A sphere, a cube and a disc all of the same material, quality and volume are heated to 900 K and left in air. Which of these have the lowest rate of cooling

Concept:

The same material, quality, and volume are heated to 900 K and left in the air, then these will be cooled by convection and follows Newton's law of cooling. 

Newton's law of cooling:

• It states that the temperature of a body changes at a rate proportional to the difference in temperature between the body and its surroundings.

• From Newton’s law of cooling:

  Q = hA (Tω – T∞)

• Therefore, 

• So, for a given volume, the cooling rate depends upon the surface area.
• Out of the given option, the sphere has a minimum surface area. So the cooling rate will be least for the sphere.