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 vibrationFree 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 gases Does 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/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