mechanisms of energy transfer by heating: conduction, convection and radiation. They also give the exact chains of reasoning that mark schemes reward in explain-questions. The topic leads directly into Topic 9 (Thermal Properties of Matter).
What these notes cover
- •Direction of energy transfer
- •Heat vs temperature
- •Thermal equilibrium
- •Conduction: lattice vibration and free electrons
- •Wax-and-pins demonstration
- •Everyday conductors and insulators
- •Convection and the density-change chain
- •Placement logic: coolers high, heaters low
- •Radiation and the three surface factors
- •Dull black vs shiny silver plate demonstration
- •Applications: vacuum flask, sea breeze, heat sinks
- •Common mistakes and exam technique
Key definitions
- Thermal equilibrium
- Two bodies are in thermal equilibrium when they are at the same temperature, so there is no net transfer of thermal energy between them. Particles still exchange energy, but the flows in each direction balance exactly.
- Heat (energy transferred by heating)
- Energy in transit, measured in joules (J), between regions at different temperatures. Objects contain internal energy, not heat.
- Conduction
- The transfer of thermal energy through a medium, from a region of higher temperature to a region of lower temperature, without any net movement of the medium’s particles.
- Convection
- The transfer of thermal energy by the movement of the heated fluid itself. It occurs only in liquids and gases, never in solids or a vacuum.
- Radiation
- The transfer of thermal energy by infrared electromagnetic waves. It needs no medium and is the only mechanism that works through a vacuum.
Key formulas
Direction of energy transfer by heating
net thermal energy flows from T(higher) → T(lower), until T(higher) = T(lower)
The direction depends only on which region is at the higher temperature, never on which object stores more internal energy. When the temperatures are equal, the net transfer is zero (thermal equilibrium).
Density
ρ = m ÷ V
ρ is density, m is mass and V is volume. When a fluid is heated, m stays the same and V increases, so ρ decreases.
Convection chain (heating)
fluid heated → expands → density decreases → rises
Write the steps in this exact order. Cooler, denser fluid sinks to replace the rising fluid, which forms a convection current.
Convection chain (cooling)
cooled → contracts → denser → sinks
Use this chain for cooling devices, such as an air-conditioner near the ceiling or a freezer compartment at the top of a refrigerator.
Worked example
On a hot afternoon at East Coast Park, a steady breeze blows from the sea towards the land. Explain, in terms of density changes, how this sea breeze forms. State and explain what happens at night.
- 1During the day, the land heats up faster than the sea, because less energy is needed to raise the temperature of the land than of the same mass of water.
- 2Air in contact with the warm land is heated by conduction. The air expands, so the same mass of air occupies a larger volume and its density decreases.
- 3The less dense air over the land rises.
- 4Cooler, denser air from over the sea moves in to replace the rising air. This horizontal flow from sea to land is the sea breeze, and it completes a convection current.
- 5At night, the land cools down faster than the sea. The air over the sea is now warmer and less dense, so it rises. Cooler, denser air from over the land flows out to sea to replace it, and this is a land breeze.
Watch out: Writing “hot air rises” on its own scores poorly. Show that the air is heated, expands and becomes less dense before it rises, and say what replaces it.
Common mistakes
Writing “heat rises”
Wrong: Heat rises, so the top of the room is warmer.
Right: The heated air rises because it expands and becomes less dense. Thermal energy itself has no preferred direction, and conduction and radiation can transfer energy in any direction, including downwards.
Thinking shiny surfaces radiate best
Wrong: The shiny silver surface radiates energy best because it looks bright.
Right: A dull black surface is the best emitter and absorber of infrared radiation. A shiny silver surface is a poor emitter and a poor absorber, but a good reflector.
Saying particles travel along the rod
Wrong: In conduction, the hot particles move along the rod to the cold end.
Right: Particles vibrate about fixed positions and pass energy to their neighbours through collisions. Only the free electrons in a metal actually move through the lattice.
Questions students ask
What is the difference between heat and temperature?
Temperature (in °C or K) measures how hot a body is and tells you the direction energy will flow. Heat, more precisely called energy transferred by heating, is energy in transit (in joules) between regions at different temperatures. Objects contain internal energy, not heat, so write “at a higher temperature” rather than “has more heat”.
Why does a metal bench feel colder than a wooden bench?
Both benches are in thermal equilibrium with the surroundings, so they are at the same temperature. Metal is a better conductor, and its free electrons carry thermal energy away from your hand at a faster rate. Your sense of touch detects the rate of energy transfer, not temperature.
How does a vacuum flask keep drinks hot?
The vacuum between the double walls contains no particles, so it prevents conduction and convection, which both need a medium. The silvered surfaces are poor emitters and good reflectors of infrared radiation, so they reflect radiation back inwards. The plastic stopper is a poor conductor and seals the mouth, which stops convection and evaporation.
Why is an air-conditioner placed high on the wall?
Air near the unit is cooled, contracts, becomes denser and sinks. Warmer, less dense air below rises towards the unit and is cooled in turn. This sets up a convection current that circulates cool air around the whole room; if the unit were near the floor, the cold, dense air would simply pool at ankle level.
In the full PDF
- •Labelled diagrams: cooling-to-equilibrium graph, wax-and-pins rod, convection current in a beaker, black and silver plates, vacuum flask cross-section
- •Further worked examples on conduction, convection and radiation, including heat-sink fins, the marathon foil blanket and the kettle element
- •Word-for-word exam-ready paragraphs for conduction by lattice vibration and by free electrons
- •The remaining misconceptions that cost marks, with corrections
- •Five-habit exam-technique checklist with weak vs full-mark answers
- •Summary table comparing conduction, convection and radiation, plus the answering chains to memorise