2.3.2Thermal Physics
Thermal Conduction — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 10 parts Lesson 35 of 45 1 quiz
Lesson summary
2.3.2 Thermal Conduction — IGCSE PHYSICS notes: Explain conduction in metals and non-metals using lattice vibrations and free electrons.Definition — transfer of thermal energy from one region to another through lattice vibrations and, in metals, free (delocalised) electronsDirect…
- Main method
- Heated end
- Passing on
- Energy transfer
- Both mechanisms
- Trapped air
Part 1 of 8Conduction defined
Definition
- Definition transfer of thermal energy from one region to another through lattice vibrations and, in metals, free (delocalised) electrons
Direction
- Direction energy flows from the hotter region to the cooler region with no overall movement of the material
Main method
- Main method conduction is the main method of energy transfer in solids
Rate
- Rate a bigger temperature difference gives a faster flow
Part 2 of 8Conduction in non-metals
Lattice
- Lattice particles are held in a lattice and can only vibrate
Heated end
- Heated end particles vibrate with larger amplitude
Passing on
- Passing on they collide with neighbours and pass on vibrational energy along the material
Result
- Result the process is slow, so most non-metals are poor conductors
Part 3 of 8Conduction in metals
Free electrons
- Free electrons delocalised electrons move randomly among the positive ions
Heated end
- Heated end the free electrons gain kinetic energy and move faster
Energy transfer
- Energy transfer they travel far and collide with distant ions, passing energy over a long distance
Speed
- Speed far faster than lattice vibrations alone, so metals are the best thermal conductors
Both mechanisms
- Both mechanisms metals use both, but the free-electron process dominates
Part 4 of 8Liquids and gases
Liquids
- Liquids most have no free electrons and particles slide past each other, so energy passes poorly
Gases
- Gases particles are far apart and move randomly, so there are no lattice vibrations and collisions are less frequent
Exception
- Exception liquid metals still contain free electrons and conduct well
Trapped air
- Trapped air an excellent insulator when it cannot circulate
- Wool, fur and feathers
- Foam and double glazing
- Trapping air also prevents convection
Part 5 of 8Factors affecting the rate
Temperature difference
- Temperature difference larger difference gives a faster rate
Cross-sectional area
- Cross-sectional area larger area gives more conducting paths and a faster rate
Thickness
- Thickness greater thickness reduces the rate
Material
- Material metals conduct best, so thick loft insulation reduces energy loss more
Part 6 of 8Applications
Cooking pans
- Cooking pans copper or aluminium base to heat quickly, plastic handles to stay safe
Cavity walls
- Cavity walls trapped air reduces conduction
Heat sinks
- Heat sinks aluminium fins conduct energy away from computer chips
Winter clothing
- Winter clothing traps layers of air next to the skin
Part 7 of 8Conduction compared with other transfers
| Method | What moves | Medium needed |
|---|---|---|
| Conduction | Energy passed on, particles stay in place | Yes, mostly solids |
| Convection | The fluid itself moves with the energy | Yes, liquids and gases |
| Radiation | Waves carry the energy | No, works in a vacuum |
Part 8 of 8More from the specification
1Lattice vibration transfer between neighbouring atoms
- Lattice vibration transfer between neighbouring atoms thermal energy conducts through metal solids via colliding kinetic atoms and mobile delocalised electrons.
2Thermal Radiation and Black Surfaces
- A dull, matt black surface is both the best emitter and best absorber of infrared electromagnetic radiation,
- whereas a shiny white or silver surface reflects radiation.
3Leslie's Cube Radiation Experiment
- Comparing cooling rates of identical flasks filled with hot water shows that vessels with dark, rough surfaces emit infrared radiation significantly faster than silvered ones.
4The Greenhouse Effect and Earth's Climate
- Short-wavelength solar radiation absorbed by Earth's surface is re-emitted as longer-wavelength infrared,
- which is absorbed and trapped by greenhouse gases.
5Conduction vs Radiation
- Unlike thermal conduction which requires physical atomic or electron contact in solids, infrared radiation propagates through vacuum at the speed of light.
6Infrared
- The part of the electromagnetic spectrum emitted by warm objects.
7Absorbed
- Taken in by an object, with light energy transferred to thermal energy.
8Infrared radiation
- Long-wavelength thermal radiation emitted by the Earth
9Greenhouse effect
- Warming of the Earth because gases absorb and re-emit its infrared radiation
10Convection current
- Circulating flow of fluid caused by density differences
11Thermal equilibrium
- Constant temperature because energy is absorbed and lost at equal rates
12Potassium permanganate
- A purple compound used to show convection currents in water.
Key terms
| Term | Meaning |
|---|---|
| Conduction | Transfer of thermal energy by lattice vibrations and free electrons |
| Lattice vibrations | Vibrations of atoms or ions about fixed positions that pass energy on |
| Free electrons | Delocalised electrons that move through a metal carrying energy |
| Thermal insulator | Material that conducts thermal energy poorly |
| Thermal equilibrium | Same temperature throughout, reached after energy has been passed on |
Avoid these mistakes
1
Saying particles travel along the solid carrying energy — in non-metals they only vibrate and pass energy on.
2
Saying free electrons exist in all solids — only metals have them.
3
Forgetting to say what carries the energy — state lattice vibrations or free electrons.
4
Saying conduction needs the material to move — there is no net movement of the material.
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