A spherical steel ball of 12 mm diameter is initially at

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A spherical steel ball of 12 mm diameter is initially at 1000 K. It is slowly cooled in a surrounding of 300 K. The heat transfer coefficient between the steel ball and the surroundings is 5 W/m²K. The thermal conductivity of steel is 20 W/mK. The ternperature difference between the centre and the surface of the steel ball is

1. large because conduction resistance is far higher than the convective resistance.
2. large because conduction resistance is far less than the convective resistance.
3. small because conduction resistance is far higher than the convective resistance.
4. small because conduction resistance is far less than the convective resistance.

Correct Option: D

Bi =	hL	=	5 ×0.002	= 0.0005
	K		20

For the given condition, Biot number tends to zero, i.e. conduction resistance is far less than convection resistance. Therefore temperature between centre and surface is very small.