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Transfer Of Thermal Energy

邊玩邊學

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Thermal Conduction

  • Conduction is the transfer of thermal energy through a solid via particle vibrations and the movement of free electrons.
  • Metals are good conductors because they have many free electrons that transfer energy through collisions.
  • Non-metals and organic materials are poor conductors (insulators) because they lack free electrons.
  • In conduction, atoms at the hotter end vibrate more and collide with neighbours, transferring energy until thermal equilibrium is reached.

Conduction: the atoms in a solid vibrate and collide, transferring energy

Conduction: the atoms in a solid vibrate and collide, transferring energy

Convection

  • Convection is the main method of thermal energy transfer in fluids (liquids and gases).
  • When a fluid is heated, it expands, becomes less dense, and rises; cooler, denser fluid sinks to replace it, creating a convection current.
  • Convection cannot occur in solids because particles are fixed in position.
  • Common examples: radiators heating a room, steam rising from hot coffee.

When a liquid or gas is heated, it becomes less dense and rises

When a liquid or gas is heated, it becomes less dense and rises

Thermal Radiation

  • Thermal radiation is the transfer of energy by infrared waves; it does not require a medium and can travel through a vacuum.
  • All objects emit thermal radiation; hotter objects emit more radiation.
  • Black, dull surfaces are good absorbers and good emitters of radiation.
  • White, shiny surfaces are poor absorbers and poor emitters (they reflect radiation).

Effect of surface colour and texture on thermal radiation

Effect of surface colour and texture on thermal radiationColour /textureAbsorbingEmittingBlackGood absorberGood emitterDull / darkReasonable absorberReasonableemitterWhitePoor absorberPoor emitterShinyVery poor absorber(reflects)Very pooremitter

Investigating Thermal Radiation

  • Experiment: fill identical flasks painted black, grey, white, and silver with hot water at the same initial temperature.
  • Measure temperature at regular intervals (e.g., every 30 s for 10 min) to compare cooling rates.
  • The black flask cools fastest (best emitter), the silver flask cools slowest (poorest emitter).
  • Control variables: same amount of water, same starting temperature, same time interval.

Different coloured flasks for investigating infrared radiation

Different coloured flasks for investigating infrared radiation

The Greenhouse Effect

  • The Earth absorbs radiation from the Sun and emits its own thermal radiation.
  • Greenhouse gases (e.g., CO₂, water vapour) absorb and re-emit some of Earth's radiation, trapping heat and warming the surface.
  • Without greenhouse gases, Earth's surface would be extremely cold.
  • Excess greenhouse gases increase the amount of radiation reflected back, leading to global warming.

The greenhouse effect: radiation from the Sun and Earth

The greenhouse effect: radiation from the Sun and Earth

Consequences of Thermal Energy Transfer

  • Conduction applications: metal pans for quick heating; plastic handles and double-glazed windows as insulators.
  • Convection applications: radiators use convection currents to heat rooms; steam rising cools hot liquids.
  • Radiation applications: black car bodies absorb sunlight and become hot; fire radiates heat to surroundings.

Double-glazed windows use a layer of air to reduce heat loss

Double-glazed windows use a layer of air to reduce heat loss

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練習題

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  1. 1.Which method of thermal energy transfer does not require a medium?

    Easy
    • AConduction
    • BConvection
    • CRadiation
    • DEvaporation
  2. 2.Convection can occur in solids.

    Easy

    True or false?

  3. 3.State one factor that affects the rate of thermal radiation emitted by a surface.

    Easy
  4. 4.Complete the sentence about conduction in metals.

    Easy

    In metals, thermal energy is transferred by ____ and by the movement of ____.

  5. 5.A black surface and a white surface are placed in sunlight. The black surface absorbs 90% of incident radiation and the white surface absorbs 20%. How many times more radiation does the black surface absorb compared to the white surface?

    Medium
    • A6.5
    • B4.5
    • C5.5
    • D3.5
  6. 6.In a convection current, a fluid expands and becomes less dense when it is heated. What happens next?

    Easy
    • AIt sinks
    • BIt rises
    • CIt stays in place
    • DIt condenses
  7. 7.Arrange the following steps in the correct order to describe how a radiator heats a room by convection.

    Easy
    • Hot air rises
    • Cooler air moves in to replace it
    • Air near the radiator is heated
    • Hot air cools and sinks
  8. 8.Match each surface type to its property regarding thermal radiation.

    Easy
    • Black, dull surface
    • White, shiny surface
    • Silver, polished surface
    • Good absorber and good emitter
    • Poor absorber and poor emitter
    • Very poor absorber and very poor emitter

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