A wall has two layers A and B each made of different materials. Both the layers have the same thickness. The thermal conductivity of materials A is twice of B. Under thermal equilibrium temperature difference across the layer B is `36^@C`. The temperature difference across layer A is

Options

A. `6^(@)C`
B. `9^(@)C`
C. `18^(@)C`
D. `27^(@)C`

Answer

B

Explanation

The two are in series.
`K_(A)=3K_(B)`
i.e., thermal resistance of A (say `R_(A)`) is one third that of B (say `R_(8)`)
i.e., `R_(B)=3R_(A)`
`:.` Thermal current =`36/(4R_(A))=9/(R_(A))`
`:.` Temperature difference across 'A' `=(9/(R_(A))).R_(A)=9^(@)C`