2.3.3Thermal Physics
Convection — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions
5 min read 8 parts Lesson 12 of 16
Section 1 of 8Convection
Convection
Convection the transfer of thermal energy in a liquid or gas by the movement of the fluid itself.
Where it occurs
- Where it occurs only in fluids, so it cannot happen in solids or in a vacuum.
Key idea
- Key idea any description must refer to changes in temperature causing changes in density.
Section 2 of 8Convection current in a heated fluid
1
The fluid is heated and its particles gain kinetic energy and move further apart.
2
The fluid expands, so its density becomes lower than that of the surroundings.
3
The less dense hot fluid rises.
4
Cooler, denser fluid moves in to take its place.
5
The rising fluid cools, contracts and becomes denser, so it sinks again.
- Convection current
- the circulating movement produced by this process.
Section 3 of 8Convection current in a cooled fluid
1
The fluid is cooled and its particles lose kinetic energy and move closer together.
2
The fluid contracts, so its density becomes greater than that of the surroundings.
3
The dense cold fluid sinks.
4
Warmer, less dense fluid moves in to take its place and is cooled in turn.
- Example
- an air conditioning unit placed high on a wall cools the air, which sinks and sets up a current.
Section 4 of 8Demonstrating convection
In a liquid
- In a liquid place a potassium permanganate crystal at the bottom of a beaker of water and heat the water just below the crystal.
- The purple dye dissolves, rises with the warm water, spreads across the top and sinks again as the water cools.
In a gas
- In a gas place lit candles under a column with angled blades.
- The heated air becomes less dense, rises and pushes the blades sideways so they spin.
Section 5 of 8Applications of convection
Room radiator
- Room radiator a radiator heats the air next to it, which expands, becomes less dense and rises, while cooler air flows in and the whole room warms.
Heater position
- Heater position heaters are placed low in a room so that the warm air can rise and circulate.
Hot drink
- Hot drink warm air above the surface rises and carries thermal energy away.
Insulation
- Insulation foam and fibreglass trap air in small pockets so that convection currents cannot form.
Coolers
- Coolers air conditioners and freezer compartments are placed high so the cooled air sinks through the space.
Section 6 of 8More from the specification
More from the specification
Thermal vibrational diffusion between atoms
- Thermal vibrational diffusion between atoms conductive heat transport occurs through kinetic energy exchanges between vibrating lattice atoms.
Convection
- Convection heat transfer in fluids where warmer, less dense fluid rises and cooler, denser fluid sinks, forming convection currents
Conduction
- Conduction energy passes through a solid by vibrating particles and free electrons, and metals are good conductors
Radiation
- Radiation infrared waves transfer energy without a medium;
- dull black surfaces absorb and emit best, shiny surfaces are poor emitters
Greenhouse effect
- Greenhouse effect gases in the atmosphere absorb infrared emitted by the Earth and re-emit some back, warming the surface
Experiment
- Experiment compare cooling of identical cans with black and shiny surfaces to show how surface affects emission of infrared
Carbon dioxide
- Gas that turns limewater cloudy.
Thermal equilibrium
- A state in which objects are at the same temperature.
Convection
- Convection heat transfer in fluids where warmer, less dense fluid rises and cooler, denser fluid sinks, forming convection currents
Conduction
- Conduction energy passes through a solid by vibrating particles and free electrons, and metals are good conductors
Radiation
- Radiation infrared waves transfer energy without a medium;
- dull black surfaces absorb and emit best, shiny surfaces are poor emitters
Greenhouse effect
- Greenhouse effect gases in the atmosphere absorb infrared emitted by the Earth and re-emit some back, warming the surface
Experiment
- Experiment compare cooling of identical cans with black and shiny surfaces to show how surface affects emission of infrared
Conduction
- Thermal energy transfer by particles vibrating in a solid.
Conductor
- Material that lets current flow.
Infrared
- Electromagnetic radiation just beyond the red end of visible light.
Electron
- Negative particle that moves around the nucleus.
Absorbed
- Taken in by a surface instead of being reflected.
Lattice
- Regular pattern of particles in a giant structure.
Metal
- Element on the left of the Periodic Table.
Vibration
- Rapid backwards and forwards movement.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Convection | Transfer of thermal energy by the movement of a heated or cooled liquid or gas. |
| Convection current | The circulation of a fluid caused by differences in density. |
| Fluid | A liquid or a gas. |
| Density | Mass per unit volume; it decreases when a fluid is heated. |
| Expansion | The increase in volume of a fluid when heated. |
Section 8 of 8Exam technique
1
Saying hot air rises because heat rises — hot air rises because it is less dense.
2
Saying the particles get lighter when heated — the particles are the same, but they are further apart.
3
Saying convection happens in solids — it needs a fluid that can flow.
4
Explaining a convection current without mentioning density.
5
Saying cold air sinks because it is heavier without referring to the same volume of air being denser.
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