Chapter 11 Heat

Heat Heat is transferred thermal (internal) energy resulting in a temperature change. Heat is transferred thermal (internal) energy resulting in a temperature change. Heat is similar to Work, which is a transfer of mechanical energy. Heat is similar to Work, which is a transfer of mechanical energy. Heat is measured in Joules or calories or kilocalories. Heat is measured in Joules or calories or kilocalories.

calories and kilocalories 1 calorie is the amount of energy needed to raise the temperature of 1g of H2O by 1 degree C. 1 calorie is the amount of energy needed to raise the temperature of 1g of H2O by 1 degree C. 1 kcal is the amount of energy needed to raise the temperature of 1 kg of H2O by 1 ºC. 1 kcal is the amount of energy needed to raise the temperature of 1 kg of H2O by 1 ºC. Food energy is measured in Calories (kilo calories) Food energy is measured in Calories (kilo calories) 1 cal = 4.18 J 1 cal = 4.18 J 1 kcal = 4180J 1 kcal = 4180J

Example If you eat a slice of cake with an energy value of 400 Calories, how long will you have to ride your bike in order to work this off? Assume the body’s power output while riding is 200 Watts. If you eat a slice of cake with an energy value of 400 Calories, how long will you have to ride your bike in order to work this off? Assume the body’s power output while riding is 200 Watts.

Specific Heat Heat, Q, is transferred energy and is measured in [Joules] or [kcal]. Heat, Q, is transferred energy and is measured in [Joules] or [kcal]. Different materials require different amounts of heat to change the temperature by one degree. Different materials require different amounts of heat to change the temperature by one degree. c = Q/mДT = specific heat c = Q/mДT = specific heat Specific heat give the heat required to change the temperature of one kg by one degree Celsius Specific heat give the heat required to change the temperature of one kg by one degree Celsius

Common Specific Heats Aluminum920 J/kgºC0.220 cal/gºC Copper390 J/kgºC cal/gºC Water 4186 J/kgºC 1.00 cal/gºC See Table 11.1 page 370 for other specific heats

Calculating Heat Transfer Q = mcДT gives the heat transferred as a mass, m, undergoes a temperature change ДT.

Example At the birthday party, another student ate the same slice of cake (400 Cal) but reasoned that if she drank enough ice water at 0ºC, the Calories would be used in warming up the ice water. How much ice water would she need to drink? At the birthday party, another student ate the same slice of cake (400 Cal) but reasoned that if she drank enough ice water at 0ºC, the Calories would be used in warming up the ice water. How much ice water would she need to drink?

Calorimetry Calorimetry is a technique for measuring heat exchanges by minimizing heat transfer with the environment. Calorimetry is a technique for measuring heat exchanges by minimizing heat transfer with the environment. In an isolated container in which no heat is exchanged with the environment, the total heat lost by an object is equal to the total heat gained by another. In an isolated container in which no heat is exchanged with the environment, the total heat lost by an object is equal to the total heat gained by another. Σ Q = 0 where Q = mcДT Σ Q = 0 where Q = mcДT

Phase Changes Melting - solid to liquid Melting - solid to liquid Freezing – liquid to solid Freezing – liquid to solid Boiling – liquid to gas Boiling – liquid to gas Condensing – gas to liquid Condensing – gas to liquid Sublimation – solid to gas Sublimation – solid to gas

Latent Heat Latent Heat is the heat required by a mass to change phase. During a phase change, temperature remains constant in spite of heat being added (or removed). The added heat goes into breaking bonds instead of changing temperature. Latent Heat is the heat required by a mass to change phase. During a phase change, temperature remains constant in spite of heat being added (or removed). The added heat goes into breaking bonds instead of changing temperature. L f –Heat of Fusion – solid to liquid phase change [J/kg] L f –Heat of Fusion – solid to liquid phase change [J/kg] Lv – Heat of Vaporization – liquid to gas change [J/kg] Lv – Heat of Vaporization – liquid to gas change [J/kg] Q = mL heat required to change phase Q = mL heat required to change phase

Latent Heat For Water: L f = 3.33 X 10 5 J/kg L v = 22.6 X 10 5 J/kg

Example Heat is added to 2kg of ice at -10ºC. How much heat is needed to generate steam at 110º C?? Heat is added to 2kg of ice at -10ºC. How much heat is needed to generate steam at 110º C??

Phase Change Diagram for 1kg H2O

Phase Change Diagrams

Example A liver for transplant is packed in ice in an insulated container. Liver has mass 0.50kg and is 29º C when packed. It is surrounded by 2.0kg of ice at -10º C. If the specific heat of liver is c L = 3500 J/kgºC, find the equilibrium temperature of the ice and liver. A liver for transplant is packed in ice in an insulated container. Liver has mass 0.50kg and is 29º C when packed. It is surrounded by 2.0kg of ice at -10º C. If the specific heat of liver is c L = 3500 J/kgºC, find the equilibrium temperature of the ice and liver.

Heat Transfer Conduction – heat transfer by direct contact; energy is transferred by molecular collisions. Conduction – heat transfer by direct contact; energy is transferred by molecular collisions. Convection – heat is transferred indirectly via currents set up in a fluid; energy is transferred via molecular collisions. Convection – heat is transferred indirectly via currents set up in a fluid; energy is transferred via molecular collisions. Radiation – transfer of heat involving NO mass but instead electromagnetic energy Radiation – transfer of heat involving NO mass but instead electromagnetic energy