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# Thermal energy transfer

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Thermal energy transfer
Describe qualitatively the processes of conduction, convection and radiation. Mike Dickinson 2002

Figure 3.4 - Three ways to score a goal!
The ball is passed from player to player, from one end of the pitch to the other. The players do not move. A single player dribbles the ball himself from one end of the pitch to the other through the other players. A player, in his own half, kicks the ball to score the goal. No other players are involved. Mike Dickinson 2002

Conduction, Covection & Radiation
Heat moves by molecular collisions from one end of a thermal conductor (large molecular oscillation) to the other due to a difference in temperature between the two ends When a fluid is heated it expands and becomes less dense. The part of the fluid where the molecules have a lower density will “float” up above the part with the higher density. Energy from a hot object can be radiated in the form of an electromagnetic wave. Radiation from the sun consists of many different wavelengths. The infra-red (IR) wavelengths are responsible for heating the Earth Conduction happens in solids (mostly). Convection happens in fluids (liquids and gases) Radiation happens in the absence of matter. Mike Dickinson 2002

Thermal energy transfer
Describe examples of conduction, convection and radiation. Mike Dickinson 2002

Conduction Let’s consider a metal spoon in a pan of hot liquid. When a good conductor (the spoon) is placed in contact with something hot (the liquid), then the molecules in direct contact with the hotter substance start to vibrate more vigorously, increasing their kinetic energy and therefore, by definition, increasing the temperature at that end of the conductor. Mike Dickinson 2002

Figure 3.5 - Conduction Movement of heat Cold end Hot end
Mike Dickinson 2002

These increased vibrations cause collisions between neighbouring molecules and the vibrations are passed on from molecule to molecule (like a football being passed from player to player). The molecules in the entire conductor will soon be vibrating vigorously and the other end of the spoon will feel hot. Mike Dickinson 2002

Convection In fluids (liquids and gases), the particles are free to move about and change places. If the temperature in a hot oven is measured, it is found that it is hotter near the top of the oven than at the bottom. As the air molecules gain more energy through heating, the air expands and becomes less dense than the cooler air around it. This causes the warmer air to rise above the cooler air. Mike Dickinson 2002

Convection can also be made to work in reverse
Convection can also be made to work in reverse. The ice-box in a refrigerator is placed at the top so that air that has been cooled by the ice-box contracts and therefore becomes more dense. The more dense, cool air then falls through the cavity of the fridge (chilling the food as it goes). Warmer air is displaced upwards to replace the cool air and is in turn chilled by the ice-box. This circulation of air in a system due to differences in densities is known as a convection current. Mike Dickinson 2002

Figure 3.6 – Convection current
ice-box Mike Dickinson 2002

When cooking vegetables in a pan of water, the vegetables can be seen to circulate around the pan as they move with the convection current that has been set up in the water. Mike Dickinson 2002

Before the advent of forced ventilation systems, mine shafts were ventilated using a convection current. A fire was built under one opening from the mine. The warm air above the fire would rise and be replaced with cool fresh air from an other opening in the mine. Mike Dickinson 2002

Figure 3.7 – Convection current
HEAT Mike Dickinson 2002

Figure 3.8 – Mine ventilation
Mike Dickinson 2002

These convection currents can be created naturally also
These convection currents can be created naturally also. Consider the different winds that blow near the coast in the evening compared with the morning. Figure Sea breezes Mike Dickinson 2002

At sunset, after a day where the sun has been heating the land, warm air rises above the land and is replaced by the air above the (relatively) cool sea. This causes a breeze blowing from the sea to the land. In the morning, after the land has had time to cool down, warm air rises above the (relatively) warm sea and is replaced by air from above the land. This causes a breeze blowing to the sea from the land. Mike Dickinson 2002

Radiation Stand by a bonfire and it is difficult to get close to it because of the radiant heat that the fire is emitting. The hotter an object is, the more radiant heat is given out from it. On a sunny day, if you go outside wearing a black tee-shirt, the shirt will absorb more radiant heat than a white tee-shirt and you will feel very hot because of this. Mike Dickinson 2002

Black surfaces absorb radiant heat better than white or shiny surfaces
Black surfaces absorb radiant heat better than white or shiny surfaces. Radiation is an electromagnetic wave (like light), in the infra-red region of the electromagnetic spectrum and therefore it has similar properties to light. White or shinny surfaces will reflect these waves better than black and therefore will absorb less heat. For example on a cloudy night, radiant heat emitted from the earth is reflected back to the earth from the clouds. The night temperature does not fall as low as on a clear night, when this radiant heat can simply escape. Mike Dickinson 2002

Similarly, black surfaces are good emitters of radiant heat, while white and shiny surfaces do not emit radiant heat well. Kettles and saucepans usually have shiny outer surfaces so that heat is not wasted by radiation from the pan surface. Mike Dickinson 2002

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