Two rods of equal length and diameter have thermal conductivities 3 & 4 units .it they joined in series . Find the thermal conductivity of the combination.

Explanation

Given `L_(1) = L_(2) = L, A_(1) = A_(2) = A`
`k_(1) = 3` units `k_(2) =4 ` units
If `R_(1)` & `R_(2)` are the thermal resistances of two rods then
`R_(1) = (L_(1))/(k_(1)A_(1)) = (L)/(3xx A) , R_(2) = (L_(2))/(k_(2)A_(2)) = (L)/(4xx A)`
If `R_(s)` is the equivalent thermal resistance of 2 rods inn series and `k_(s)` is the equivalent thermal conductivity
`R_(s) = (L_(1)+ L_(2))/(k_(s) xx A) = (2L)/(K_(s)A)`