Question

A composite wall 8 m long and 4 m high consists of 4 cm of brick, 8 cm of concrete, 6 cm of fiberglass and 10 cm of corkboard. The inside temperature is 18°C, and the outside temperature is 40°C. How much heat is flow into the house through this wall in 10 hours?

Answer 1

Heat flow through walls in 10 hours is

`Q = 6.1 * 10^6 J`

Step-by-step explanation:

Thermal conductivity of all the materials is as follows

1). Concrete = 0.8

2). Brick = 0.6

3). Fibreglass = 0.04

4). Corkboard = 0.04

Area of the wall is given as

`A = (8 * 4) = 32 m^2`

now the thermal resistance due to each wall is given as

`R_1 = (0.04)/(0.6(32)) = 2.08 * 10^(-3)`

`R_2 = (0.08)/(0.8(32)) = 3.125 * 10^(-3)`

`R_3 = (0.06)/(0.04(32)) = 46.87 * 10^(-3)`

`R_2 = (0.10)/(0.04(32)) = 78.13 * 10^(-3)`

Now total thermal resistance of all walls

`R = R_1 + R_2 + R_3 + R_4`

`R = (2.08 + 3.125 + 46.87 + 78.13) * 10^(-3)`

`R = 0.130`

now rate of heat transfer per second is given as

`(dQ)/(dt) = (T_1 - T_2)/(R)`

`Q = (40 - 18)/(0.130)(10 * 3600)`

`Q = 6.1 * 10^6 J`