What we are going to learn
How heat spreads from one region to another.
Howthe wholecopperrod get hot whenwe heat one end
Howdoes theheat spread throughout the whole pot of waterwhenwe are
Howdoes theheat fromthe Sun reachesus
What is happening?
All things are made up of molecules
When things get heated, they absorb heat energy.
This means that the molecules are absorbing the
With more energy, the molecules are able to
When the molecules move faster, the temperature
of the object increases.
Temperature increase means the object gets
3 Processes of Heat Transfer
Occurs mainly in solids
Two types of conduction
Free electron diffusion
Note: Conduction is not the main form of heat
transfer in liquids and gases because their
molecules are spaced further apart.
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 molecule gains energy and starts to
vibrate more vigorously. The cycle continues.
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
Occurs in metals only
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
Materials that do not conduct heat easily
(eg. Water, air, plastic) are known as
Applications of conduction
Trapping air as insulation
Different sensations from conductors and
Uses of good conductors: cooking utensils
Uses of good insulators: table mats,
Occurs in liquids and gases
Does 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
Convection currents are set up.
Refer to the animation on the right.
The animation shows the process of
convection taking place.
The blue circles represent unheated
The redder the molecules get the more heat
they are absorbing.
Applications of convection
Air con is usually placed at the top of a
Heating coil of a kettle is usually at the
Formation of land and sea breezes
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
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 surface
Shiny, silver surface
Factors affecting radiation
Colour and texture of the surface (refer to
Higher surface temperature, higher rate of
Larger surface area, higher rate of transfer
Applications of radiation
Colour and texture of clothings
The vacuum flask
The vacuum prevents
The silvered surfaces
Cap and base are
made of good
insulators to reduce