Heat and Mass Transfer
A composite slab has two layers of different materials with thermal conductivities k₁ and k₂. If each layer has the same thickness, then the equivalent thermal conductivity of the slab will be

2 k₁ k₂/ (k₁ + k₂)
k₁ k₂
(k₁ + k₂)
(k₁ + k₂)/ k₁ k₂

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Heat and Mass Transfer
Kirchhoff's law states that

The total radiation from a black body per second per unit area is directly proportional to the fourth power of the absolute temperature
The wave length corresponding to the maximum energy is proportional to the absolute temperature
None of these
The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body

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