Transfer of Heat Heat: It is the average kinetic energy of the particles of a substance. There are three methods by which heat (kinetic energy) gets transferred from the hot region to the cold region. (REMEMBER: Heat always travels from hot region to a cold region). 1.Conducion Bear trap: Temperature is not heat
2.Convection 3.Radiation
1. Conduction: It is a method of heat transfer in solids. In metals and in insulators, there is conduction of heat due to the vibration of the atoms. As atoms closest to the heat source absorb heat/thermal energy, they make their neighbouring atoms vibrate more rapidly which then in turn make their neighbouring atoms vibrate more. This is how heat gets transferred to the cold end from the hot end.
In metals, there is also conduction through the movement of free electrons. This method of conduction does not operate in non-metals because there are no free electrons (other than graphite). When a metal is heated, the electrons closest to the heat source vibrate more rapidly. Electrons then collide with these atoms and gain more kinetic energy. The electrons therefore move around faster and collide with other free electrons which then gain more kinetic energy. Kinetic energy is therefore transferred between the electrons and through the metal from the point closest to the heat source towards points further away. The electrons all travel very short distances but are very fast moving therefore conduction of heat takes place very quickly. Conductors:
Materials which transfer heat quickly such as metals.
Insulators:
Materials which do not transfer heat quickly such as plastics, glass, water, air.
Examples of conduction
The wire gauzes used on tripods are metal therefore they are good heat conductors. Gauzes on cookers are also metal so that heat is conducted quickly and food is cooked fast. Poor thermal conductors (insulators) are used for saucepan handles so that they don't heat up and can still be handled. Metals are used for the containers which heat liquids e.g. pans, kettles on hobs Air is a poor conductor therefore m aterials that trap air are used for insulation in lofts and hot water cylinders. Trapped air is used in expanded polystyrene, fibre, wool, carpets to help them insulate better. Birds also “fluff” their feathers to trap air in cold weather. Cavity walls use trapped air in expanded polystyrene to insulate rooms. Double glazed windows also use trapped air as an insulating material.
Double glazed window
cavity wall
2. Convection: This is the method of heat transfer in fluids (liquids and gases) Convection involves the movement of the medium itself due to density changes. When air/liquid is heated the cool particles gain kinetic energy from the source and expand as it heats up. The particles become less dense than the surrounding cold air/liquid and rise. Cold particles are more dense therefore they fall and move towards the heat source to take the place of the warm particles. They heat up and rise while other particles cool down and fall. This sets up a convection current in the air/liquid.
Convection current: This is the movement of the fluid due to changes in its density.
Examples of convection 
Radiators use convection to heat up a room.
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Convection is used in fridges to cool it down. Heat is carried away, therefore the back of fridges are always warm.
Land & sea breezes are due to convection Sea Breeze
Land Breeze
Atmospheric winds. Hot water systems.
Difference between conduction and convection Conduction involves the vibrations of the atoms of the medium whereas in convection the whole medium itself moves due to density changes.
3. Radiation: It is the direct transfer of heat energy from the source. It involves the transfer of infra red rays from the hot object to the cold object. Radiation is direct and instantaneous. Direct means that radiation does not heat up the air in the sorroundings and it does not involve vibrations or movement of particles. Radiation passes through spaces in atoms and can even pass through vacuum. Instantaneous means that radiation is fast. You go out in the sun you receive radiation (infra-red) instantaneously. You don’t have to wait for the air around you to heat up.
Examples of Radiation 1. 2. 3. 4. 5.
white shiny suit of an astronaut
Radiant Heaters (radiators). Sun rays heating up objects on the earth. Silvery, shiny reflectors of heater reflect radiant heat back into the room. Silvery, shiny reflectors on the windscreen of cars. Heating units on the roofs of houses absorb radiation from sun and use it to heat water inside copper tubes.
Heating Unit on the Roof of a House
The hotter the object, the more the radiation it emits. A heat sensitive camera can show hot spots. The cat's nose in the picture below is the hottest part.
The Vacuum Flask A vacuum flask prevents heat loss by conduction, convection and radiation. Below you can see the design features of a vacuum flask and how it prevents heat loss.
Heat loss by Conduction
Convection
Radiation
Design feature preventing it 1. The vacuum between the glass walls prevents heat loss by conduction as there are no particles to conduct the heat away. 2. The plastic stopper is an insulator therefore it reduces heat loss by conduction. 1. The vacuum between the glass walls (shell). 2. The rubber bung does not allow the convection currents to build up. Heat loss cannot take place by convection unless hot air escapes out and cold air comes in through the mouth of the bottle. 1. The glass walls are painted silver on both sides so as to reflect the heat back into the hot liquid.
Thermal (infra red) radiation is the transfer of energy by electromagnetic waves. All bodies emit and absorb thermal radiation. The hotter a body is the more energy it radiates. Dark, matt surfaces are good absorbers and good emitters of radiation. Light, shiny surfaces are poor absorbers and poor emitters of radiation. The transfer of energy by conduction and convection involves particles and how this transfer takes place. Under similar conditions different materials transfer heat at different rates. The shape and dimensions of a body affect the rate at which it transfers heat. The bigger the temperature difference between a body and its surroundings, the faster the rate at which heat is transferred.
Some more examples of Objects using the three methods of heat transfer The shape of an object affects the rate at which it absorbs or emits heat. The heat-sink in an electronic circuit is: made of metal so that it conducts well; fin-shaped so that the area for cooling by convection is increased; painted black so that heat can pass by radiation.
A heat-sink is a device placed in circuits to prevent the circuit from heating up. It works just like the black fins of a refrigerator. Difference between conduction, convection and radiation.