Figure 3 Radiation National Oilheat Research Alliance 5 iron are good conductors of heat, while most non-metals (plastic and air) are poor conductors. Electricity and heat are both energy in motion. Good conductors of electricity are also good conductors of heat. Materials that are poor conductors, such as plastic or air, are considered insulators. Generally, good electric insulators will also be good heat insulators. Convection Air and water are heat sponges. They suck up and hold a great deal of heat. If they are moving, they will carry the heat with them as they move. Convection depends on motion of a fluid (liquid or gas) from one space to another, carrying heat with it. When a fluid is heated it expands, becoming less dense, causing it to be lighter. As a result, if will rise through cooler fluid. When the water in a pot is heated at the bottom, a natural flow from the bottom to top of the water column is created by differences in density caused by thermal expansion. As air or water moves, it heats the air or water it is moving through. This is called free convection or gravity flow. Old gravity warm air and gravity hot water systems depend on natural convection to circulate the air or water. (Figure 2). A pump circulating the water in a heating system, or a fan circulating air in a furnace and ducts, causes forced convection flow. As that water or air moves, it warms the water and air it moves through. You can feel convection taking place when air flows over your skin. As cooler air passes over your skin, it draws heat from your body and you feel cool. If warmer air flows over your skin, you draw heat from the air and feel warm. Radiation Radiation is transfer of heat by electromagnetic waves of energy. These waves may be visible light, infrared or ultraviolet. Heat from the sun travels through space to warm the earth. The heat travels in the form of electromagnetic waves. Every warm object gives off this type of heat, and cool objects absorb these waves of radiation. Figure 3. Waves of heat travel in a straight line. This is why when facing an open fire, your face will be warm and your back will be cold. You have to be able to see the heat source to receive radiant heat from it. Radiative heat transfer involves the production of heat at a source, its movement through Figure 2 Convection
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