Every explanation in this topic begins with the same idea: thermal energy flows from a hotter region to a colder region. Thermal energy travels by conduction, convection and radiation. When an object gains or loses heat, there are three possible effects: a temperature change, a change of state, or expansion and contraction.
Getting heat, temperature and thermal equilibrium precise now will carry you through Sec 3/4 Thermal Physics. The same ideas explain everyday puzzles, from why a vacuum flask keeps Milo hot to why power lines sag on a hot afternoon. Schools test them mostly through short explain questions, and these are marked against fixed chains of reasoning.
What these notes cover
- •Heat, temperature and the particle picture
- •Direction of heat flow and thermal equilibrium
- •Conduction in solids and free electrons in metals
- •Conductors and insulators in daily life
- •Convection currents in fluids
- •Sea breeze, land breeze and air-conditioner placement
- •Radiation and the three factors affecting its rate
- •The vacuum flask and other everyday applications
- •Greenhouses and the parked car
- •Effects of heat gain and loss: temperature change and change of state
- •Expansion and contraction: railway gaps, power lines and jar lids
- •The bimetallic strip in fire alarms and thermostats
Key definitions
- Heat
- Thermal energy transferred from a hotter body to a colder body. SI unit: joule (J).
- Temperature
- A measure of the degree of hotness of a body — qualitatively, the average kinetic energy of its particles. Unit: degree Celsius (°C).
- Conduction
- The transfer of thermal energy through a medium, from a region of higher temperature to a region of lower temperature, without any flow of the medium itself.
- Convection
- The transfer of heat energy by the bulk movement of a fluid, caused by differences in density within the fluid. It occurs only in liquids and gases.
- Radiation
- The transfer of thermal energy by means of infrared waves, a type of electromagnetic wave. It does not require a medium and can pass through a vacuum.
Key formulas
Density
ρ = m ÷ V
ρ is density, m is mass and V is volume. When a fluid is heated it expands, so V increases while m is unchanged. Its density therefore decreases, which is why heated fluid rises in a convection current.
Worked example
A vacuum flask keeps hot drinks hot and cold drinks cold. Explain how, naming each feature of the flask and the heat-transfer process it reduces.
- 1The vacuum between the double walls contains no particles (no medium), so heat transfer by conduction and convection through the walls cannot occur.
- 2The silvered shiny surfaces are poor emitters and good reflectors of infrared radiation, so heat transfer by radiation across the gap is greatly reduced.
- 3The cork or plastic stopper is a poor conductor, which reduces conduction through the top; it also seals the flask so warm air cannot escape, which prevents convection at the mouth.
- 4These features reduce heat transfer in both directions: heat cannot leak out when the contents are hotter than the surroundings, and it cannot leak in when the contents are colder. The flask does not create heat or cold; it only slows the transfer.
Watch out: Do not write that the vacuum stops all heat transfer. Radiation needs no medium and passes straight through a vacuum. The vacuum stops conduction and convection only; the silvered surfaces deal with radiation.
Common mistakes
Mixing up heat and temperature
Wrong: Heat is measured in °C.
Right: Heat is energy and is measured in joules (J). °C measures temperature.
Skipping the density link in convection
Wrong: Hot water rises.
Right: The water near the heat source gains heat, expands, becomes less dense and rises. Cooler, denser water sinks to replace it, forming a convection current.
Saying the particles expand
Wrong: When heated, the particles expand and get bigger.
Right: The particles stay the same size. They vibrate more vigorously and move slightly further apart, so the spacing between them increases.
Questions students ask
What is the difference between heat and temperature?
Heat is thermal energy transferred from a hotter body to a colder body, and it is measured in joules (J). Temperature is a measure of the degree of hotness of a body, which is the average kinetic energy of its particles, and it is measured in °C. A bathtub of warm water stores far more thermal energy than a teaspoon of boiling water, even though the teaspoon has the higher temperature.
Why are metals better conductors of heat than non-metals?
Metals contain free electrons. At the heated end, these electrons gain kinetic energy, travel quickly through the metal and collide with atoms and other electrons in cooler regions. This transfers thermal energy much faster than particle vibrations alone. When comparing a metal with a non-metal, you must mention free electrons to earn full marks.
Why is an air-conditioner placed high on the wall?
The cold air from the unit is denser than the warm air in the room, so it sinks. Warmer, less dense air near the floor rises towards the ceiling, where it is cooled in turn. This sets up a convection current that cools the whole room. For the same reason, a heater is placed low, near the floor.
Why does a bimetallic strip bend when heated?
A bimetallic strip is made of two metals, such as brass and iron, joined together. For the same temperature rise, brass expands more than iron, so the strip bends with brass on the outside of the curve, because the outside of a curve is longer. In a fire alarm, the bending strip touches a contact and completes the circuit, so the bell rings.
In the full PDF
- •15 further worked examples with model answers and B1 mark points, including black and silver cans, the parked car, railway gaps and the stubborn jar lid
- •Labelled diagrams of conduction along a rod, a convection current in a beaker, a vacuum flask cross-section and a bimetallic strip
- •Temperature–time graphs for thermal equilibrium and for cooling cans with different surfaces
- •The full table of the three factors affecting the rate of radiation, with everyday examples
- •The remaining common mistakes and phrases that lose marks in every section
- •End-of-section summaries for all six sections