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Transcript
Heat and Temperature Energy that makes things hot! Outline Review of Principle of Conservation of Energy. Temperature and Thermal Equilibrium. Heat Capacity Thermal Expansion Temperature Measurements Conservation of Energy E total = Ek + EP Energy conversions: The Faith Pendulum. Hydroelectric Power EP work turning a turbine. Wind Power Ek work turning the rotor blades. Heat Power ? Work turning the fan Thermal Equilibrium Hot chili and cold coke example Definition: Heat is what flows between two objects when there is a difference in temperature. Heat flows from the higher temperature to the lower temperature, so that the higher temperature falls, and the lower temp. rises! Heat flows until the two temperatures are the same, and then it stops. Heat and Temperature Heat = Q (units?) Temperature = T (degrees C for now) Heat Capacity = c: Q = c m ÄT “Heat” = “Heat Capacity” times “mass” times “change in Temperature” Answers the question: How much heat must I add to raise or lower the temperature? Heat Capacity demonstrations Heating a wax paper cup Heating a wax paper cup with water Heating a balloon Heating a balloon with water Which cools off first? The aluminum pan or the cheese on the pizza? Heat Units A calorie: the amount of heat needed to raise 1 gram of water from 14.5 C to 15.5 C So, c = 1 cal/g-C A Btu (British thermal unit): the amount of heat needed to raise 1 pound of water from 63 to 64 F. Then c = 1 Btu/lb-F A Food calorie is 1 kilocalorie or 1,000 calories. Heat is energy! 1 calorie = 4.18 Joules; 1 Btu = 1055 Joules. Heat Capacity examples Heat capacity of water c = 1 cal/g-C; of gold, c = .03 cal/g-C. How much heat is needed to raise the temperature of 100 g of water 10 C? Q = 1cal/g-C x 100 g x 10 C = 1,000 cal How much heat needs to be released to raise the temperature of 100g of gold by 10 C? Q = .03cal/g-C x 100 g x 10 C = 30 cal Thermal Equilibrium Two objects at different temperatures are placed together. By energy conservation – Q lost by the hotter one must equal Q gained by the lower one. The final temperature of the two objects will be the same. The ratio of heat capacities (and masses) tells the relative temperature change. For equal masses of gold and water: c water / c gold = 1 / .03 = 33 --- gold will change in temperature 33 times as much as the water.