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Transcript
Energy in Thermal Processes Chapter 11 Internal Energy Internal energy, U, is the sum total of all types of energy possessed by a system of atoms or molecules. Internal energy can include random kinetic energy of linear motion, rotational motion, vibrations, and intermolecular potential energy. James P. Joule Studied the relationship between heat and mechanical energy The unit of energy was named after him 1 calorie = 4.186 Joules 1 Calorie = 4186 Joules Heat Capacity Heat Capacity is a measure of how much heat energy must be added to raise the temperature of an object. A large heat capacity means that a lot of heat must be added transferred to raise the temperature of the object by a given amount. A bigger object of the same material has a bigger heat capacity. Specific Heat Capacity Is the heat capacity per unit mass of a material. It does not depend on how large the object is – only what it is made of. Its name is often shortened to just “specific heat”. Specific Heats for Various Materials Note that water has one of the highest values of specific heat. Materials with heavy elements, such as gold and lead have much smaller specific heats. Latent Heat Q = ± m L is the amount of heat added or removed during a phase transition. Heat Conduction When two materials at different temperatures are placed in contact, heat flows from the hotter one to the cooler one by the process of conduction. Heat Conduction The rate of heat conduction depends on the difference in temperature across the material, the thickness, the area the heat if flowing through and the thermal conductivity, k. Thermal Conductivity The thermal conductivity of a material is a measure of how well it conducts heat. Insulation Poor thermal conductors reduce the amount of heat conducted. The fiberglass insulation works by trapping a layer of air (which is a poor heat conductor). Double-paned windows work the same way R-Values R = L / k is a measure of the resistance to heat flow of a material. Higher R-Values mean better insulation The R-Values of successive layers add together.