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HEAT EXCHANGE Change process internal energy body without doing work THERMAL CONDUCTIVITY CONVECTION RADIATION
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THERMAL CONDUCTIVITY
Thermal conductivity is a type of heat transfer in which energy is transferred from a warmer part of the body to a less heated one, due to the movement and interaction of body particles. Typical for solids.
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Hot water was poured into aluminum and glass pans of the same capacity. Which of the pots will heat up faster to the temperature of the non-pot? Aluminum conducts heat faster than glass, so the aluminum pan heats up faster to the temperature of the water poured into it
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CONVECTION
Convection is a type of heat transfer in which energy is transferred by streams (jets) of matter. Typical for liquids and gases.
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In industrial refrigerators, the air is cooled by pipes through which chilled liquid flows. Where should these pipes be located: at the top or bottom of the room? To cool the room, the pipes through which the cooled liquid flows must be placed at the top. Hot air, in contact with cold pipes, will cool down and, under the influence of the force of Archimedes, go down.
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RADIATION
Radiation is a type of heat transfer in which energy is transferred using electromagnetic waves (mainly infrared). Can take place in a vacuum.
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Which dress is less hot in summer: white or dark? Explain the answer. Light surfaces reflect solar radiation better, dark ones absorb better. Therefore, it is less hot in a white dress in summer than in a dark one.
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Heat transfer is a spontaneous, irreversible process of energy transfer from more heated bodies or parts of the body to less heated ones. Heat transfer is a way to change the internal energy of a body or system of bodies. Heat transfer determines and accompanies processes in nature, in technology and in everyday life. There are three types of heat transfer: heat conduction, convection and radiation.
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What is heat transfer and thermal conductivity? Sineva Klara Mikhailovna MOU secondary school No. 5, Baltiysk
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Heat transfer The process of changing internal energy without performing work on the body or the body itself Thermal conductivity Radiation Thermal conductivity Radiation
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The process of transferring heat from more heated bodies to less heated ones is called heat transfer.
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Dip a cold metal spoon in hot water. After a while, the spoon will heat up. In this case, you can make sure that the transfer of heat does not occur immediately to all parts of the spoon at the same time, but gradually. First, the part of the spoon that is directly in hot water is heated, and then gradually the whole spoon. The process of transferring heat from more heated parts of the body to less heated as a result of thermal motion and interaction of the particles that make up the body is called heat conduction.
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THERMAL CONDUCTIVITY - the transfer of energy from more heated parts of the body to less heated ones as a result of thermal motion and interaction of microparticles (atoms, molecules, ions, etc.). Leads to an equalization of body temperature. Not accompanied by substance transfer! This type of transfer of internal energy is characteristic of both solids and liquids and gases. The thermal conductivity of various substances is different. There is a dependence of thermal conductivity on the density of a substance.
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Try placing a piece of ice in hot water in a small vessel. After a while, the temperature of the ice will begin to rise and it will melt, and the temperature of the surrounding water will drop. If you put a hot spoon in cold water, it turns out that the temperature of the spoon will start to decrease, the temperature of the water will rise, and after a while the temperature of the water and the spoon will become the same. Now put a wooden stick in the hot water. You can immediately notice that the wooden stick heats up much more slowly than the metal spoon (Fig. 134). Hence, we can conclude that bodies made of different substances have different thermal conductivity.
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The thermal conductivity of various substances is different. Metals have the highest thermal conductivity, and different metals have different thermal conductivity. Liquids have less thermal conductivity than solids, and gases less than liquids. When heating the upper end of a test tube closed with a finger with air inside, you can not be afraid to burn your finger, because the thermal conductivity of gases is very low.
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Observe and explain. Determine the type of heat transfer by which heat is transferred when heating a metal rod over a fire
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Substances with low thermal conductivity are used as heat insulators. Heat insulators are substances that do not conduct heat well. Air is a good heat insulator, so window frames are made with double glazing so that there is a layer of air between them. Wood and various plastics have good thermal insulation properties. You can pay attention to the fact that the handles of teapots are made of these materials (fig. 136), so as not to burn your hands when the teapot is hot.
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To create warm clothes, substances that do not conduct heat well, such as felt, fur, cotton wool, feathers and down of various birds, are widely used. Such clothing helps to keep the body warm. Felt and cotton gloves are used when working with hot objects, for example, to remove hot pots from the stove. All metals, glass, water conduct heat well and are poor heat insulators. Never remove hot objects with a cloth soaked in water. The water in the rag will instantly heat up and burn your hand. Knowledge about ability different materials different ways to transfer heat will help in the hike. For example, in order not to burn yourself on a hot metal mug, its handle can be wrapped with insulating tape, which is a good heat insulator. In order to remove a hot pot from the fire, you can use felt, cotton or canvas mittens.
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A hot pot hangs over the fire. Next to the fire lie: a felt glove, a sheet of foil and a wet rag. Which of these items can you remove the pot from the fire? Explain your answer.
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THERMAL CONDUCTIVITY 1. Wrap a thick nail or metal rod with a strip of paper in one layer. Hold the candle over the flame until it ignites, time it. Why didn't the paper light up right away? 2. ... in the kitchen, when lifting hot dishes, so as not to burn yourself, you can only use a dry cloth. The thermal conductivity of air is much lower than that of water! And the structure of the fabric is very loose, and all the gaps between the fibers are filled with air in a dry cloth, and with water in a wet cloth. Be careful not to burn yourself!
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TASKS FOR THINKING! Prepare three identical pieces of ice, wrap one of them in foil, the second in paper, the third in cotton wool and leave on saucers in the room. Determine the time for complete melting. Explain the difference. If in winter you put your finger on the frozen glass (covered with frost) of a tram or bus and press a coin with the other finger, then the thawing area under the coin will be larger. Why?
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Partridges, ducks and other birds do not freeze in winter because the temperature of their paws can differ from body temperature by more than 30 degrees. Low temperature paws greatly reduces heat transfer. These are the body's defenses! IF ... ... put a piece of styrofoam (or wood) and a mirror on the palm of your hand on the table next to it, then the sensations from these objects will be different: the styrofoam will seem warmer, and the mirror will be colder. Why? After all, the ambient temperature is the same! Glass is a good heat conductor (it has a high thermal conductivity), and it will immediately begin to "take away" heat from your hand. The hand will feel cold! Polyfoam conducts heat worse. He, too, will, while heating, "take away" the heat from the hand, but more slowly, therefore it will seem warmer.
