Transfer of Thermal Energy

Sec 3 Physics

What we are going to learn

The 3 methods of heat transfer and

how they are applied in everyday application.

3 Processes of Heat Transfer

Conduction

Convection

Radiation

CONDUCTION

Occurs mainly in solids

Conduction takes place when there is

Molecular vibration

Free electron diffusion

Molecular Vibration

When one end of a solid is heated, the molecules at the hot end gain energy and their kinetic energy increases.

They start to vibrate more vigorously and in the process, collide with their neighboring molecules. In doing so, some energy is transferred to the neighbours.

The neighbour molecules gains energy and starts to vibrate more vigorously. The cycle continues until the other end is also heated.

Free electron Diffusion

Free Electron diffusion takes place only in metals as only metals have free moving electrons.

When heated, the free electrons gain energy and move quickly towards the cold end. In the process, they collide with molecules and transfer energy to them. This helps to increase the rate of heat transfer.

Conduction

Question:Why is conduction not the main form of heat transfer in liquids and gases?

Answer: Liquid and gas molecules are spaced further apart so it is very slow to transfer energy by molecular vibration.

Comparing the 2 mechanisms

Molecular vibration Free electron diffusion

Occurs in all solids

Slow process

Occurs in metals only

Fast process

Metals are better conductor because they have 2 mechanisms of conduction taking place at the same time whereas non-metals only have one.

Conductors and Insulators

Materials that can conduct heat fast are known as conductors (e.g. metal).

Materials that are slow in conducting heat are known as insulators (e.g. water, trapped air, wood, plastic)

Applications of good conductors

Good conductors are used when heat transfer needs to be fast.

Uses of good conductors: metal cooking utensils, copper pipes in car radiators and water heater and heat sinks*

*Heat sinks are components that conducts heat away from electronic equipment to prevent overheating.

Applications of poor conductors

Materials that trap air are good insulators e.g. wool, styroform and double glazed window.

They are used when heat transfer needs to be slowed down.

Uses of good insulators: wooden handle of cooking utensils, containers to hold hot drinks (styrofoam cups), winter wear made of wool etc.

Convection

Occurs in liquids and gasesDoes not occur in solids because the

molecules are not free to move around

What happens during convection

Water at the bottom is heated and it expands.

When water expands density decreases.

Heated water starts to rise because of its lower density

Cooler water of higher density will be displaced to the bottom.

The cooler water gets heated and the cycle repeats.

Convection currents are set up to heat the water evenly.

Applications of convection

Air con is usually placed at the top of a room.

Heating coil of a kettle is usually at the bottom

Formation of land and sea breezes

Radiation

Radiation is the transfer of heat by electromagnetic wave (Infrared Radiation).

When an object absorbs radiation, it will convert it into thermal energy.

Radiation does not require a medium to transfer heat. (can occur in a vacuum)

Emitters and Absorbers

An object will absorb heat when it is

Good & Poor Emitters/Absorbers

A good emitter would also be a good absorber.A poor emitter would be a poor absorber.

Good emitter/absorber Poor emitter/absorber

Dull, black surfaceRough surface

Shiny, silver surfaceSmooth surface

Factors affecting radiation

Colour and texture of the surface (refer to previous slide)

Surface temperature

Higher surface temperature, higher rate of transfer.

Surface area

Larger surface area, higher rate of transfer

Applications of radiation

TeapotsThe greenhouseColour and texture of clothingsSkin cancer

The vacuum flask

The vacuum prevents conduction and convection

The silvered surfaces reduces radiation

Cap and base are made of good insulators to reduce conduction