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Use the worked example above to help you solve this problem. Find the energy transferred in 1 h by conduction through a concrete wall 2.3 m high, 3.20 m long, and 0.20 m thick if one side of the wall is held at 20°C and the other side is at 5°C. 1391040 X Your response differs from the correct answer by more than 10%. Double check your calculations. J

Use the worked example above to help you solve this problem. Find the energy transferred in 1 h by conduction through a concrete wall 2.3 m high, 3.20 m long, and 0.20 m thick if one side of the wall is held at 20°C and the other side is at 5°C. 1391040 X Your response differs from the correct answer by more than 10%. Double check your calculations. J

College Physics
College Physics
11th Edition
ISBN:9781305952300
Author:Raymond A. Serway, Chris Vuille
Publisher:Raymond A. Serway, Chris Vuille
Chapter1: Units, Trigonometry. And Vectors
Section: Chapter Questions
Problem 1CQ: Estimate the order of magnitude of the length, in meters, of each of the following; (a) a mouse, (b)...
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EXAMPLE 11.9 Energy Transfer Through a Concrete Wall Goal Apply the equation of heat conduction. Problem Find the energy transferred in 1.00 h by conduction through a concrete wall 2.0 m high, 3.65 m long, and 0.20 m thick if one side of the wall is held at 20°C and the other side is at 5°C. Strategy P = kA(Th - Tc)/L gives the rate of energy transfer by conduction in joules per second. Multiply by the time and substitute given values to find the total thermal energy transferred. SOLUTION Q = PAt = kA(Th - Tc /L)At Multiply the energy transfer equation by At to find an expression for the total energy Q transferred through the wall. Substitute the numerical values to Q = (1.3 J/s · m °C)(7.3 m²)(15°C/0.20 m)(3600 s) = 2.6 x 106 J obtain Q, consulting the table for k. LEARN MORE Remarks Early houses were insulated with thick masonry walls, which restrict energy loss by conduction because k is relatively low. The large thickness L also decreases energy loss by conduction, as shown by energy transfer equation. There are much better insulating materials, however, and layering is also helpful. Despite the low thermal conductivity of masonry, the amount of energy lost is still rather large, enough to raise the temperature of 600 kg of water by more than 1°C. Question True or False: Materials having high thermal conductivities provide better insulation than materials having low thermal conductivities. False. A good insulator does not transfer energy by heat readily through it, and has low thermal conductivity. True. The rate of heat flow in a material is proportional to its thermal conductivity. True. Thermal conductivity measures a material's usefulness as heat insulation. False. A good insulator transfers energy by heat effectively, and has low thermal conductivity. PRACTICE IT Use the worked example above to help you solve this problem. Find the energy transferred in 1 h by conduction through a concrete wall 2.3 m high, 3.20 m long, and 0.20 m thick if one side of the wall is held at 20°C and the other side is at 5°C. 1391040 Your response differs from the correct answer by more than 10%. Double check your calculations. J EXERCISE HINTS: GETTING STARTED | I'M STUCK! A wooden shelter has walls constructed of wooden planks 1.00 cm thick. If the exterior temperature is -20.0°C and the interior is 5.00°C, find the rate of energy loss through a wall that has dimensions 1.95 m by 2.30 m. 2466 Do not follow the example too closely. What question is asked in the example? What question is asked in this exercise? How are these related? W
Transcribed Image Text:EXAMPLE 11.9 Energy Transfer Through a Concrete Wall Goal Apply the equation of heat conduction. Problem Find the energy transferred in 1.00 h by conduction through a concrete wall 2.0 m high, 3.65 m long, and 0.20 m thick if one side of the wall is held at 20°C and the other side is at 5°C. Strategy P = kA(Th - Tc)/L gives the rate of energy transfer by conduction in joules per second. Multiply by the time and substitute given values to find the total thermal energy transferred. SOLUTION Q = PAt = kA(Th - Tc /L)At Multiply the energy transfer equation by At to find an expression for the total energy Q transferred through the wall. Substitute the numerical values to Q = (1.3 J/s · m °C)(7.3 m²)(15°C/0.20 m)(3600 s) = 2.6 x 106 J obtain Q, consulting the table for k. LEARN MORE Remarks Early houses were insulated with thick masonry walls, which restrict energy loss by conduction because k is relatively low. The large thickness L also decreases energy loss by conduction, as shown by energy transfer equation. There are much better insulating materials, however, and layering is also helpful. Despite the low thermal conductivity of masonry, the amount of energy lost is still rather large, enough to raise the temperature of 600 kg of water by more than 1°C. Question True or False: Materials having high thermal conductivities provide better insulation than materials having low thermal conductivities. False. A good insulator does not transfer energy by heat readily through it, and has low thermal conductivity. True. The rate of heat flow in a material is proportional to its thermal conductivity. True. Thermal conductivity measures a material's usefulness as heat insulation. False. A good insulator transfers energy by heat effectively, and has low thermal conductivity. PRACTICE IT Use the worked example above to help you solve this problem. Find the energy transferred in 1 h by conduction through a concrete wall 2.3 m high, 3.20 m long, and 0.20 m thick if one side of the wall is held at 20°C and the other side is at 5°C. 1391040 Your response differs from the correct answer by more than 10%. Double check your calculations. J EXERCISE HINTS: GETTING STARTED | I'M STUCK! A wooden shelter has walls constructed of wooden planks 1.00 cm thick. If the exterior temperature is -20.0°C and the interior is 5.00°C, find the rate of energy loss through a wall that has dimensions 1.95 m by 2.30 m. 2466 Do not follow the example too closely. What question is asked in the example? What question is asked in this exercise? How are these related? W
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