a) A cubic container full of hot water at a temperature of 90 °C is completely l

Question

a) A cubic container full of hot water at a temperature of 90 °C is completely lagged with an insulating material of thermal conductivity 6.4x10-4 Wcm-1K-1 (6.4x10-2 Wm-1K-1). The edges of the container are 1.0 m long and the thickness of the lagging is 1.0 cm. Estimate the rate of flow of heat through the lagging if the external temperature of the lagging is 40 °C. Mention any assumptions you make in deriving your result.

b. Discuss qualitatively how your result will be affected if the thickness of the lagging is increased considerably assuming that the temperature of the surrounding air is 18 °C.

Explanation / Answer

temperature of water = T1 = 90

temperature of lagging = T2 = 40

thickness = t = 1 cm = 0.01 m

length = l = 1m

total area of the surface = A = 6l^2 = 6

the rate of flow of heat is

directly proportiona to temperature difference dT = (T1-T2)

directly proportional to area of the surface

inversely proportional to thickness

H = K*A*dT/t

K = thermal conductivity

H = (6.4*10^-2*6*(90-40))/0.01

H = 1920 W

part(b)

H = (6.4*10^-2*6*(90-18))/0.01

H = 2765 W

flow rate increases

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