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Transcript
Chapter 15 – Section 2 Heat Pages 438 - 443 Heat and Thermal Energy • Heat is the transfer of thermal energy from one object to another when the objects are at different temperatures. • The amount of heat that is transferred when two objects are brought into contact depends on the difference in temperature between the objects. Transfer of Thermal Energy • When heat is transferred, thermal energy always moves from warmer to cooler objects. • This process of heat transfer can occur in three ways—by conduction, radiation, or convection. convection Conduction • Transfer of heat by direct contact is called conduction. conduction • Conduction occurs when the particles in a material collide with neighboring particles. particles • Examples: 1) A frying pan on a stove. 2) Feet on hot sand or a cold tile floor. 3) An ice cube melting in your hand. Radiation • Heat is transferred from the Sun to Earth by radiation. radiation • Heat transfer by radiation occurs when energy is transferred by electromagnetic waves. waves • There is no direct contact with radiation. • Other examples besides the Sun: 1) A fireplace 2) A lamp Convection • In a gas or liquid, liquid molecules can move much more easily than they can in a solid. • The transfer of thermal energy by the movement of molecules from one part of a fluid to another is called convection. convection • Hot fluids become less dense and rise and cooler fluids are more dense and sink. sink • This creates density currents. currents Natural Convection • Natural convection occurs when a warmer, less dense fluid is pushed away by a cooler, denser fluid. • Wind movement near a lake or ocean can result from natural convection. • Thunderstorms and hurricanes are powered by convection. Diagram of a Convection Cells of a Hurricane Thermal Conductors • A thermal conductor is any material that easily transfers heat. • Metals are typically good thermal conductors because they have loosely held electrons that bump into other materials and transfer heat. • Gold and copper are the best conductors of heat. Thermal Insulators • A thermal insulator is a material that does not transfer heat easily. • Liquids and gases are usually better insulators than solids are. • Fluffy insulation is put in-between walls to reduce the amount of heat transfer between the inside and outside of your house. Heat Absorption • Why is pavement hotter than grass? • The change in temperature of an object as it absorbs heat depends on the material it is made of. of • The specific heat of a substance is the amount of heat needed to raise the temperature of 1 kg of that substance by 1°C. • The specific heat of grass is higher than pavement so it takes more heat to increase the temperature of the grass than it does the pavement.