CHAPTER FIFTEEN : Transmission of Heat EnergyPresentation Transcript
Transmission of Heat Energy CHAPTER FIFTEEN
You will learn
the meaning of conduction, convection and radiation
some everyday applications of conductors and insulators
about the formation of convection currents in liquids and gases
some everyday applications of convection
about the type of surface which makes it a good or bad absorber or radiator of heat
some applications of good and bad absorbers and radiators of heat
The Movement of Heat Definition: Heat is a form of energy that can move from the hotter object to the cooler one. The temperature of the cooler object rises while that of the hotter one falls until both are at the same temperature. What is Heat ???
The Movement of Heat
Three methods of heat transfer are:
conduction convection radiation
15.1 Conduction Definition: Conduction is the movement of heat through a material without the obvious movement of the material itself. What is Conduction ???
Good conductors are materials (eg. metals) that allow heat to flow through easily. Insulators (or poor conductors) are materials (eg. plastics) that do not allow heat to flow through them easily. metal spoon very hot water plastic spoon
Some metals conduct heat better than others. Silver, gold and copper are the three best conductors of heat. Comparing Conductors and Insulators copper rod iron rod Bunsen burner wax melts first wax melts slower tripod stand
Non-metals such as wood, plastics and glass are poor conductors of heat. Liquids and gases are good insulators of heat. Vacuum is the best insulator because an empty space cannot conduct heat. Comparing Conductors and Insulators matchstick does not burn matchstick burns metal rod Bunsen burner
Uses of Conductors and Insulators Conductors and Insulators of Heat cooking pan penguin electric iron oven gloves rabbit wool blanket icebox
15.2 Convection Definition: Convection is the movement of heat through a material by the movement of the material itself. What is Convection ???
The transfer of heat in a liquid is demonstrated below: cold water coloured crystal cold water sinks hot water rises coloured crystal dissolves When the hot water rises, heat energy is carried from one place to another. This movement of heat is called convection .
The transfer of heat in a gas is demonstrated below: The movements of the hot and cold air set up a convection current . glass chimney smoke particles smouldering paper box glass front convection air
An Explanation for Convection When a substance is heated, it expands and its volume increases. Density decreases. HOT liquid/gas becomes less dense than COLD liquid/gas hot and less dense parts of the liquid or gas will rise while the surrounding cooler and denser parts will sink Convection currents are set up which transfer heat from the hotter regions to the cooler regions. cooler water is more dense and falls hotter water is less dense and rises
Uses and Effects of Convection Applications of convection in water: hot water system electric kettle hot water tank boiler expansion pipe cold water tank cold water sinks cold water sinks water hot water rises
Uses and Effects of Convection Applications of convection in air: glider air-conditioner heater cold air warm air cold air hot air
Uses and Effects of Convection Sea breeze cooler air takes the place of the hotter air cooler air COOLER HOTTER hotter air rises
Uses and Effects of Convection Land breeze cooler air takes the place of the warmer air warm air rises COOLER WARMER cooler air falls
Uses and Effects of Convection Convection currents cause the monsoon winds that blow across many parts of Asia twice a year.
large areas of northern Asia become very hot
hot air rises and air from the oceans blows towards the land to replace the hot air
the water in this air condenses to form clouds and the heavy rains fall during the summer monsoons
land areas become colder than the oceans
the air above the oceans rises and cool dry winds blow across the land to the oceans to replace this air
Differences between Conduction and Convection flow of heat hot cold metal rod cold fluid hot fluid
Hot fluid moves freely and carries energy from one place to another
Energy is transferred through the solid without the solid itself moving
Heat flows upwards
Heat can flow in any direction
Occurs mainly in fluids
Occurs mainly in solids
15.3 Radiation Definition: Radiation is the transfer of energy through a vacuum in the form of waves. What is Radiation ???
All hot objects give out energy as radiation. The radiation moves in all directions. sun radiation radiation radiation Radiation itself is not hot, but when absorbed, is changed to heat.
Heat from a Fire The fire warms the boy because most of the heat reaches him by radiation. Very little heat comes from conduction or convection.
Absorption of Heat All objects are able to absorb radiation. Both the black (or dull) flask and the white (or shiny) flask absorb heat, but the black flask absorbs heat faster. air inside
Radiation of Heat All hot objects are able to radiate heat. Both the black (or dull) flask and the white (or shiny) flask radiate heat, but the black flask radiates heat faster, and cools more quickly than the white flask. hot water inside
Radiation of Heat
Shiny or white surfaces:
reduce the amount of energy absorbed from the surrounding by reflecting radiation
reduce the amount of heat lost from a container by reflecting heat from the walls and back into the container
Good absorbers of heat are also good radiators of heat.
Applications of Absorbers and Radiators of Heat Uses of absorbers and radiators of heat: white clothes solar water heater hot water container fuel storage tank refrigerator
15.4 More Applications of Heat Transfer Keeping Food Warm A plastic food box helps to reduce heat loss in three ways: 1) Conduction – styrofoam is a poor conductor of heat 2) Convection – box is covered by a lid 3) Radiation – white surface is poor absorber and good reflector of heat
A vacuum flask keeps liquids hot or cold by preventing heat from getting in or out in three ways: The Vacuum Flask 2) Convection – vacuum between silvered and glass walls 3) Radiation – shiny, smooth surface on silvered walls silvered walls vacuum between the walls glass walls stopper (cork or plastic)
– vacuum between silvered and glass walls
– glass walls reduce heat gain or heat loss
– cork and plastic are insulators
Design of Spacesuits Temperatures in space range from very hot to very cold depending on whether they are facing the sun. helmet white outer layer layers of insulating material As astronauts on space missions have to work outside the space shuttle, their spacesuits must be able to keep their bodies at a comfortable temperature.
Keeping Buildings in Singapore Cool Some ways to reduce heat transfer: concrete air-conditioning double-glazing glass walls