1 11 Heat Homework: 1, 3, 4, 5, 6, 9, 11, 21, 23, 54, 63, 64. 2 Heat Heat is energy transferred due to temperature difference. Symbol, Q [J] Ex. 4186J heat needed to raise 1kg of water one degree C. 3 c = Q/m T [J/(kgK)] heat needed per kg to raise temperature by 1 degree C or K. slope warming water = T/Q = 1/(mc) specific heat 4 example cs in J/(kg-C): aluminum 920 copper 390 ice 2100 water 4186 5 Example: A student wants to check c for an unknown substance. She adds 230J of heat to 0.50kg of the substance. The temperature rises 4.0K. 6 Calorimetry literally: meter the calories emitted by a substance as it cools. Ex. Heated object is added to water. change in temperature of water determines specific heat of object. 7 Example Calorimetry 2kg of substance-A heated to 100C. Placed in 5kg of water at 20C. After five minutes the water temp. is 25C. heat lost by substance = heat gained water. 8 continued: 9 L = Q/m [J/(kg)] heat needed per kg to melt (f) or vaporize (v) a substance latent heat 10 example Ls in J/kg: melting (f) vaporization (v) alcohol 100, ,000 water 333,000 2,226,000 11 Example: How much heat must be added to 0.5kg of ice at 0C to melt it? Q = mL = (0.5kg)(333,000J/kg) = 167,000J same amount of heat must be removed from 0.5kg water at 0C to freeze it. 12 Heat Transfer Conduction Convection Radiation 13 Conduction Heat conduction is the transmission of heat through matter. dense substances are usually better conductors most metals are excellent conductors 14 conduction equation heat current = energy/time [watts] heat current = kA T/L k = thermal conductivity & T = temperature difference, L below 15 conduction example some conductivities in J/(m-s-C): silver 429 copper 401 aluminum 240 Ex: Water in aluminum pot. bottom = 101C, inside = 100C, thickness = 3mm, area = 280sq.cm. Q/t = kA(Th-Tc)/L = (240)(0.028)( )/(0.003) = 2,240 watts heat current 16 Convection Convection transfer through bulk motion of a fluid. Natural, e.g. warm air rises, cool falls Forced, e.g. water-cooled engine 17 Radiation Heat transfer by electromagnetic radiation, e.g. infrared. Examples: space heaters with the shiny reflector use radiation to heat. If they add a fan, they use both radiation and convection 18 Greenhouse Effect dirtier air must be at higher temperature to radiate out as much as Earth receives higher temperature air is associated with higher surface temperatures, thus the term global warming very complicated model! 19 Summary T measured in C, K, F. Use K for gas laws. thermometry uses thermometric properties change in length is proportional to change in temperature for many solids c: heat needed to raise 1kg by 1C. L: heat needed to melt or vaporize 1kg. Heat transfer 20 Phase Change freeze (liquid to solid) melt (solid to liquid) evaporate (liquid to gas) sublime (solid to gas) phase changes occur at constant temperature 21 Temperature vs. Heat (ice, water, water vapor) 22 Heat and Phase Change Latent Heat of Fusion heat supplied to melt or the heat removed to freeze Latent Heat of Vaporization heat supplied to vaporize or heat removed to liquify. 23 Newtons Law of Cooling For a body cooling in a draft (i.e., by forced convection), the rate of heat loss is proportional to the difference in temperatures between the body and its surroundings rate of heat-loss ~ T 24 Real Greenhouse covering allows sunlight to enter, which warms the ground and air inside the greenhouse. the house is mostly enclosed so the warm air cannot leave, thus keeping the greenhouse warm (a car in the sun does this very effectively!) 25 Solar Power Solar Constant Describes the Solar Radiation that falls on an area above the atmosphere = 1.37 kW / m. In space, solar radiation is practically constant; on earth it varies with the time of day and year as well as with the latitude and weather. The maximum value on earth is between 0.8 and 1.0 kW / m.Solar Radiation see: solarserver.de