Question

Three conducting rods of same material and cross-section are shown in figure. Temperatures of A,D and C are maintained at `20^(@)C, 90^(@)C and 0^(@)C`. The ratio of lengths BD and BC if there is no heat flow in AB is

A. `2/7`
B. `7/2`
C. `9/2`
D. `2/9`

Answer 1

Correct Answer - B
There will be no heat flow through AB, if temperature of B is also `20^(@)`C and heat current ini DB is equal to heat current in BC.

`therefore` `H_(DB) = H_(BC)`
or `(70)/(I_(BD)//KA) = 20/(I_(BC)//KA)` or `(I_(BD)/I_(BC)) = 7/2`