5. Transfer of thermal energy

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1 5. Transfer of thermal energy A bird can reduce the heat loss from its body during cold weather by fluffing up its feathers. What are the processes of heat transfer? Conduction Convection Radiation

2 transfer of thermal energy Heat travels from a place of higher temperature to a place of lower temperature. Three processes by which heat may be transmitted conduction convection radiation simple demonstration of the three processes: conduction, convection and radiation

3 how conduction works conduction Conduction is the process by which heat is transmitted through a medium from one particle to another. Heat is transmitted from the hot to the cold end. direction of heat transfer along the rod heat travels from the hot end of the rod to the cooler end high temperature low temperature

4 conduction in solids conduction atoms are strongly held in fixed positions; they vibrate about their fixed positions atoms are quite close together solid particles

5 conduction in solids conduction Metals are different from other kinds of solids because they also have electrons that move about freely. Most of them are good thermal conductors because heat can be conducted in two ways: by the vibration of particles like atoms and molecules by the movement of free electrons through the metal conduction in good thermal thermometers

6 conduction in solids conduction Poor thermal conductors have electrons that are held firmly to the atoms. Heat is transmitted only through the vibration of atoms and molecules. heat is transmitted very slowly from particle to particle plastics

7 conduction in liquids and gases Liquids are poor thermal conductors. conduction Generally, substances (except for mercury) which are liquids at room temperatures are poor conductors. particles are further apart than those in solids particles have freedom to move about a little more than solid particles; little energy transfer conduction is limited liquid particles

8 conduction in liquids and gases Gases are also poor thermal conductors. conduction Thermal conductivity of gases is even lower than that of liquids. particles have much less contact with each other compared to liquids and solids particles that absorb heat energy move about quite independently and freely; very little energy transfer conduction is very limited gas particles

9 uses of good conductors conduction Good conductors are used in situations where heat has to be transmitted quickly. kettle pan soldering solder

10 uses of poor conductors (insulators) conduction Poor conductors (insulators) are used in situations where unwanted heat has to be kept away or to prevent heat loss. ceiling tiles contain air bubbles double glazing insulated cavity wall wallpaper contains air bricks contain air

11 convection A process by which heat is transmitted from one place to another by the movement of heated particles of a gas or liquid, due to density changes liquid or gas occurs in fluids (liquids and gases), but not in solids density changes cause convection currents; hot fluids rise and cold fluids sink cool (higher density) hot (lower density) transfer of heat by convection

12 convection in liquids convection flask purple streak water potassium permanganate crystals small flame The circulation of a liquid in this manner is called a convection current.

13 hot water system convection A hot water system makes use of the principle of convection. expansion pipe ball valve cold tank hot water storage tank hot water rising cold water drawn down boiler

14 convection cooling system in a car the radiator When a car engine is running for a long time, a lot of heat energy is produced. It is necessary to cool the engine so that it does not overheat. hot water hot water rises and enters cooling fins air flowing into radiator copper tubes with cooling fins cool water pump engine cool water absorbs heat from engine

15 convection in gases convection Convection occurs more readily in gases than in liquids because they expand much more than liquids when their temperature rises. smouldering paper glass cylinders smoke lighted candle glass window box convection currents in air

16 convection convection in gases convection current slightly rigid plastic bottle hot water hot water bottle is deformed due to the effects of convection

17 air-conditioners convection Air-conditioners are best positioned high, near the ceiling of a room. cold air which is denser sinks warm air which is less dense rises cycle repeats until room air temperature is the same as the temperature set on the thermostat of the airconditioner airconditioner cold air warm air

18 sea breeze land heats up faster than the sea air above land heats up and rises cool air above sea rushes in convection warmer cooler sea breeze

19 land breeze sea cools slower than the land warm air above sea rises cool air above land moves to the sea convection cooler warmer land breeze

20 radiation Radiation is a method of heat transfer by which a heat source transmits electromagnetic waves. (in the form of infra-red radiation). does not require any medium can take place in a vacuum

21 emission of radiation radiation boiling water Which side feels hotter? shiny dull black the hotter an object is, the more energy it radiates dull black surfaces are good emitters or radiators than shiny ones the greater the surface area and temperature of the object, the faster is the rate of heat transfer from it

22 uses of good and poor emitters radiation Good emitters are used in situations where heat has to be quickly emitted. Good Emitters Poor Emitters Cooling fins at the back of a refrigerator painted dull black A shiny metal teapot cooling fins at the back of a refrigerator painted dull black refrigerator

23 absorption of radiation radiation polished dull black wax melts first cork held by wax cork falls off dull black surfaces are good absorbers of radiation than shiny ones in general, good emitters or also good absorbers

24 uses of good and poor absorbers radiation Good absorbers are used in situations where heat has to be quickly absorbed. Good Absorbers Solar heating panels are painted in dull black paint solar panels Poor Absorbers Houses in hot countries and factory roofs are painted in white, lightcoloured paint, aluminium paint Light colours are chosen for clothes and cars in hot weather