Heat Transfer
Nusselt number is the ratio of the

Temperature difference to the temperature gradient at the wall
Temperature gradient of the wall to that across the entire pipe
None of these
Heat flux at the wall to that across the entire pipe

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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 B & C
Both the spheres will emit equal amount of radiation per unit time in the beginning
Both will absorb equal amount of radiation from the surrounding 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

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