Heat Transfer Processes
FoundationHeat always flows from hot to cold in three ways: conduction passes vibrations through particles and free electrons, convection carries heat by circulating fluid, and radiation travels as electromagnetic waves needing no medium — the vacuum flask cuts off all three.
- Heat flows spontaneously from hot to cold; conduction, convection and radiation often occur together
- Conduction passes vibrational energy from particle to particle; metals add fast-conducting free electrons
- Convection carries heat by circulating flow — hot fluid rises, cooler fluid sinks
文體小知識：科學知識說明文——分述傳導、對流、輻射三種熱轉移過程的機理與應用，並以保溫瓶作綜合分析。
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1Heat flows spontaneously from hotter objects to colder ones until their temperatures are equal. Heat is transferred in three ways: conduction, convection and radiation. The three often happen at the same time — when a bowl of hot soup cools, conduction carries heat through the bowl, convection carries heat away from the surface with the air, and radiation sends energy out continuously.
2Conduction mainly occurs in solids: heated particles vibrate more vigorously and pass energy on to their neighbours. Metals conduct heat especially well because, besides particle vibration, large numbers of free electrons carry energy quickly through the metal. For example, a metal spoon in hot soup soon becomes warm at the handle, while a wooden spoon stays cool to hold for a long time — their conducting powers differ enormously.
3Convection happens only in fluids — liquids and gases: heated fluid expands, becomes less dense and rises, while cooler, denser fluid flows in to take its place, setting up a circulating convection current. When water boils, hot water rises from the bottom and cooler water sinks, and the whole pot gradually heats; air-conditioners are fitted high up so the cool air sinks — both are applications of convection.
4Radiation is the only way of transferring heat that needs no medium: hot objects emit energy as electromagnetic waves (mainly infra-red), and it works even in a vacuum — that is how the Sun's heat reaches the Earth. The nature of an object's surface affects emission and absorption of radiation: dull, black, rough surfaces are the strongest, shiny, light-coloured surfaces the weakest.
5A vacuum flask cuts off all three processes one by one: the vacuum between the walls stops conduction and convection; the silvered surfaces reflect radiation back; the stopper seals the mouth so warm air cannot escape by convection. Once you can analyse the three processes, winter clothing, car radiators and household insulation all make physical sense.
1Summary 重點摘要
Heat always flows from hot to cold in three ways: conduction passes vibrations through particles and free electrons, convection carries heat by circulating fluid, and radiation travels as electromagnetic waves needing no medium — the vacuum flask cuts off all three.
2Vocabulary cards Vocabulary
The transfer of heat through a solid by particle-to-particle passing of vibrational energy; metals also conduct quickly via free electrons.
The circulating flow set up when heated fluid rises and cooler fluid sinks, carrying heat through liquids and gases.
The transfer of energy by electromagnetic waves (mainly infra-red), which needs no medium at all.
The main component of thermal radiation; its wavelength is longer than visible light and it is invisible to the eye.
An electron in a metal that can move freely; free electrons make metals conduct heat and electricity especially well.
Mass per unit volume; heating makes fluid expand and become less dense, which drives convection.
A space with almost no matter; it blocks conduction and convection but radiation passes through.
The best emitter and absorber of radiation; shiny, light-coloured surfaces are the worst at both.
A container that cuts off conduction, convection and radiation at once with a vacuum jacket, silvered surfaces and a stopper.
The substance needed for a process; conduction and convection require a medium, radiation does not.
3Step by step Steps
- Step 1: Where does heat goHeat transfer has only one direction: spontaneously from hot to cold, and net heat flow stops when temperatures are equal. This rule is the starting point of every transfer question. The three processes — conduction, convection and radiation — often occur together, so describe them separately in answers. For example, touching a window in winter feels cold not because 'coldness' flows in, but because heat conducts from your hand into the colder glass; standing before a campfire feels warm because radiation reaches you directly — the air in between is not dragged along.
- Step 2: Conduction — particles passing energy onConduction mainly happens in solids. Particles at the hot end vibrate more vigorously and jostle their neighbours, passing energy along while the material itself stays put. Metals conduct especially well because they teem with free electrons: heated electrons race through the metal, carrying energy quickly to the cold end. For example, rest one end of a metal ruler over a candle flame and the far end burns your hand within seconds; try a glass rod or a wooden stick of the same thickness and it stays cool to hold for a long time — that is how widely solids' conducting powers differ.
