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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