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Heat Transfer One of the most striking properties of heat is that it is constantly in motion. Heat is a form of energy, and it moves from warmer materials to cooler materials or from warm areas to cool areas in the same material. It will keep moving until there is no longer a temperature difference between the two. This flow or transmission of heat is called heat transfer. People spend much of their time trying to control the movement of heat so that they can get it into places where it is needed and keep it out of places where it is not wanted. They introduce heat into their dwellings in cold weather and seek to prevent its entry in hot weather. Winter clothes are designed to prevent the escape of heat from the body; summer clothes, to promote its flow away from the body. Effective methods of transferring heat are essential in hundreds of industrial operations, such as the tempering of steel, the molding and curing of plastics and rubber, the processing of glass and pottery, and the preparation of chemicals. Heat is transferred in three ways: by conduction , by convection , and by radiation . In some situations, one or more of these processes may be involved at the same time ( Figure 1 ). Conduction When heat is applied to a material, the material's temperature rises. This happens because molecules close to the heat source vibrate more rapidly as they absorb heat energy. Although they remain in their places, they cause neighboring molecules to vibrate, thus transmitting heat energy. In this way, the vibrations spread out from the heat source, causing the temperature increase. Heat is transferred from molecule to molecule, much as pails of water are passed from one person to the next in a bucket line. All substances conduct heat, some much more effectively than others do. Copper, for example, is an excellent conductor; glass, a poor one. If you were to take a copper rod about the size of a pencil and hold one end of the rod in a flame, the other end would soon become too hot to handle. Heat would quickly be transmitted from the end in the flame to the other end. If you were to hold a glass rod of similar dimensions in a flame in the same way, the heat would pass along the rod very slowly. You would probably be able to hold the unheated end of the rod even when the other end became very hot. Very poor conductors, such as glass, are called insulators . The coefficient of heat conduction , or coefficient of thermal conductivity , indicates a material's ability to conduct heat. A high coefficient means that heat moves very quickly through the material; a low coefficient means that heat moves very slowly. As a rule, solids conduct heat better than liquids do, and liquids conduct heat better than gases do. Some solids, particularly metals, are fine conductors. Silver, copper, and aluminum are the best of all. A simple experiment ( Figure 2 ) shows that liquids such as water are poor conductors of heat. Place a small piece of ice at the bottom of a test tube and hold the ice down by means of a small lead weight or a wad of steel wool. Fill the tube about three­quarters full of water, and hold it over a hot flame in such a way that only the upper portion of the water is heated. Because water is such a poor conductor, the ice will not be melted even after the water at the top of the tube is boiling. Water conducts heat only about 1/650 as well as silver does. Air is even more effective as an insulator—it conducts heat only about 1/25 as well as water does. The insulating properties of mineral wool, felt, and cork are due largely to the fact that they are full of small spaces containing air. Air is also trapped in the fur of animals and the feathers of birds. This is why fur and feathers provide excellent protection against cold. Before the days of electric refrigeration and home freezers, people would cut large blocks of ice from rivers and ponds in winter. The blocks would be stacked in a building called an icehouse and packed in sawdust, which contains countless tiny pockets of trapped air. The insulation provided by the sawdust preserved the ice for months. Conduction in Everyday Life You encounter examples of conduction and insulation every day. A tiled bathroom floor feels colder to your bare feet than a carpeted floor, for example. The cold sensation is actually a loss of body heat—the carpet conducts little heat from your body, while the tile is a much better conductor. A blanket or a quilt provides warmth because the air that is trapped among its fibers prevents the loss of body heat. A pot holder protects your hand from a hot pan because the pot holder is made of insulating materials. In winter cold and summer heat, the insulating properties of air help keep houses and other buildings comfortable. Insulated houses, for example, have double walls, with a space between the interior wallboard or plaster and the exterior siding or brick. Some sort of insulation material, such as fiberglass batting or mineral wool, fills the space. These materials trap tiny pockets of dead air, preventing heat transfer by conduction or convection. Insulation is also placed between the joists in an attic floor or between the rafters of a roof to prevent the escape of heat. After a snowfall, you can often spot which houses in a neighborhood are well insulated. Their roofs remain covered with snow, while, on houses with poorly insulated roofs, the snow is quickly melted by escaping heat. Trapped air also helps stop heat loss at doors and windows. Storm windows set outside regular windows provide a nonconducting air space between two panes of glass. Double­ and triple­glazed windows and patio doors have two or three