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Chapter 22 - Heat Transfer p.430 Discover activity If several objects of different temperatures are brought near to one another, eventually, they will all have the same temperature. There are three ways that heat is transferred: conduction, convection, and 22.1 Conductio n Conduction is the movement of energy within one substance or between materials that are touching. Conductors are simply materials that conduct or move heat easily – like metals. ”In conduction, collisions between particles transfer thermal energy without any overall transfer of matter.” When a metal is heated the atoms move more rapidly and collide more frequently with nearby atoms causing them to gain energy. Because metals have free electrons they conduct heat and electricity easily. radiation. Touching a piece of metal will feel colder than touching a piece of wood that is the same temperature. Heat flows easily from your hand to the metal, but not the wood. This is because the electrons in the wood are tightly bound to the atoms, making it a poor conductor – or an insulator. An insulator is simply a material that conducts heat poorly or delays the transfer of heat. Air and materials containing air spaces (such as down) make good insulators. While heat is energy, cold is not a real thing but the absence of heat – like dark being the absence of light. Insulation in houses slows down the flow of heat from the house. 1 22.2 Conduction moves energy Convection without moving matter. Convection is the movement of test tube energy by moving the heated with ice on matter, for example, currents bottom in a fluid. ”In convection, heat is transferred by movement of the hotter substance from one place to another.” Convection or the movement of energy through fluid materials depends largely on buoyancy and gravity. For this reason matches or candles on the space station will quickly extinguish for lack of oxygen. Warmer fluids are less dense and are buoyed upward while cooler, denser fluids fill in the space below them. Convection currents are the source of wind patterns near the shore. The land heats up more in the day and cools more in the night while the air over water changes less. So winds go from the cool sea to the warm land in the day and from the cool land to the warmer sea in the night. 22.3 Radiation polar During the daytime warm air rises and expands. As air expands, it cools. There is lower pressure near the surface of the land and high pressure aloft. (We can experience the cooling of air as it expands by blowing through pursed lips or releasing air from a bicycle tire.) Heat from the sun cannot reach Earth through either conduction or convection because there is no matter between us. Energy transferred or transmitted by electromagnetic waves is called radiation. The energy transmitted through radiation is called radiant 2 regions are cooler because of angle of sun. Demo angled paper with ‘rain’ or horizontal energy. ”In radiation, heat is transmitted in the form of radiant energy or electromagnetic waves.” 22.4 Emission of Radiant Energy ”All substances continuously emit radiant energy in a mixture of wavelengths.” Objects that are cooler are moving less and make longer waves, such as microwaves or radio. Object with higher temperatures emit waves that have higher frequencies and smaller wavelengths. Think about an electric stove heating up. It begins to emit infrared waves and then glows as it increases in temperature. The frequency of radiant energy is directly proportional to temperature. Stellar radiation is the radiant energy emitted by stars. The color of the star can tell us its temperature. Red stars are much cooler than blue stars. We know this because blue is produced by more energetic movement of atoms producing higher frequency light. The earth emits radiation, too, but this is at a much lower frequency than the radiation from the sun. This is called terrestrial radiation. Most of this terrestrial radiation is in the infrared part of the spectrum that is invisible to our eyes. 22.5 Absorption of Radiant Radiation from the sun is produced by thermonuclear fusion. This is hydrogen being fused into helium atoms. The Earth produces radiation from radioactive decay in its interior (fission). Both the sun and the Earth produce radiant energy, but Earth’s radiation is at low infrared frequencies. Some radiant energy is absorbed and some is reflected. All objects both absorb and radiate or emit energy. When the rate of absorption and the rate of emission are the same, the object is in thermal equilibrium with its environment. If the objects absorbs 3 Energy Collom’s white roof Burning paper w/glass 22.6 Newton’s Law of Cooling more than it emits, the temperature increases. If absorption decreases relative to radiation, then temperature will be lower. ”Good emitters of radiant energy are also good absorbers; poor emitter are poor absorbers.” Radiant energy can reflect repeatedly until the energy is absorbed and nothing reflects. A good reflector typically absorbs little radiant energy. Light colored objects or clothes reflect energy without absorbing much and are cooler. The surface of Earth absorbs energy in the day and emits it at night. Clear nights will be colder because clouds reflect energy back to the surface. In New Mexico it is much cooler in the shade than in the sun. Not so in Florida. Why is this? Rate of cooling is how many temperature degrees of change happen in a unit of time. The rate of cooling depends on how much difference there is between objects. “The colder an object’s surroundings, the faster the object will cool.” Newton’s law of cooling says that the rate of cooling is proportional to the temperature difference. Chilling wort from a boil to 80°F shows a rapid temperature change at first because the difference is great. This slows as the wort approaches the cool water temperature. 4 22.7 Global Warming and the Greenhous e Effect try colored films to show selective transparen cy When radiation is trapped by the atmosphere and causes the planet’s surface to warm, this is the greenhouse effect. (To an extent understanding the greenhouse effect depends on understanding the idea of transparency which we will discuss in depth in chapter 27.) The sun being hot (high energy & atomic movement) produces short wavelength radiation that passes through the atmosphere and is absorbed by Earth. The Earth re-emits this energy but at a longer wavelength—because it is cooler. Our atmosphere is transparent to the short wave radiation from the sun but more opaque to the longer wavelength energy being emitted from the Earth. Certain molecules such as methane and CO2 increase temperatures by holding more reemitted terrestrial radiation. Over long averages, the amount of heat in the form of terrestrial radiation leaving the earth balances the solar radiation coming in, so the temperature remains stable and supports life. ”The near unanimous view of climate scientists is that human activity is a main driver of global warming and climate change.” Increased levels of CO2 also leads to more water vapor in the air. As more energy is trapped patterns in storms and climate such as rainfall. Remember these are long term patterns or cycles, not short-term weather. The Rocky Mountains experienced flooding in the fall of 2013 even though our continuing pattern is a long-term drought. 5 6