Transfer of thermal energy
Introduced| English | Meaning |
|---|---|
| conduction/kənˈdʌkʃn/ | Transfer of thermal energy through a material without bulk movement of the material. |
| convection/kənˈvekʃn/ | Heat transfer through the bulk movement of a fluid. |
| radiation/ˌreɪdɪˈeɪʃn/ | The emission or transmission of energy through electromagnetic waves or particles. |
| insulators/ˈɪnsjuːleɪtəz/ | A material in which electric charge does not move readily under ordinary conditions. |
| infrared/ˌɪnfrəˈred/ | Electromagnetic radiation with wavelengths longer than visible red light and shorter than microwaves. |
| emitter/ɪˈmɪtə/ | A surface giving out radiation; its temperature and surface properties affect the rate. |
Three ways heat travels
- A metal spoon left in hot soup soon burns your hand.
- Yet the Sun warms you across millions of kilometres of empty space.
- Thermal energy always flows from hot to cold — by three routes: conduction 热传导, convection 对流 and radiation 辐射.
Heat transfer lab
Compare the routes by which thermal energy moves.
Conduction
- Conduction carries energy through a material without the material moving.
- Heated particles vibrate more and pass the energy to their neighbours.
- Metals are the best conductors: their free (delocalised) electrons carry energy quickly.
- Insulators 绝缘体 (air, wood, plastic) conduct badly — which is why we use them to keep things warm.
Conduction: vibrating particles pass energy along a metal bar
Why are metals such good conductors of thermal energy?
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Metals have free (delocalised) electrons that move through the metal and carry energy rapidly from the hot end to the cold end.
Convection
- Convection happens in liquids and gases.
- Heated fluid expands, becomes less dense, and rises; cooler, denser fluid sinks to take its place.
- This circulation is a convection current — it carries energy around the fluid.
- It cannot happen in a solid, because the particles cannot move from place to place.
In convection, heated fluid rises because it has become:
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Heating makes the fluid expand, so it is less dense and floats upward; cooler, denser fluid sinks to replace it.
Convection can happen in a solid.
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Convection needs particles to move from place to place, so it only happens in liquids and gases — not solids.
Radiation
- Thermal radiation is energy carried by infrared 红外线 waves.
- All objects emit it, and it needs no material — it can cross empty space (this is how the Sun's energy reaches us).
- A dull, black surface is a good emitter 发射体 and absorber of infrared.
- A shiny, white surface is a poor emitter and a good reflector — which is why hot-water tanks and spacecraft are often shiny.
A dull, black surface emits (and absorbs) infrared much better than a shiny, white one
Which method of heat transfer can travel through empty space (a vacuum)?
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Radiation is carried by infrared waves, which need no material — so the Sun's energy reaches us across empty space.
Match each method of heat transfer to how it works.
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Conduction works through a material; convection moves the fluid itself in a current; radiation is infrared waves that need no material.
Which surface is the best emitter of thermal radiation?
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Dull black surfaces are the best emitters (and absorbers) of infrared; shiny, light surfaces are poor emitters and good reflectors.
Worked examples
1. A metal spoon and a wooden spoon sit in the same room, so both are at room temperature. The metal feels colder because it conducts energy away from your warm hand much faster.
2. A vacuum flask: the vacuum stops conduction and convection across the gap; the shiny silver walls reduce radiation; the stopper stops convection out of the top.
"Cold" does not travel. Energy always moves from the hotter object to the colder one. A coat keeps you warm by slowing the energy leaving your body, not by keeping cold out.
You've got it
- thermal energy always flows from hot to cold
- conduction: through a material, particle to particle; metals best (free electrons)
- convection: warm fluid rises, cool sinks — a current; never in solids
- radiation: infrared, needs no material; dull black = best emitter/absorber
Write from a conduction rate of 30 joules per second
- A metal bar conducts 900 J from the hot end to the cold end in 30 s.
- Find the rate. Say which process can cross a vacuum.
A metal bar conducts 900 J in 30 s. What is the rate, and which thermal process can transfer energy across a vacuum?
900 divided by 30 is 30 joules per second. Energy still flows from hot to cold.
What is the rate of energy transfer along the bar?
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900 divided by 30 is 30 joules per second.