Heat and Mass Transfer
According of Kirchhoff's law

Emissive power depends on temperature
Emissive power and absorptivity are constant for all bodies
Radiant heat is proportional to fourth power of absolute temperature
Ratio of emissive power to absorptive power for all bodies is same and is equal to the emissive power of a perfectly black body

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Heat and Mass Transfer
The heat transfer by conduction through a thick cylinder (Q) is given by (where T₁ = Higher temperature, T₂ = Lower temperature, r₁ = Inside radius, r₂ = Outside radius, l = Length of cylinder, and k = Thermal conductivity)

Q = [2πlk (T₁ - T₂)]/2.3 log (r₂/r₁)
Q = = 2πlk/2.3 (T₁ - T₂) log (r₂/r₁)
Q = [2π (T₁ - T₂)]/2.3 lk log (r₂/r₁)
Q = 2.3 log (r₂/r₁)/[2πlk (T₁ - T₂)]

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