Three rods of lengths `L,21` and `3L` having thermal conductivities `3K,2K` and `K` are connected end to end If cross sectional areas of three rods are equal If cross sectional areas of three rods are equal then equivalent thermal conductivity of the system is .

Options

A. `18K//13`
B. `36K//133`
C. `9K//13`
D. `12K//13`

Answer

A

Explanation

`R_(eff)=R_(1)+R_(2)+R_(3)`
`(l_(1)+l_(2)+l_(3))/(K_(eff))=(l_(1))/(K_(1))+(l_(2))/(K_(2))+(l_(3))/(K_(3)` .