Thermal Processes: Complete O Level Physics Cheatsheet
O Level Physics, Chapter 9 · Read time: ~8 minutes
These notes come from TGC Academy's O Level Physics tuition in Singapore.
Key Formulas
Thermal equilibrium
Energy is transferred by heating from a region of higher temperature to a region of lower temperature.
The transfer continues until both regions are at the same temperature. They are then in thermal equilibrium, and there is no further net transfer of energy between them.
Trap: Cold does not flow. When you hold an ice cube, energy flows from your hand into the ice, which is why your hand feels cold. Always describe the energy moving from hot to cold.
Conduction
In all solids: particles at the hot end vibrate more vigorously and pass energy to neighbouring particles through collisions, so energy travels along the solid. This is slow.
In metals: there are also free (delocalised) electrons. They gain kinetic energy at the hot end and move quickly through the metal, transferring energy to particles further along. This is why metals are good conductors.
Liquids are poor conductors and gases are very poor conductors, because their particles are further apart. Materials that trap air, like wool and polystyrene, are good insulators.
Trick: For a 'why do metals conduct better' question, two marks usually need both mechanisms: lattice vibrations passed between neighbouring particles, and free electrons moving through the metal carrying energy. Mentioning only one usually caps you at one mark.
Convection
Convection happens in fluids (liquids and gases) only. When part of a fluid is heated, it expands, becomes less dense and rises. Cooler, denser fluid sinks to take its place. This sets up a convection current.
This is why a heating element sits at the bottom of a kettle and an air conditioner is mounted high on a wall: cool air from the air conditioner is denser and sinks to fill the room.
Large-scale examples include sea breezes, monsoon winds and ocean currents.
Trap: Never write 'heat rises'. It is the heated fluid that rises, because it becomes less dense. Examiners look for the chain: heated, expands, less dense, rises.
Radiation
All objects emit infrared radiation, an electromagnetic wave. It needs no medium, which is how energy from the Sun reaches the Earth through the vacuum of space.
The rate at which an object emits or absorbs radiation depends on its surface colour and texture (dull black surfaces are the best emitters and absorbers, shiny white or silver surfaces are the worst), its surface temperature (hotter surfaces emit faster) and its surface area (larger area, faster transfer).
Trick: A good absorber is also a good emitter. Dull black is best at both, shiny silver is worst at both. That one idea answers most 'which container cools fastest' questions.
Worked Example
Two identical metal cans are filled with the same mass of hot water at 80°C. Can A is painted dull black and can B is shiny silver. Both are left on a wooden bench in the same room.
(a) State which can cools faster, and explain why. [2]
Can A (dull black) cools faster.
A dull black surface is a better emitter of infrared radiation than a shiny silver surface, so energy is lost from can A at a greater rate.
(b) Explain how the air just above the cans carries energy away from them. [3]
Air next to the hot cans is heated, gains energy and expands.
It becomes less dense than the surrounding cooler air and rises.
Cooler, denser air moves in to replace it, setting up a convection current that carries energy away.
(c) Suggest why the cans are placed on a wooden bench rather than a metal tray for a fair comparison. [1]
Wood is a poor conductor, so very little energy is lost by conduction through the base, and the comparison mainly tests radiation.
Why this question is a good test of the topic: it tests all three processes in one setup. Students who only think about radiation miss the convection marks, and the wooden bench part checks whether they understand conduction well enough to control it.
Frequently Asked Questions
What is thermal equilibrium?
When two regions are at the same temperature, so there is no further net transfer of energy between them by heating.
Why are metals good conductors of heat?
Metals have free electrons that gain energy at the hot end and move quickly through the metal, in addition to energy being passed on by vibrating particles.
Why can't convection happen in a solid?
Convection needs the heated material itself to move as a current. Particles in a solid are fixed in place and can only vibrate, so they cannot flow.
Which surface is the best emitter of infrared radiation?
A dull black surface. It is also the best absorber. Shiny silver or white surfaces are the poorest emitters and absorbers.
How does energy from the Sun reach the Earth?
By radiation. Infrared waves need no medium, so they travel through the vacuum of space, where conduction and convection cannot happen.
Still mixing up conduction, convection and radiation?
Small group O Level Physics classes at TGC Academy's Bishan, Bukit Timah and Potong Pasir centres, helmed by Andrew Seah, MOE Award-Winning Teacher and Marshall Cavendish textbook author.