layers of glass, with a thin air space between each pane. Instead of air, the gas argon—an even better insulator than air—is sometimes used to fill the spaces between the panes. Low­e (low­emission) glass has a special coating that also helps keep heat from escaping. Convection Heat circulates through liquids and gases by convection, which is the movement of heated particles from one place to another. Convection cannot take place in solids because the molecules that make up a solid are fixed in place. Liquids and gases are fluids, and their molecules can slide around. In a fluid, warmer molecules rise, and cooler molecules sink. This process usually begins with conduction. For example, if you set a pot of water on a hot burner, heat from the burner is transferred through the bottom of the pot by conduction. The water in contact with the bottom of the pot is also heated by conduction. As the molecules of water gain heat energy and vibrate more vigorously, the water in this bottom layer expands in all directions and becomes less dense. Because it is less dense, the layer of water at the bottom of the pot is now lighter than the cooler water above. Thus, the bottom layer of water rises, and the cooler, denser water sinks to take its place. When the warm water reaches the surface, it gives up some of its heat to the air (through conduction). As it cools, this layer of water sinks again; and its place is taken by more warm water moving up from below. In this way, a complete circuit of moving water is set up. Circulatory movements of this kind are called convection currents . Convection currents are set up in air in much the same way as in water. However, the flow of air in such currents is more rapid than the flow in water. The reason is that air is much lighter and easier to move than water, and air expands more when it has been heated. Convection currents can carry heat over great distances; they are common in nature and account in large part for winds and for Earth's patterns of weather and climate. In the atmosphere, winds are created when air is heated by the Sun. The warm air rises, and cool air rushes in to take its place. This is why coastal areas often have cooling sea breezes on hot summer days. The air over the land heats up faster than the air over the sea, so it rises and the cooler air flows in from the sea to take its place ( Figure 3a ). At night, the land cools more quickly than the sea. The situation is reversed, so warm land breezes blow ( Figure 3b ). Convection in Home Systems When you open the hot­water tap in the kitchen or bathroom, you are putting the convection principle to work. In most homes, the tap draws from a hot­water heater. The heater is a large tank with electric heating elements or, in some cases, a gas or oil­fired burner. Cold water enters the heater through a pipe near the bottom of the tank. As this water is heated, it rises, setting up convection currents that carry heat throughout the tank. Hot water is drawn off for use through a pipe at the top of the tank and is replaced by more cold water piped in at the bottom. This water is heated in turn. Most home heating systems also depend on the action of convection currents. In a hot­water heating system ( Figure 4 ), for example, water is heated in a boiler and rises through pipes to radiators in the different rooms of the house. There, it gives up its heat and then returns by other pipes to the base of the boiler. The heated radiators, meanwhile, create convection currents in the air. Air around the radiator is warmed first. It expands, rises, and spreads over the ceiling, while colder air from near the floor moves in around the radiator. This cold air is heated in turn and also rises. In this way, convection currents are set up in the room, circulating and warming all the air. In hot­air heating systems, air heated in a furnace is distributed to the rooms of the house through wide ducts and enters the rooms through grilles, also called registers, set in the floor or at the base of the walls. The hot air rises and spreads through the room. As it cools, the air sinks back toward the floor. There, the air enters separate ducts that carry it back to the furnace, where it is heated once again. Modern heating systems are much more efficient than those of the past. A room can actually lose heat through an open fireplace ( Figure 5 ), for example. This happens because a convection current carries warm air up the chimney when logs are blazing away in the fireplace. Before the fire is started, the air inside the chimney has the same density as the air outside it. Once the fire is going, the air above the flames is warmed, expands, becomes lighter, and, finally, rises up the chimney. As it does, the rising air creates a pressure differential called a draft, which draws air from the room into the fireplace and upward into the flue. The hotter the air in the chimney compared to the air outside, the stronger the draft. The height of the chimney also affects the draft. A tall chimney draws better than a short one—that is, a stronger convection current is set up within the structure. In a tall chimney, there is a greater difference in pressure between the column of gases in the chimney and the outside air. If warm air from the room goes up the chimney along with heat from the fire, why do you still perceive warmth when you sit in front of a roaring fire? You are feeling the effects of the third method of heat transfer, radiation. Radiation When you step outside on a sunny day, you feel the warmth of the Sun on your skin. The Sun's heat cannot reach you through conduction or convection because the space between Earth and the Sun is a vacuum. There are no molecules of air or