Transfer Of Thermal Energy

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Notes

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.

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.

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).

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.

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**.

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.

Particle arrangement in solids, liquids, and gases. Conduction occurs mainly in solids; convection in liquids and gases.

Refraction of light: thermal radiation is part of the electromagnetic spectrum and can be reflected/refracted.

Practice questions

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1.Which method of thermal energy transfer does not require a medium?
Easy
- AConduction
- BConvection
- CRadiation
- DEvaporation
2.Convection can occur in solids.
Easy
True or false?
3.State one factor that affects the rate of thermal radiation emitted by a surface.
Easy
4.Complete the sentence about conduction in metals.
Medium
In metals, thermal energy is transferred $by ____ and$ by the movement $of ____$ .
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.In a convection current, a fluid expands and becomes less dense when it is heated. What happens next?
Medium
- AIt sinks
- BIt rises
- CIt stays in place
- DIt condenses
7.Arrange the following steps in the correct order to describe how a radiator heats a room by convection.
Hard
- Hot air rises
- Cooler air moves in to replace it
- Air near the radiator is heated
- Hot air cools and sinks
8.Match each surface type to its property regarding thermal radiation.
Medium
- 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

Slides

Transfer of Thermal Energy

Thermal energy can be transferred by three processes: conduction, convection and radiation.
Conduction mainly occurs in solids, convection in fluids (liquids and gases), and radiation can travel through a vacuum.
This lesson covers each process and their everyday applications.

Thermal Conduction

Conduction is the transfer of thermal energy through a material without the material itself moving.
In solids, atoms vibrate and collide, transferring energy from hotter to cooler regions.
Metals are good conductors because they have free electrons that move and transfer energy quickly.
Non-metals are poor conductors (insulators) because they lack free electrons.

Which of the following is the best thermal conductor?

Wood
Plastic
Copper
Air

Convection

Convection is the transfer of thermal energy by the movement of a fluid (liquid or gas).
When a fluid is heated, it expands, becomes less dense and rises. Cooler, denser fluid sinks to replace it.
This creates a convection current that circulates and transfers heat throughout the fluid.
Convection does not occur in solids because particles are fixed in position.

In convection, heated fluid becomes less $____ and ____$ , while cooler fluid $____ to$ take its place.

Thermal Radiation

Thermal radiation is the transfer of energy by infrared waves, part of the electromagnetic spectrum.
All objects emit thermal radiation; hotter objects emit more.
Radiation does not require a medium – it can travel through a vacuum (e.g., from the Sun to Earth).
The amount of radiation emitted or absorbed depends on surface colour and texture.

Which surface is the best emitter of thermal radiation?

Shiny white
Dull black
Shiny silver
Dull grey

Investigating Thermal Radiation

An experiment can compare how different surfaces radiate heat.
Four identical flasks painted black, grey, white and silver are filled with hot water at the same temperature.
Temperature is recorded at regular intervals (e.g., every 30 s for 10 min).
The black flask cools fastest because it is the best emitter; the silver flask cools slowest.

Which of these are true about the radiation experiment?

All flasks should start at the same temperature.
The silver flask cools fastest because it reflects radiation.
The black flask is the best emitter of thermal radiation.

The Greenhouse Effect

The Earth absorbs thermal radiation from the Sun and emits its own infrared radiation.
Greenhouse gases (e.g., carbon dioxide, water vapour) absorb and re-emit some of this radiation back to Earth.
This keeps the Earth's surface warmer than it would be without an atmosphere.
Excess greenhouse gases increase the amount of radiation trapped, leading to global warming.

What is the main reason the Earth's surface temperature increases with more carbon dioxide?

More carbon dioxide absorbs more sunlight directly.
More carbon dioxide traps more of the Earth's emitted radiation.
Carbon dioxide is a good conductor of heat.
Carbon dioxide increases convection in the atmosphere.

Everyday Applications of Conduction

Good conductors: metal pans heat food quickly; metal radiators transfer heat to air.
Bad conductors (insulators): plastic handles on saucepans prevent burns; air gaps in double glazing reduce heat loss.
Double-glazed windows have a layer of air (or gas) between two panes of glass to insulate.

Double-glazed windows use a layer $of ____$ between panes to reduce ____ transfer.

Everyday Applications of Convection

Room radiators heat air by convection: hot air rises, cool air sinks, creating a current.
Steam rising from a hot drink is a convection current carrying thermal energy away.
Despite their name, radiators transfer most heat by convection, not radiation.

Why does a radiator placed near the floor heat a room more effectively?

It radiates heat directly to all objects.
Hot air rises and sets up a convection current.
It conducts heat through the floor.
It emits more infrared radiation than if placed high.

Everyday Applications of Thermal Radiation

A black car becomes hot in sunlight because black surfaces absorb thermal radiation well.
As you move closer to a fire, you feel more heat mainly due to increased thermal radiation.
Shiny surfaces (e.g., silver foil) reflect radiation and are used in emergency blankets to keep people warm.

Which statements about thermal radiation are correct?

A shiny surface absorbs more radiation than a black surface.
Thermal radiation can travel through a vacuum.
All objects emit thermal radiation.

Summary: Three Methods of Heat Transfer

Conduction: transfer through solids by particle vibrations and free electrons.
Convection: transfer in fluids by movement of fluid due to density changes.
Radiation: transfer by infrared waves, no medium needed.
Understanding these helps explain everyday phenomena and design better insulation.

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