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Matter and Energy

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Drill What mode of heat transfer best describes the scenarios below: Energy transfer through a brick wall Air blowing over a hot surface The sun warming your face on a cold day

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Matter and Energy Heat (Q) – energy transfer caused by a temperature difference Thermodynamics - “the study of systems and energy transfer” ConductionConvectionRadiation

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Matter and Energy Energy is transferred from a warm room at 20C inside a house to the outside air at -10C through a single-pane window. The glass is 5mm thick with an area of 0.5 m 2 and a conductivity of 1.4 W/m K. What is the rate of heat transfer through the glass? Known: Heat flows through a window pane (A=0.5m 2, ∆ x = 5 mm, k = 1.4 W/m K) Find: Sketch: Assumptions: Closed System Solution: T room = 20C = 293 K T outside = -10C = 263 K ∆x

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Matter and Energy Cold air at -10C blows over a warm window-pane with a surface temperature of 12C. The glass has a surface area of 0.5 m 2 and the convective heat transfer coefficient is h =100 W/m 2 K. What is the rate of heat transfer through the glass? Known: Heat flows through a window pane (A=0.5m 2, 100 W/m 2 K) Find: Sketch: Assumptions: Closed System Solution: T glass = 12C = 285 K T outside = -10C = 263 K

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Matter and Energy A small light bulb with a surface area of 0.025m 2 and an emissivity of 0.6 fluoresces at a temperature of 100C. What is the rate of heat transfer from the light bulb? Known: Heat radiates from a bulb (A=0.025m 2, T bulb = 100 W/m 2 K) Find: Sketch: Assumptions: Closed System Solution: T bulb = 100 C = 373 K

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Matter and Energy Thermodynamics - “the study of systems and energy transfer” A piston cylinder containing a gas was compressed over a period of 20 seconds. The change in energy of the system was 300 kJ and the average rate of heat transfer from the cylinder was 12 kW. Find the work done on the gas in kJ. Known: A gas is compressed inside of a piston-cylinder Find: W, [kJ] Sketch: Assumptions: Closed System Solution:

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Matter and Energy A vertical piston-cylinder device (D = 5 cm) contains a trapped mixture of gases. The walls of the cylinder are insulated, but heat is allowed to enter through the floor of the cylinder at a rate of 2.75 W over 20 seconds. The absolute gas pressure is 101.3 kPa, initially. If the change in energy of the system is 41 J, how far does the piston rise (in cm) during this process? Known: A gas mixture is heated and expands inside of a piston-cylinder Find: rise of piston, h [cm] Sketch: Assumptions: Closed System, Solution: Vertical-piston cylinder -> Isobaric

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