Early houses were Insulated with thick masonry walls, which restrict energy loss

Question

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 degree C. True or False: Materials having high thermal conductivities provide better insulation than materials having low thermal conductivities. True. Thermal conductivity measures a material's usefulness as heat insulation. False. A good insulator does not transfer energy by heat readily through it, and has low thermal conductivity. False. A good insulator transfers energy by heat effectively, and has low thermal conductivity. True. The rate of heat flow in a material is proportional to its thermal conductivity. Use the worked example above to help you solve this problem. Find the energy transferred in 1 h by conduction through a concrete wall 1.8 m high, 3.75 m long, and 0.20 m thick if one side of the wall is held at 20 degree C and the other side is at 5 degree C. Your response differs significantly from the correct answer. Rework your solution from the beginning and check each step carefully. J

Explanation / Answer

we know, thermal conductivity of concrete, k = 1.5 W/(mk)

A = 1.8*3.75 = 6.75 m^2

L = 0.2 m

rate of heat transfered, dQ/dt or P = k*A*dT/L

= 1.5*6.75*(20-5)/0.2

= 759 J/s

heat tranfered during an hour, Q = (dQ*dt)*dt

= 759*60*60

= 2.73*10^6 J

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