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THERMAL (HEAT) ENERGY What is HEAT? • Particles move around more and take up more room if heated – this is why things expand when they’re hot • It is also why substances change from: solids liquids gases when heated Heat and Temperature The temperature of an object tells us how HOT it is Measured in degrees Celsius - °C Heating and Cooling If an object has become hotter, it means that it has gained heat energy. If an object cools down, it means it has lost energy Heating and Cooling cont… Thermal energy always moves from: HOT object COOLER object HEAT ENERGY 3 Types of Thermal Energy Transfer Conduction Convection Radiation Conduction Heat is transferred through a material by being passed from one particle to the next Particles at the warm end move faster and this then causes the next particles to move faster and so on. In this way heat in an object travels from: the HOT end the cold end Conduction cont… Occurs by the particles hitting each other and so energy is transferred. Can happen in solids, liquids and gases, Happens best in solids, where particles very close together Conduction does not occur very quickly in liquids or gases Conductors Materials that conduct heat quickly are called conductors All metals are good conductors of heat Copper and aluminium are very good conductor of heat Pans for cooking are usually made with a copper or aluminium bottom and plastic handles Insulators/Poor Conductors Materials that conduct heat slowly or poorly are called insulators Glass, wood, plastic and rubber are poor conductors (good insulators) Nearly all liquids including water are poor conductors (good insulators) Gases, including air are poor conductors,e.g., wool feels warm because it traps a lot of air A refrigerator has insulation material around it to keep it cold – it reduces amount of heat conducted to inside from the warmer room Convection Takes place in material where particles can move around inside the material, i.e. liquid or gas The heat is carried by the particles themselves moving Occurs because an area with warm particles expands and becomes less dense than the cooler areas nearby. The warm particles rise. Cooler particles fall into the space left by the warm particles. This process is called a convection current. Convection Currents Hot liquids and gases expand and rise while the cooler liquid or gas falls 2. Goes across 3. Then down 1. Hot air rises 4. And across Convection cont… The sun can cause large convection currents - WINDS During daytime the land warms up more than the ocean. The warm air rises over the land and cool air falls over the ocean, so we feel a sea breeze. Radiation Transfer of heat directly form the source to the object by a wave, travelling as rays Thermal radiation is also known as INFRA-RED RADIATION All objects that are hotter than their surroundings give out heat as infra-red radiation. Heat transfer by radiation is the only way energy can be transferred across empty space. Thermal Radiation Bright shiny materials are poor radiators Dull dark materials are good radiators Absorbers Shiny, bright can Poor absorber Dull black can Good absorber Key Words Temperature Cold Insulator Radiation Transfer Heat Convection Conduction Conductor