Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
16-6 what is convection? obje*ilru Describe how heat travels through gases and liquids. Key Ttzrms convection (kuhn-VEHK-shuhn): transfer of heat in gases and liquids Convection currents are found in 1iquids as well as in air. In bodies of water, warmer water rises to the surface and carries heat with it. Cooler water sinks to the bottom. Convection currents caltse heat to be transferred through bodies of water. When water is heated in a pot, convection currents form. Figure 16-20 shows convection currents in a pot of boiling water. convection current: movement of gases or liquids caused by drfferences in density Heat Transfen in Gases and Liquids The transfer of heat in gases and liquids is called convection. \i\4ren gases and liquids are heated, the particles gain energy and move faster and farther apart. Because these particles are farther apart, the heated gas or liquid becomes less dense than the cooler, surrounding gas or liquid. This makes the less-dense gas or liquid rise and the cooler, denser gas or liquid moves in to take its place. As it rises, the gas or liquid carries heat with it. ffi orrtrur: What is convection? eonveetion Culrrents Heat is carried in gases and liquids by means of convection currents. Convection currents are movements of gases or liquids caused by differences in density. Convection currents transfer heat. Air is a gas. Air at Earth's surface is warmed by the Sun and then rises. Cool air abor.e the North Pole and the South Pole sinks. The rising of warm air and the sinking of cool air cause convection currents. Certain birds, such as eagles and hawks, use rising warm air in convection currents to soar above Earth's surface. I Figure 16-19 An eagle uses convection currents to soar. 342 '\ r" u 44 t "*/ t\*." t "i Figure 16-20 Convection currenls transfer heat in a liquid. Just below Earth's crust is a layer of rock called the mantle. The mantle is heated from below by heat from Earth's core. Rock in the upper part of the mantle behaves like a very thick liqr_rid. Scientists believe that convection currents traveling through the upper mantle are responsible for the very slow morrement of large sections of Earth's crust. This concept is known as the theory of plate tectonics. Hp rxpUtl: How do convection currents transfer heat in liquids? Uses of €onveetion is used in Heat transfer by convection some home heating systems. In a hot-water heating system, water is heated in a hot-water heater. The hot water is then pumped through pipes to each room in the house. The hot water flows through heaters near the floor of each room. The hot water warms the air near the floor by conduction. The warm air rises, carrying heat through each room by means of convection currents. Heat circulates through each room in this way. After the water loses some of its heat, it returns to the hot-water heater to repeat the process. 3. Warm water rises and cool watet 4. Convection takes place in gases and M rHrrrrrcNc cRtncALLY room, should an air conditior-rer be placed at the bottom of a wall or at the top of a r,t all? Explain your answer. ANALYZE:To cool a r' t____a-:t .tll:,::i ::l::::,: Expansion tank Smoke Pump ,i heat transferred through solids by convection? Explain. 6. INFER: Is outlet tliALTH Figure 16-21 ln a holwater heating system, heated water Smoke and gases from the burners of hot-water heaters contain pollutants. These pollutants are sent through an exhaust pipe that runs out through a chimney or vent in the roof. Hot air containing the pollutants rises up the chimney. Cool, clean air moves in from outside to take its place. Why do you think these pollutants need to be removed from a house? travels through a system of pipes to heat a home, f,p torrurtrY: What method of heat transfer is used in a hot-water heating system? CHECKING CONCEPTS 1. Heat travels tl-rrough gases by dense than cold air. 2. Warm air is -=- Inteyating Earth, 'r.,\ '\3 AND sAFETY TrP Convection currenl Soi.P-*t cs T*FIC: plate teetonies CONVECTION CURRENTS INSIDE THE MANTLE Differences in density within a gas or liquid can cause convection currents within the gas or liquid. Under Earth's crust is a thick layer of rock called the mantle. The part of the mantle closest to Earth's core acts like a thick, hot liquid. The part of the mantle that is farthest from the core ,r, Figure 16-22 Convection currents in has the properties of a thick, cooler liquid. Scientists mantle the believe that the differences in the density in the mantle cause convection currents. The thick, hot liquid of the mantle rises and cools. The cooler liquid of the mantle sinks. As the cooler liquid gets closer to the core, it heats up and rises. The process repeats over and over again. Scientists also believe that these convection currents cause the plates that make up the crust to continuously move. The crust floats on top of the mantle. The convection currents in the mantle carry the plates along so that the continents that sit on top of the plates slowly move, too. This concept supports the theory of plate tectonics. The theory of plate tectonics explains how Earth has changed over time. Thinking Critically How do convection currents support the theory of plate tecton ics? CHAPTER 16: Heat 343