Effect of a Window in a Door. A carpenter builds a solid wood door with dimensions 2.00 m × 0.95 m × 5.0 cm. Its thermal conductivity is k = 0.120 W/m · K. The air films on the inner and outer surfaces of the door have the same combined thermal resistance as an additional 1.8-cm thickness of solid wood. The inside air temperature is 20.0°C, and the outside air temperature is −8.0°C. (a) What is the rate of heat flow through the door? (b) By what factor is the heat flow increased if a window 0.500 m on a side is inserted in the door? The glass is 0.450 cm thick, and the glass has a thermal conductivity of 0.80 W/m · K. The air films on the two sides of the glass have a total thermal resistance that is the same as an additional 12.0 cm of glass.
Effect of a Window in a Door. A carpenter builds a solid wood door with dimensions 2.00 m × 0.95 m × 5.0 cm. Its thermal conductivity is k = 0.120 W/m · K. The air films on the inner and outer surfaces of the door have the same combined thermal resistance as an additional 1.8-cm thickness of solid wood. The inside air temperature is 20.0°C, and the outside air temperature is −8.0°C. (a) What is the rate of heat flow through the door? (b) By what factor is the heat flow increased if a window 0.500 m on a side is inserted in the door? The glass is 0.450 cm thick, and the glass has a thermal conductivity of 0.80 W/m · K. The air films on the two sides of the glass have a total thermal resistance that is the same as an additional 12.0 cm of glass.
Effect of a Window in a Door. A carpenter builds a solid wood door with dimensions 2.00 m × 0.95 m × 5.0 cm. Its thermal conductivity is k = 0.120 W/m · K. The air films on the inner and outer surfaces of the door have the same combined thermal resistance as an additional 1.8-cm thickness of solid wood. The inside air temperature is 20.0°C, and the outside air temperature is −8.0°C. (a) What is the rate of heat flow through the door? (b) By what factor is the heat flow increased if a window 0.500 m on a side is inserted in the door? The glass is 0.450 cm thick, and the glass has a thermal conductivity of 0.80 W/m · K. The air films on the two sides of the glass have a total thermal resistance that is the same as an additional 12.0 cm of glass.
The exhaust duct from a heater has an inside diameter of 114.3 mm with ceramic walls 6.4 mm thick. The average k = 1.52 W/mK. Outside this wall, an insulation of rock wool 102 mm thick is installed. The thermal conductivity of the rock wool is k = 0.046+1.56*10-4T (°C) (W/mK). The inside surface temperature of the ceramic is T1= 588.7 K, and the outside surface temperature of the insulation is T3= 311 K. Calculate the heat loss for 1.5 m of duct and the interface temperature T2between the ceramic and the insulation.Assumesteady heat transfer.Hint: The correct value of km for insulation is that evaluated at the mean temperature of T2+T3/2. Hence, for the first trial assume a mean temperature of, say, 448 K. Then, calculate the heat loss and T2. Using this new T2, calculate a new mean temperature and proceed as before.
Animal tissue helps prevent excessive heat loss by conduction. In species 1, the outer tissue has thermal conductivity k1, cross-sectional area A1, and thickness x1. In species 2, the outer tissue has thermal conductivity k2, cross-sectional area A2, and thickness x2. For the same difference between internal and external temperatures, how is the rate of heat loss in species 2 (P2) related to that in species 1 (P1)?
Group of answer choices
P2 = (A1/A2) (x1/x2) (k2/k1) P1
P2 = (A2/A1) (x1/x2) (k2/k1) P1
P2 = (A1/A2) (x2/x1) (k2/k1) P1
P2 = (A2/A1) (x2/x1) (k2/k1) P1
P2 = (A2/A1) (x1/x2) (k1/k2) P1
The walls of a house consists of L=0.02 m thick plywood backed byinsulation with the same thickness. The temperature of the inside surface of the wall (the insulation side) is T2=25 oC, while the temperature at the outside surface (the plywood side) is T1= -15 oC, both being constant. The thermal conductivities of the plywood and insulation are, respectively k1=0.09 J/(s m oC) and k2=0.03 J/(s m oC).a) Find the temperature T at the plywood – insulation interface. b) If the total surface area of the walls is 70 m2, find the amount of heat the house looses in 24 hours. (Assuming all the heat losses are due to the thermal conductivity of the walls).c) Does the entropy of the house change? If it does, by how much?d) Does the entropy of the Universe change? If it does, by how much?e) If the cost of electric power is $0.15 per kilowatt hour, how much one has to pay to keep the house warm for 24 hours?
Chapter 17 Solutions
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