Heat Transfer
The variation of thermal conductivity of a metal with temperature is often correlated using an expression of the form K = K₀ + at. where, K is the thermal conductivity and T is the temperature (in °K). The units of 'a' in SI system will be

W/m.k²
None, A is just a number
W/m
W/m.k

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Heat Transfer
A hollow sphere and a solid sphere of the same material and equal radii are heated to the same temperature. In this case,

Both will absorb equal amount of radiation from the surrounding in the beginning
Both the spheres will emit equal amount of radiation per unit time in the beginning
The cooling rate will be the same for the two spheres and hence the two spheres will have equal temperatures at any instant
Both B & C

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Heat Transfer
Extended heat transfer surface like fins are used to increase the heat transfer rate. Fin efficiency is defined as the ratio of heat transferred across the fin surface to the theoretical heat transfer across an equal area held at the

Average temperature of the fin
Surrounding temperature
Constant temperature equal to that of the base
Temperature of the fin end

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