Transfer of Thermal Energy

Sec 3 Physics

What we are going to learn

How heat spreads from one region to another.

Examples:

How the whole copper rod get hot when we heat one end

How does the heat spread throughout the whole pot of water when we are only heating the bottomHow does the heat from the Sun reaches us

What is happening?

All things are made up of moleculesWhen things get heated, they absorb heat energy.This means that the molecules are absorbing the

heat energyWith more energy, the molecules are able to move

fasterWhen the molecules move faster, the temperature

of the object increases.Temperature increase means the object gets hotter.

3 Processes of Heat Transfer

Conduction

Convection

Radiation

CONDUCTION

Occurs mainly in solids

Two types of conduction

Molecular vibration

Free electron diffusion

Note: Conduction is not the main form of heat transfer in liquids and gases because their molecules are spaced further apart.

Molecular Vibration

When heat is supplied to one end, the molecules at the hot end start to vibrate more vigorously.

In the process, they ‘bump’ into their neighboring molecules. In doing so, some energy is transferred to the neighbour.

The neighbour molecule gains energy and starts to vibrate more vigorously. The cycle continues.

Conduction Animation

The conduction animation is on the left.Notice the movement of the molecules

along the rod.

Click here for animation

Free electron Diffusion

This form of conduction takes place only in metals. As only metals have free electrons.

The electrons are freed from the molecule when heated and they travel towards the cold end.

At the cold end they collide into a molecule therefore passing all their energy to the molecule.

Comparing the 2 mechanisms

Molecular vibration Free electron diffusion

Occurs in all solids

Slow process

Occurs in metals only

Fast process

This explains why metals heat up faster:

1. Metals have 2 mechanisms of conduction occuring at the same time.

2. In metals, free electron diffusion is the main mechanism, which is faster.

Conductors and Insulators

Materials that can conduct heat easily and readily (eg. Metals) are known as conductors.

Materials that do not conduct heat easily (eg. Water, air, plastic) are known as insulators.

Applications of conduction

Trapping air as insulationDifferent sensations from conductors and

insulatorsUses of good conductors: cooking utensilsUses of good insulators: table mats,

handles

Convection

Occurs in liquids and gasesDoes not occur in solids because the

molecules are not free to move around

What happens during convection

Taking the example of heating water Water at the bottom is heated first Heated water expands When water expands density decreases Heated water of lower density starts to rise Cooler water of higher density rushes in from

sides to take its place The cooler water gets heated and the cycle repeats. Convection currents are set up.

Convection Animation

Refer to the animation on the right.The animation shows the process of

convection taking place.The blue circles represent unheated

molecules.The redder the molecules get the more heat

they are absorbing.

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

Sun releases electromagnetic waves (heat is contained in the waves as infra-red)

Hotter objects radiates more heat.

Emitters and absorbers

The Sun gives out the heat.It is known as an emitter / radiator

The Earth takes in the heat.It is known as an absorber.

Good and Bad 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