407,642 views
3 votes
3 votes
Consider two walls, A and B, with the same surface areas and the same temperature drops across their thicknesses. The ratio of their thermal conductivities is kA/kB=4 and the ratio of the wall thickness is LA/LB=2. The ratio of heat transfer rates through the walls qA/qB is:

(a) 0.5

(b) 1

(c) 2

(d) 4

(e) 8

(f) None of them

User Parris
by
3.2k points

1 Answer

15 votes
15 votes

Answer:

(c) 2

Step-by-step explanation:

Heat transfer across the walls due to conduction is given by:


image

where,

q = heat transfer rate

K = thermal conductivity

A = Area

ΔT = change in temperature

L = thickness

For wall A:


image

For wall B:


image

Because the change of temperature and area of walls are the same. Dividing both terms:


image

using values given in the question:


image

Therefore, the correct answer is:

(c) 2

User Fabio
by
2.5k points