Thermal processes
Thermal equilibrium, conduction, convection, radiation and their everyday applications.
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Before you begin
A cup of tea cools down and an ice cube in your hand melts because energy moves from hotter things to colder ones. The lessons start with thermal equilibrium and conduction through solids, then explain convection in liquids and gases and radiation, which can cross empty space. They finish by using all three to explain everyday designs, from vacuum flasks to cooking pots.
Be comfortable with: the kinetic particle model, because conduction and convection are explained by the motion of particles.
Learning goals
- Explain heating from higher to lower temperature until thermal equilibrium
- Describe conduction in solids through particle vibration and mobile electrons
- Describe convection in fluids through density changes and bulk motion
- Explain that energy transfer by electromagnetic radiation needs no material medium
- Explain how surface colour, texture, temperature and area affect radiation transfer rate
- Apply conduction, convection and radiation in everyday systems
Syllabus statements covered
- show an understanding that energy is transferred (by heating) from a region of higher temperature to a region of lower temperature until thermal equilibrium is achieved between the two regions
- describe, in microscopic terms, how conduction occurs in solids (via vibration of atoms/molecules and movement of electrons)
- describe, in terms of density changes, how convection occurs in fluids
- explain that energy transfer by electromagnetic radiation does not require a material medium and that this rate of energy transfer to/from a body is affected by its: (i) surface colour and texture (ii) surface temperature (iii) surface area
- apply the concepts of conduction, convection and radiation in everyday examples
Lessons
Work through them in order.
- Thermal equilibrium and conductionPredict the direction and end point of heating, and describe how conduction transfers energy through a solid.
- Convection in liquids and gasesDescribe a convection current in a liquid or gas in terms of density changes.
- Radiation and surface propertiesExplain how radiation crosses a vacuum and compare the effects of surface finish, temperature and area.
- Controlling heat transfer in everyday designsExplain how everyday designs use or reduce conduction, convection and radiation.
Practise and check
Or choose
Topic reference
Quick reference
| Process | How energy is transferred | Where |
|---|---|---|
| conduction | vibrating particles pass energy to their neighbours; free electrons carry it quickly in metals | mainly in solids |
| convection | warmer fluid expands, becomes less dense and rises; cooler, denser fluid sinks | liquids and gases |
| radiation | infrared waves, which need no medium | anywhere, including a vacuum |
| Surface | Emits and absorbs radiation |
|---|---|
| dull, black | well |
| shiny, white | poorly |
Core knowledge to remember
- Energy is transferred by heating from a region of higher temperature to a region of lower temperature until thermal equilibrium is reached.
- Conduction in solids happens through the vibration of atoms and molecules and the movement of free electrons.
- Convection in fluids happens because heating changes the density of part of the fluid, which then rises or sinks.
- Energy transfer by radiation does not need a material medium. Its rate depends on a body’s surface colour and texture, surface temperature and surface area.
Common mistakes
- Saying “cold” flows into an object. Energy flows out of the hotter object.
- Saying heat rises. The warmer, less dense fluid rises.
- Saying convection happens in solids. The particles in a solid cannot flow.
- Saying radiation needs air to travel through.
- Saying a shiny surface absorbs radiation well. Dull, black surfaces are the better absorbers and emitters.