- Step 3: Convection — fluids carrying heatConvection occurs only in fluids — liquids and gases. Fluid heated at the bottom expands, becomes less dense and rises; cooler, denser fluid above sinks to replace it, forming a circulating current that transports heat in bulk. For example, when water boils, water at the bottom of the pot heats first and rises while cooler water sinks, and soon the whole pot is boiling; air-conditioners are mounted high so the dense cool air sinks and cools the whole room evenly. Liquids and gases are poor conductors — convection is their main way of moving heat.
- Step 4: Radiation — a journey needing no mediumRadiation transfers energy as electromagnetic waves, mainly infra-red, and needs no medium: it works even in a vacuum, making it the only one of the three processes that can move heat across empty space. Every object emits radiation all the time, and hotter objects emit more. Surface matters for emission and absorption: dull black, rough surfaces are strongest; shiny, light-coloured surfaces are weakest. For example, the Sun's heat crosses the vacuum of space by radiation; in summer a black shirt absorbs far more heat than a white one, while firefighters' silvery suits reflect radiation away.
- Step 5: Factors affecting radiationThe curriculum asks you to weigh the factors governing emission and absorption of radiation: first, surface nature — dull black and rough is strongest, shiny and light-coloured weakest; second, temperature — the hotter the surface, the more radiation it emits; third, surface area — larger areas emit and absorb more. For example, heat-sink fins are made into thin plates to enlarge the area and shed heat faster by radiation and convection; the silvered jacket of a vacuum flask does the opposite, making the surface shiny so radiation is reflected back. In answers, list surface, temperature and area together.
- Step 6: The vacuum flask's three defencesThe vacuum flask combines all three processes and appears in exams constantly. Its double glass wall has a vacuum between the layers: with no matter there, neither conduction nor convection can operate. Both faces are silvered so the shiny surfaces emit poorly and reflect infra-red back. The cork or plastic stopper seals the mouth, cutting the last convection path, and the stopper itself conducts poorly. For example, pour boiling water in and it is still warm six hours later because all three channels are blocked; equally, a cold drink stays cold, since heat from outside cannot get in either.
- Step 7: Transfer design around usOnce you can analyse the three processes, everyday designs explain themselves. Slowing transfer: down jackets trap air (a poor conductor) to keep you warm, and double-glazed windows insulate with an air layer. Speeding transfer: car radiators use metal fins to enlarge the area, and computer coolers add fans to strengthen convection. For example, a metal ladle in hot soup scalds your hand while a wooden spoon barely warms — wood conducts slowly; for the same reason a metal railing feels much colder than a wooden one in winter, because metal whisks heat away from your hand.
4Diagrams Diagrams
5Worked example Example
Example: a vacuum flask consists of an inner wall, a vacuum jacket, silvered outer layers and a stopper. State, one by one, how conduction, convection and radiation are each cut off, with reasons.
- Conduction: the jacket is evacuated, and with almost no particles there is nothing to pass vibrations on; the stopper is made of poorly conducting cork or foam, slowing conduction through the mouth
- Convection: convection needs a moving fluid; the vacuum separates the air inside and outside completely, so warm air cannot rise away, and the stopper seals the mouth to block the last convection path
- Radiation: both faces of the jacket are silvered — shiny surfaces are poor emitters and reflect infra-red back, greatly reducing radiative loss
- Check: match each of the three transfer modes to a design feature of the flask; the vacuum jacket blocks both conduction and convection, and the two must be explained separately.
- Answer: The flask cuts off all three processes at once: the vacuum blocks conduction and convection, the silvered surfaces reflect radiation, and the stopper seals off convection — so the water inside loses internal energy very slowly and stays warm for hours. In answers, pair each process with 'structure + principle'.
6Common mistakes Mistakes
7Quick check Quick check
Q1Name the three processes of heat transfer.
Q2Why do metals conduct heat especially well?
Q3How is a convection current formed?
Q4Why can the Sun's heat reach the Earth?
Q5How does a vacuum flask cut off all three transfer processes?
8Recap Recap
- Heat flows spontaneously from hot to cold; conduction, convection and radiation often occur together
- Conduction passes vibrational energy from particle to particle; metals add fast-conducting free electrons
- Convection carries heat by circulating flow — hot fluid rises, cooler fluid sinks
- Radiation transfers heat by electromagnetic waves (infra-red) with no medium; dull black surfaces emit and absorb best
- Vacuum flask = vacuum against conduction and convection + silvering against radiation + stopper against convection