Heat Transfer Heat is another word for thermal energy. Heat energy always moves from warm to cold, until the system reaches equilibrium (balance). Heat Transfer Heat can be transferred (moved) 3 ways: conduction: direct contact between molecules. convection: up-and-down movement of fluids (liquids and gases) due to differences in temperature and density (hot fluids rise , cold fluids sink ). radiation: electromagnetic (EM) waves that travel through space, and transfer their energy as heat when they reach an object. Everyday Examples: CONDUCTION the metal spoon in your hot chocolate gets hot after a while a cold can of soda gets warm after it’s been out of the refrigerator for a while CONVECTION the attic of your house is always warmer than the basement. drafts of cold air come in at the bottom of the door, not the top. RADIATION solar panels collecting and storing solar energy dark clothes are hotter on a sunny day than white clothes. Conductors & Insulators If a material allows heat to pass through it quickly, it is called a good conductor. (Examples: metal, glass) If a material does not allow heat to pass through it quickly, it is called a good insulator. (Examples: plastic, wood) QuickTime™ and a TIFF (Uncompressed) decompre ssor are neede d to see this picture. Heat Transfer by Conduction • Fast-moving, high-energy molecules collide with the slow-moving, low-energy molecules, transferring their energy (heat) to them. Heat Transfer by Convection • When molecules absorb heat, they move faster, spread out and rise because they are less dense. • When molecules lose heat, they slow down, come closer together and sink because they http://www.wisc- online.com/Objects/View are now more dense. Object.aspx?ID=SCE304 This is why we say “Heat rises, cold http://www.echalk.co.uk/S sinks.” It’s actually more accurate cience/physics/convection/ to say “High-energy molecules rise, convection.html low-energy molecules sink,” but that takes way too long to say. Heat Transfer by Radiation • Radiation moves energy without any help from particles of matter. • The Sun’s energy radiates (travels in waves) through space to reach Earth. • Energy that travels by radiation is called electromagnetic (EM) waves. Light is one kind of EM radiation we can see, but visible light is just one tiny part of the EM spectrum. QuickTime™ and a TIFF (Uncompressed) decompress or are needed to see this picture. Radiation http://www.spaceflight.esa.int/i mpress/text/education/Heat%20 Transfer/Radiation.html Electromagnetic (EM)spectrum micrometer As the wave’s wavelength decreases (shortens), the amount of energy it carries increases. Photo of infra-red radiation (heat) coming from a cat Although we can’t see it, the heat we feel on our skin when we stand in the sun or put our hands over a hot stove is caused by infrared radiation, another type of EM radiation. The 3 Kinds of Heat Transfer http://www.wisc-online.com/Objects/ViewObject.aspx?ID=SCE304 All 3 kinds Fire transfers heat energy 3 ways: QuickTime™ and a TIFF (Uncompressed) decompre ssor are neede d to see this picture. Convection Convection currents: currents in fluids (liquids or gases) that transfer heat as warm fluids rise and cold fluids sink. http://www.spaceflight.esa.int/i mpress/text/education/Heat%20 Transfer/Convection%20in%20 a%20Liquid.html Convection Currents in the Earth’s atmosphere Convection currents in the earth Convection Convection currents in the atmosphere & mantle Currents in the Earth’s mantle Most of our weather is the result of convection currents in the atmosphere. convection Forced-Air Heating Systems– fuel is burned in a furnace and a fan circulates the heat in the house Which method of heat transfer is being used here? Which of the following best explains why the end of a spoon sticking out of a cup of hot water also gets hot? The hot water causes a chemical reaction to occur in the spoon. The heat from the hot water is conducted through the spoon to the handle. The hot water heats the air surrounding the upper part of the spoon. The hot water causes a physical change in the spoon handle. When it is very hot in his room, Stuart makes it cooler by opening both the top and the bottom of his window. Which statement best explains why this works? Air comes in the top to cool the ceiling and in the bottom to cool the floor. Cooler air comes in the bottom and warm air goes out through the top. Warmer air goes out through both the bottom & top. It does not make any difference because if there is no wind, the air will not come in. In which situation has heat energy moved mainly by conduction? A radiator heated with hot water warms a room. The person sitting closest to the blazing fire finds it necessary to move farther from the heat. The handle of a cast iron skillet being used to cook something becomes too hot to touch. The red-hot coil of an electric space heater is used to keep a small room warm. Which BEST explains how the food on the stove is being heated? conduction convection interaction radiation Which of the following is an example of heat transfer by convection? A hot wood stove heats an entire room. The handle of a pot that is being heated becomes warm. A teaspoon in a glass of hot tea becomes warm. The coils of a toaster become red. What is the process by which heat energy gets to Earth from the Sun? conduction radiation subduction convection Patti used a steel wrench to work on her bicycle on a hot, sunny day. She left the wrench on the hot concrete for an hour, then returned, to find the wrench very hot to touch. What is the BEST explanation for how the wrench got so hot? Heat transferred from the wrench to the air. Heat transferred from the bicycle to the wrench. Heat transferred from the wrench to the concrete. Heat transferred from the concrete to the wrench.