anything else to carry heat through a vacuum. Instead, heat energy from the Sun is being transferred directly to your body without the aid of any intervening molecules, through radiation. Radiation transfers heat in all directions in the form of electromagnetic waves. These waves travel in straight paths through space at the speed of light—approximately 186,000 miles (300,000 kilometers) per second. All bodies produce radiant heat to some degree. The higher the temperature of a body, the greater the amount of radiation it will emit. An object such as the Sun emits other forms of electromagnetic radiation as well. Radiant heat, or infrared radiation , is closely akin to these other forms of electromagnetic radiation, which include gamma rays, X­rays, ultraviolet rays, visible light, and radio waves. These electromagnetic radiations vary greatly in wavelength. X­rays, for example, may be only 0.003937 microinch (0.00000001 centimeter) in length. Radio waves may be several miles long. Some heat waves are longer than others. The higher the temperature of a radiating body, the shorter the waves. When radiant heat falls upon an object, some of the heat may be reflected, some may pass through, and some is absorbed. An object is warmed only by the heat that it absorbs. If an object is smooth and highly polished, most of the radiant heat is reflected and little of it is absorbed. If the surface is dull and rough, a great deal of the heat is absorbed and little is reflected. Color also plays an important part in the absorption or reflection of heat. Dark objects absorb more heat than light­colored objects do, and reflect less. This is the reason why dark garments help people stay warm in cold weather, and light­colored garments help people stay cool in warm weather. Because of the differences in their wavelengths, electromagnetic waves behave quite differently when they encounter various substances in their path. Certain materials are transparent to light but are opaque to heat, and vice versa. Clear glass, for example, is transparent to light and to short wavelengths of radiant heat, but opaque to longer wavelengths of heat. This fact makes possible what is known as the greenhouse effect . Since the Sun is very hot, most of the radiant heat that comes from it to Earth is of very short wavelengths. The glass of a greenhouse thus transmits both light and radiant heat to the inside. The soil, plants, and other objects inside are warmed and then emit radiant heat. But because their temperature is vastly lower than the temperature of the Sun, the waves of the radiant heat they emit are too long to pass through the glass. The heat is trapped within the greenhouse ( Figure 6 ). The same phenomenon explains why the interior of a parked car quickly heats up on a sunny day. The car takes in light and heat from the Sun; but radiant heat from the interior cannot escape. In Earth's atmosphere, carbon dioxide and certain other gases act like the glass in a greenhouse. They allow the Sun's radiant energy to reach Earth, but they block longer wavelengths of radiant heat from escaping into space. By holding some of the Sun's heat, even at night, these gases have allowed life as we know it to flourish on Earth. But most scientists believe that human activities, including the widespread use of fossil fuels, are producing a buildup of carbon dioxide and other so­called greenhouse gases in the atmosphere, leading to a steady warming in Earth's climate ( Figure 7 ). Using Radiant Heat Home heating systems that use radiators warm rooms by radiation as well as convection. If you hold your hand a few inches to the side of a baseboard heater, for example, you can feel this radiant warmth. Hot­water pipes or electric heat cables can also be embedded in panels set in the floors, ceilings, or walls of rooms. When the heating unit in the panel is operating, heat is radiated, warming all the objects that it strikes. If the panels are set in the floor, the air in the room will be gradually warmed and convection currents will form. Solar heating is an application of radiant heat. A passive solar home is designed to capture the Sun's heat much as a greenhouse does. Such homes are usually designed with large areas of glass facing south, to admit as much sunlight as possible. In addition, passive solar designs make use of heat­absorbent materials such as stone and, in some cases, tanks or pipes filled with water. During the day, the sunlight warms these materials, which are usually dark in color so that they soak up as much heat as possible. At night, the absorbers slowly emit the heat they have stored during the day, warming the house. Portable solar cookers use reflective surfaces to focus incoming solar radiation, achieving temperatures high enough to cook food. Solar water heaters, used in homes and businesses, use flat panels that are often mounted on south­facing roofs. Solar radiation shines through a glass cover to black absorber fins attached to thin, fluid­filled pipes. The fluid circulates to a heat exchanger, where it is used to heat water, and then returns to the solar panel. Active solar technology makes use of photovoltaic cells to turn the Sun's energy into electricity. Mirrors often are used to focus incoming solar radiation on the cell, which contains a semiconductor chip. Photovoltaic cells are used to provide auxiliary power to some homes and businesses, and they help power water pumps, cell­phone repeater stations, and similar devices in remote areas. Solar energy produces no pollution, but so far it has proved more costly than other methods of energy production. Article Citation "Heat Transfer." The New Book of Popular Science. Grolier Online, 2015. Web. 1 Dec. 2015.