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 bottom How does the heat from the Sun reaches us

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 heat energy  With more energy, the molecules are able to move faster  When 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 vibrationFree 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 insulation  Different sensations from conductors and insulators  Uses of good conductors: cooking utensils  Uses of good insulators: table mats, handles

Convection  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 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/absorberPoor emitter/absorber  Dull, black surface  Rough surface  Shiny, silver surface  Smooth surface

Factors affecting radiation  Colour and texture of the surface (refer to previous slide) previous slide  Surface temperature Higher surface temperature, higher rate of transfer.  Surface area Larger surface area, higher rate of transfer

Applications of radiation  Teapots  The greenhouse  Colour and texture of clothings  Skin 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