Question

A wall has two layers A and B each made of different materials. The layer A is 10cm thick and B is 20 cm thick. The thermal conductivity of A is thrice that of B. Under thermal equilibrium temperature difference across the wall is `35^@C`. The difference of temperature across the layer A is
A. `30^@C`
B. `14^@C`
C. `8.75^@C`
D. `5^@C`

Answer 1

Correct Answer - D
`H_A = H_(Total)`
`:. (TD)_A/R_A = (TD)/(R_A + R_B)`
`:. (TD)_A = ((R_A)/(R_A+R_B)) TD`
`=(1/(1+R_B//R_A)) TD` ……….(i)
`R_B/R_A = (l_B/l_A)(K_AA)/(K_BA) = (20/10)(3) = 6`
Substituting in Eq. (i), we get
`(TD)_A = (1/(1+6)) xx 35 = 5^@C` .