The brick wall (of thermal conductivity 0.81 W/m · ◦ C) of a building has dimensions of 6.2 m by 12 m and is 8 cm thick. How much heat flows through the wall in a 16.9 h period when the average inside and outside temperatures are, respectively, 30◦C and 8◦C? Answer in units of MJ.

The heat flows through the wall is 1008.3 MJ

Explanation:

Given;

thermal conductivity, K = 0.81 W/m · ◦ C

dimension of the brick wall, =  6.2 m by 12 m

thickness of the brick wall, L = 8 cm = 0.08 m

period of heat flow, t = 16.9 h

temperature change, Δθ = 30◦C - 8◦C = 22°C

Power generated by the brick wall:

where;

A is the area of the brick wall

A = 6.2 m x 12 m = 74.4 m²

Heat flow through the wall:

E = Pt

where;

t is period of flow

t = 16.9 h = 16.9h x 60 mins x 60 sec = 60840 seconds

E = 16572.6 x 60840 = 1008276984 J

E = 1008.3 x 10⁶ J

E = 1008.3 MJ

Therefore, the heat flows through the wall is 1008.3 MJ