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Thursday, September 4, 2008
Chapter 8: Transfer of Thermal Energy
- Thermal energy is transferred only when there is a difference in temperature. - Thermal energy always flows from a region of higher temperature to a region of lower temperature. Thermal energy is transferred by any of these three processes;
1. Conduction Conduction is the transfer of energy from one molecule to another. This transfer occurs when molecules hit against each other, similar to a game of pool where one moving ball strikes another, causing the second to move. Conduction takes place in solids, liquids, and gases, but works best in materials that have simple molecules that are located close to each other. For example, metal is a better conductor than wood or plastic. Conduction is the process of thermal energy transfer without any flow of the material medium. 2. Convection Convection is the movement of heat by a liquid such as water or a gas such as air. The liquid or gas moves from one location to another, carrying heat along with it. This movement of a mass of heated water or air is called a current. Convections requires the movement of particles, so it can only take place in liquids and gases. Convection is the transfer of thermal energy by means of currents in a fluid (liquids or gases) 3. Radiation Heat travels from the sun by a process called radiation. Radiation is the transfer of heat by electromagnetic waves. When infrared rays strike a material, the molecules in that material move faster. In addition to the sun, light bulbs, irons, and toasters radiate heat. When we feel heat around these items, however, we are feeling convection heat (warmed air molecules) rather than radiated heat since the heat waves strike and energize surrounding air molecules. Radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium. Unlike conduction and convection, radiation does not require a medium for energy transfer. This means that radiation can take place in a vacuum. Factors affecting the rate of infrared radiation Rate of infrared radiation depends on three factors:
Applications of Thermal Energy Transfer Uses of good conductors of heat If thermal energy has to be transferred quickly through a substance, good conductors of heat such as metals are used. Some examples of the uses of metals are: 1. Cooking utensils like kettles, suacepans and boilers. They are usually made of stainless steel where direct heating is involved. 2. Soldering iron rods are made of iron with the tip made of copper, as copper is a much better conductor of heat than iron. 3. Heat exchanges, such as those used in a large laundry facility, help save energy. Uses of bad conductors of heat (Insulators) Insulators are very useful if we want to minimise loss of thermal energy, or prevent thermal energy from being transferred quickly. Some common uses of insulators are: 2.Tables mats are usually made of cork so that hot kitchenware can be placed on them without damaging the tabletop 3. Sawdust is used to cover ice blocks to reduce melting because of its good insulating property. 4. Wooden ladles are very useful for stirring or scooping hot soup and also for scooping rice that has just been cooked 5. Woolen clothes are used to keep people warm on cold days. 6. Fiberglass, felt and expanded polystyrene foam which trap large amounts of air are employed as insulators in the walls of houses, ice boxes and refrigerators.
Common applications of radiation
END OF CHAPTER EIGHT :D |
